Digital Subscriber Line

Digital Subscriber Line, or DSL, is a family of technologies that provide digital data transmission over the wires used in the "last mile" of a local telephone network. Typically, the download speed of DSL ranges from 128 kilobits per second (Kb/s) to 6000 Kb/s depending on DSL technology and service level implemented. Upload speed is lower than download speed for ADSL and symetrical for SDSL.

History

The origin of Digital Subscriber Line technology dates back to 1988, when engineers at Bell Labs devised a way to carry a digital signal over the unused frequency spectrum available on the twisted pair cables running between the telephone company's telephone exchange and the customer premises. Implementation of DSL could permit an ordinary telephone line to provide digital communication without interfering with voice services. However, the management of incumbent local exchange carriers (ILEC) were not enthusiastic about it, since DSL was not as profitable as installing a second phone line for consumers who preferred simultaneous dial-up internet and voice connections, and the broadband data connection would cannibalize existing ISDN customers. This changed in the late 1990s when cable television companies began marketing broadband Internet access. Realizing that most consumers would prefer broadband Internet to dial-up Internet, ILECs rushed out the DSL technology, which they had delayed implementing, as an attempt to win market share from the broadband Internet access offered by cable television operators. As of 2005, DSL is the principal competition of cable modems for providing high speed Internet access to home consumers in Europe and North America. Older ADSL standards can deliver 8 Mbit/s over about 2 km (1.24 miles) of unshielded twisted pair copper wire. The latest standard ADSL2+ can deliver more than 20 Mbit/s over similar distances. Many customers, however, are located farther than 2 km (1.24 miles) from the telephone exchange, which reduces the amount of bandwidth available (thereby reducing the data rate) on the wires. On average, cable is faster than DSL in most commercial situations. Modern cable systems can provide 30 Mbit/s downstream, but this bandwidth is shared between all the users on the cable segment (which could be from 100 to 200 households).

Operation

The local loop of the Public Switched Telephone Network was initially designed to carry POTS voice communication and signaling, since the concept of data communications as we know it today did not exist. For reasons of economy, the phone system nominally passes audio between 300 and 3,400 Hz, which is regarded as the range required for human speech to be clearly intelligible. This is known as commercial bandwidth. Dial-up services using modems are constrained by the Shannon capacity of the POTS channel. At the local telephone exchange (UK terminology) or central office (US terminology) the speech is generally digitized into a 64 kbit/s data stream in the form of an 8 bit signal using a sampling rate of 8,000 Hz, therefore – according to the Nyquist theorem – any signal above 4,000 Hz is not passed by the phone network (and has to be blocked by a filter to prevent aliasing effects). The local loop connecting the telephone exchange to most subscribers is capable of carrying frequencies well beyond the 3.4 kHz upper limit of POTS. Depending on the length and quality of the loop, the upper limit can be tens of megahertz. DSL takes advantage of this unused bandwidth of the local loop by creating 4312.5 Hz wide channels starting between 10 and 100 kHz, depending on how the system is configured. Allocation of channels continues at higher and higher frequencies (up to 1.1 MHz for ADSL) until new channels are deemed unusable. Each channel is evaluated for usability in much the same way an analog modem would on a POTS connection. More usable channels equates to more available bandwidth, which is why distance and line quality are a factor. The pool of usable channels is then split into two groups for upstream and downstream traffic based on a preconfigured ratio. Once the channel groups have been established, the individual channels are bonded into a pair of virtual circuits, one in each direction. Like analog modems, DSL transceivers constantly monitor the quality of each channel and will add or remove them from service depending on whether or not they are usable. The commercial success of DSL and similar technologies largely reflects the fact that in recent decades, while electronics have been getting faster and cheaper, the cost of digging trenches in the ground for new wires remains expensive. All flavors of DSL employ highly complex digital signal processing algorithms to overcome the inherent limitations of the existing twisted pair wires. Not long ago, the cost of such signal processing would have been prohibitive but because of VLSI technology, the cost of installing DSL on an existing local loop, with a DSLAM at one end and a DSL modem at the other end is orders of magnitude less than would be the cost of installing a new, high-bandwidth fiber-optic cable over the same route and distance. Most residential and small-office DSL implementations reserve low frequencies for POTS service, so that with suitable filters and/or splitters the existing voice service continues to operate independent of the DSL service. Thus POTS-based communications, including fax machines and analog modems, can share the wires with DSL. Only one DSL modem can use the subscriber line at a time. The standard way to let multiple computers share a DSL connection is to use a router that establishes a connection between the DSL modem and a local Ethernet or Wi-Fi network on the customer's premises. Once upstream and downstream channels are established, they are used to connect the subscriber to a service such as an Internet service provider.

Equipment

The subscriber end of the connection consists of a DSL modem. This converts data from the digital signals used by computers into a voltage signal of a suitable frequency range which is then applied to the phone line. In the early days of DSL, installation required a technician to visit the premises. A "splitter" was installed near the demarcation point, from which a dedicated data line was installed. Today, many DSL vendors offer a self-install option, in which they ship equipment and instructions to the customer. In this case, since no changes are made to the cable plant on the customer premises, all the phone wires are carrying both POTS and DSL signal frequencies; therefore the customer generally needs to plug a DSL filter into each telephone outlet. However, this can sometimes cause degradation of the DSL signal (especially if more than 5 analogue devices are connected to the line) because the DSL signal is present on all telephone wiring in the building. A way to circumvent this is to install one filter upstream from all telephone jacks in the building, except for the jack to which the DSL modem will be connected. Since this requires wiring changes by the customer and may not work on some (poorly designed) household telephone wiring, it is rarely done. It is usually much easier to install filters at each telephone jack that is in use. Establishing new cable modem or satellite broadband service generally does require a visit by a technician to the premises, even when there is existing cable television service to this customer; this constitutes one of the major competitive advantages of DSL over cable broadband service. At the exchange a digital subscriber line access multiplexer (DSLAM) terminates the DSL circuits and aggregates them, where they are handed off onto other networking transports. It also separates out the voice component.

Protocols and configurations

Many DSL technologies implement an ATM layer over the low-level bitstream layer to enable the adaptation of a number of different technologies over the same link. DSL implementations may create bridged or routed networks. In a bridged configuration, the group of subscriber computers effectively connect into a single subnet. The earliest implementations used DHCP to provide network details such as the IP address to the subscriber equipment, with authentication via MAC address or an assigned host name. Later implementations often use PPP over Ethernet or ATM (PPPoE or PPPoA), while authenticating with a userid and password and using PPP mechanisms to provide network details. DSL also has contention ratios which need to be taken into consideration when deciding between broadband technologies.

DSL technologies

The line length limitations from telephone exchange to subscriber are more restrictive for higher data transmission rates. Technologies such as VDSL provide very high speed, short-range links as a method of delivering "triple play" services (typically implemented in fiber to the curb network architectures). Example DSL technologies (sometimes called xDSL) include:
- High-bit-rate Digital Subscriber Line (HDSL), covered in this article
- Symmetric Digital Subscriber Line (SDSL), a standardised version of HDSL
- Asymmetric Digital Subscriber Line (ADSL), a version of DSL with a slower upload speed
- Rate-Adaptive Digital Subscriber Line (RADSL)
- Very-high-bit-rate Digital Subscriber Line (VDSL)
- Very-high-bit-rate Digital Subscriber Line 2 (VDSL2), an improved version of VDSL
- G. Symmetric High-speed Digital Subscriber Line (G.SHDSL), a standardised replacement for early proprietary SDSL by the International Telecommunication Union Telecommunication Standardization Sector

Transmission methods

Transmission methods vary by market, region, carrier, and equipment.
- CAP: Carrierless Amplitude Phase Modulation - deprecated in 1996
- DMT: discrete multitone modulation, otherwise known as OFDM
- OFDM: Orthogonal frequency-division multiplexing

See also

- Broadband Internet access
- Asymmetric Digital Subscriber Line (ADSL)
- Carrierless Amplitude Phase Modulation (CAP)
- Digital subscriber line access multiplexer (DSLAM)
- DSL around the world
- IDSL
- ISDN
- Modem
- Orthogonal frequency-division multiplexing (OFDM)
- Triple play (telecommunications)

External links

- [http://www.iol.unh.edu/training/dsl/ The UNH-IOL DSL Knowledgebase (advanced tutorials)]
- [http://www.uk-bug.net The UK Broadband Usergroup]
- [http://www.howstuffworks.com/ Howstuffworks.com] - [http://electronics.howstuffworks.com/dsl.htm "How DSL Works"]
- [http://tldp.org/HOWTO/DSL-HOWTO/ DSL HOWTO for Linux]
- [http://www.t1.org/t1e1/_e14home.htm ANSI Working Group T1E1.4, a standards group for DSL]
- [http://www.dslforum.org/ DSL Forum, a promotional trade organization for the ADSL industry] Category:Telephony Category:Broadband ja:デジタル加入者線

Baud

:For the town in France, see Baud, Morbihan. In telecommunications and electronics, baud (pronounced ) is a measure of the "signaling rate" which is the number of changes to the transmission media per second in a modulated signal. It is named after Émile Baudot, the inventor of the Baudot code for telegraphy. For Example: 250 baud means that 250 signals are transmitted in one second. If each signal carries 4 bits of information then in each second 1000 bits are transmitted. This is abbreviated as 1000 bit/s. Note : Baud rate should not be confused with data rate (also called "bits per second"). Each signalling event transmitted can carry one or more bits (for example, 8 bits in 256-QAM modulation) of information. When each signalling event transmitted carries one bit the baud rate and the data rate are equal. However, it is more common to make better use of bandwidth by encoding multiple bits in one event. This reduces the transmission time required for sending information. Thus, a 2400 bit/s modem actually transmits at 600 baud, where each quadrature amplitude modulation event carries four bits of information. A clear example of the difference between the baud rate (or signalling rate) and the data rate (or bit rate) is a man using a single semaphore flag. He can move his arm to a new position once each second, so his signalling rate (or baud rate) is 1 per second. However, the flag can be held in one of eight distinct positions: Straight up, 45 degrees left, 90 degrees left, 135 degrees left, straight down, 135 degrees right, 90 degrees right, and 45 degrees right. This means each signal carries three bits of information, as it takes 3 binary digits to encode 8 distinct states - so the data rate is 3 bits per second. Modems work in the same way - a 2400 bit/s modem will typically have a baud rate of 600 per second - but each signal carries 4 bits of information encoded within it, allowing a data rate of 2400 bit/s.

See also

- Modem
- Bandwidth
- List of device bandwidths ja:ボー

100BASE-T

100BASE-T is any of several Fast Ethernet 100 Mbit/s (12.5 MB/s including overhead) CSMA/CD standards for twisted pair cables, including: 100BASE-TX (100 Mbit/s over two-pair Cat5 or better cable), 100BASE-T4 (100 Mbit/s over four-pair Cat3 or better cable, defunct), 100BASE-T2 (100 Mbit/s over two-pair Cat3 or better cable, also defunct). The segment length for a 100BASE-T cable is limited to 100 meters (as with 10BASE-T and 1000BASE-T). All are or were standards under IEEE 802.3 (approved 1995). The vast majority of common implementations or installations of 100BASE-T are done with 100BASE-TX.

References

- [http://www.inetdaemon.com/tutorials/lan/ethernet/origins.html Origins and History of Ethernet] Category:Ethernet cables

Ethernet

Ethernet is a frame-based computer networking technology for local area networks (LANs). The name comes from the physical concept of ether. It defines wiring and signaling for the physical layer, and frame formats and protocols for the media access control (MAC)/data link layer of the OSI model. Ethernet is mostly standardized as IEEEs 802.3. It has become the most widespread LAN technology in use during the 1990s to the present, and has largely replaced all other LAN standards such as token ring, FDDI, and ARCNET.

History

Ethernet was originally developed as one of the many pioneering projects at Xerox PARC. A common story states that Ethernet was invented in 1973, when Robert Metcalfe wrote a memo to his bosses at PARC about Ethernet's potential. But Metcalfe claims Ethernet was actually invented over a period of several years. In 1976, Metcalfe and his assistant David Boggs published a paper titled, Ethernet: Distributed Packet-Switching For Local Computer Networks. Metcalfe left Xerox in 1979 to promote the use of personal computers and local area networks (LANs), forming 3Com. He convinced DEC, Intel, and Xerox to work together to promote Ethernet as a standard (DIX). The standard was first published on September 30 1980. It competed with two largely proprietary systems, token ring and ARCNET, but those soon found themselves buried under a tidal wave of Ethernet products. In the process, 3Com became a major company. Metcalfe sometimes jokingly credits Jerry Saltzer for 3Com's success. Saltzer cowrote an influential paper suggesting that token-ring architectures were theoretically superior to Ethernet-style technologies. This result, the story goes, left enough doubt in the minds of computer manufacturers that they decided not to make Ethernet a standard feature, which allowed 3Com to build a business around selling add-in Ethernet network cards. This also led to the saying "Ethernet works better in practice than in theory," which, though a joke, actually makes a valid technical point: the characteristics of typical traffic on actual networks differ from what had been expected before LANs became common in ways that favor the simple design of Ethernet. Metcalfe and Saltzer worked on the same floor at MIT's Project MAC while Metcalfe was doing his Harvard dissertation, in which he worked out the theoretical foundations of Ethernet.

General description

dissertation that supports both coaxial-based 10BASE2 (BNC connector, left) and Twisted-pair-based 10BASE-T ( RJ-45 connector, right).]] Ethernet is based on the idea of peers on the network sending messages in what was essentially a radio system, captive inside a common wire or channel, sometimes referred to as the ether. (This is an oblique reference to the luminiferous aether through which 19th century physicists incorrectly theorized that electromagnetic radiation traveled.) Each peer has a unique 48-bit key known as the MAC address to ensure that all systems in an Ethernet network have distinct addresses. By default network cards come programmed with a globally unique address but this can generally be changed and there are a number of reasons for doing so. Due to the ubiquity of Ethernet and the ever-reducing cost of the hardware needed to support it, most manufacturers build the functionality of an Ethernet card directly into PC motherboards. Despite the huge changes in ethernet from a very thick coaxial cable bus running at 10 megabit to point to point links running at 1 gigabit and beyond, the different variants remain essentially the same from the programmer's point of view and are easily interconnected using readily available inexpensive hardware. It has been observed that Ethernet traffic has self-similar properties, with important consequences for traffic engineering.

CSMA/CD shared medium Ethernet

A scheme known as carrier sense multiple access with collision detection (CSMA/CD) governs the way the computers share the channel. Originally developed in the 1960s for the ALOHAnet in Hawaii using radio, the scheme is relatively simple compared to token ring or master controlled networks. When one computer wants to send some information, it obeys the following algorithm: # Start - If the wire is idle, start transmitting, else go to step 4 # Transmitting - If detecting a collision, continue transmitting until the minimum packet time is reached (to ensure that all other transmitters and receivers detect the collision) then go to step 4. # End successful transmission - Report success to higher network layers; exit transmit mode. # Wire is busy - Wait until wire becomes idle # Wire just became idle - Wait a random time, then go to step 1, unless maximum number of transmission attempts has been exceeded # Maximum number of transmission attempt exceeded - Report failure to higher network layers; exit transmit mode This works something like a dinner party, where all the guests talk to each other through a common medium (the air). Before speaking, each guest politely waits for the current guest to finish. If two guests start speaking at the same time, both stop and wait for short, random periods of time. The hope is that by each choosing a random period of time, both guests will not choose the same time to try to speak again, thus avoiding another collision. Exponentially increasing back-off times (determined using the truncated binary exponential backoff algorithm) are used when there is more than one failed attempt to transmit. Ethernet originally used a shared coaxial cable winding around a building or campus to every attached machine. Computers were connected to an Attachment Unit Interface (AUI) transceiver, which in turn connected to the cable. While a simple passive wire was highly reliable for small Ethernets, it was not reliable for large extended networks, where damage to the wire in a single place, or a single bad connector could make the whole Ethernet segment unusable. Coax was also prone to very strange failure modes when an electrical discontinuity reflected the signal in such a manner that some nodes would work just fine while others would work slowly due to excessive retries or not at all; these could be much more painful to diagnose than a complete failure of the segment. Debugging such failures often involved several people crawling around wiggling connectors while others watched the displays of computers running ping and shouted out reports as performance changed. Since all communications happen on the same wire, any information sent by one computer is received by all, even if that information was intended for just one destination. The network interface card filters out information not addressed to it, interrupting the CPU only when applicable packets are received unless the card is put into "promiscuous mode". This "one speaks, all listen" property is a security weakness of shared-medium Ethernet, since a node on an Ethernet network can eavesdrop on all traffic on the wire if it so chooses. Use of a single cable also means that the bandwidth is shared, so that network traffic can slow to a crawl when, for example, the network and nodes restart after a power failure.

Ethernet repeaters and hubs

As Ethernet grew, the Ethernet hub was developed to make the network more reliable and the cables easier to connect. For signal degradation and timing reasons, Ethernet segments have a restricted size which depends on the medium used. For example, 10BASE5 coax cables have a maximum length of 500 metres (1,640 feet). A greater length can be obtained by using an Ethernet repeater, which takes the signal from one Ethernet cable and repeats it onto another cable. Repeaters can be used to connect up to five Ethernet segments, three of which can have attached devices. This also alleviates the problem of cable breakages: when an Ethernet coax segment breaks, all devices on that segment are unable to communicate; repeaters allowed the other segments to continue working. Like most other high-speed buses, Ethernet segments must be terminated with a resistor at both ends. For coaxial cable, each end of the cable must have a 50-ohm resistor and heatsink attached, called a terminator and affixed to a male N or BNC connector. If this is not done, the result is the same as if there is a break in the cable: the AC signal on the bus will be reflected, rather than dissipated, when it reaches the end. This reflected signal is indistinguishable from a collision, and so no communication can take place. A repeater electrically isolates the segments connected to it, regenerating and retiming the signal. Most repeaters have an "auto-partition" function, which partitions (removes from service) a segment when it has too many collisions or collisions that last too long, so that the other segments are not affected by the broken one. The repeater reconnects the segment when it detects activity without collisions. People recognized the usefulness of cabling in a star topology, and network vendors started creating repeaters having multiple ports. Multi-port repeaters are now known as hubs. Hubs can be connected to other hubs and/or a coax backbone. The first hubs were known as "multiport transceivers" or "fanouts". The best-known example is DEC's DELNI. These devices allow multiple hosts with AUI connections to share a single transceiver. They also allow creation of a small standalone Ethernet segment without using a coax cable. Network vendors such as DEC and SynOptics sold hubs which connected many 10BASE-2 thin coaxial segments. 10BASE-2 The development of Ethernet on unshielded twisted-pair cables (UTP), beginning with StarLAN and continuing with 10BASE-T eventually made Ethernet over coax obsolete. These variations allowed unshielded twisted-pair Cat-3/Cat-5 cable and RJ45 telephone connectors to connect endpoints to hubs, replacing coaxial and AUI cables. Hubs made Ethernet networks more reliable by preventing problems with one cable or device from affecting other devices on the network. Twisted-pair Ethernet resolves the termination problem by making every segment point-to-point, so termination can be built into the hardware rather than requiring a special external resistor. AUI Despite the physical star topology, hubbed Ethernet networks are half-duplex and still use CSMA/CD, with only minimal cooperation from the hub in dealing with packet collisions. Every packet is sent to every port on the hub, so bandwidth and security problems aren't addressed. The total throughput of the hub is limited to the speed of a single link, either 10 or 100 Mbit/s, minus the overhead for preambles, inter-frame gaps, headers, trailers, and padding. Collisions also reduce the total throughput, especially when the network is heavily loaded. In the worst case when there are lots of hosts with long cables that transmit many short frames, excessive collisions that seriously reduce throughput can happen with loads as low as 50%. A more typical configuration can tolerate higher loads before collisions seriously reduce throughput.

Bridging and Switching

While repeaters could isolate some aspects of Ethernet segments, such as cable breakages, they still forward all traffic to all Ethernet devices. This creates significant limits on how many machines can communicate on an Ethernet network. To alleviate this, bridging was created to communicate at the data link layer while isolating the physical layer. With bridging, only well-formed packets are forwarded from one Ethernet segment to another; collisions and packet errors are isolated. Bridges learn where devices are, by watching MAC addresses, and do not forward packets across segments when they know the destination address is not located in that direction. Control mechanisms like spanning-tree protocol enable a collection of bridges to work together in coordination. Early bridges examined each packet one by one, and were significantly slower than hubs (repeaters) at forwarding traffic, especially when handling many ports at the same time. In 1989 the networking company Kalpana introduced their EtherSwitch, the first Ethernet switch. An Ethernet switch does bridging in hardware, allowing it to forward packets at full wire speed. Most modern Ethernet installations use Ethernet switches instead of hubs. Although the wiring is identical to hubbed Ethernet, switched Ethernet has several advantages over shared medium Ethernet including greater bandwidth and better isolation from misbehaving devices. Switched networks typically have a star topology, even though they may still implement a single Ethernet shared medium from the viewpoint of attached machines, if they use the half-duplex option. Full-duplex Ethernet in the 10BASE-T and later standards is not a shared-medium system. Initially, Ethernet switches work like Ethernet hubs, with all traffic being echoed to all ports. However, as the switch "learns" the end-points associated with each port, it ceases to send non-broadcast traffic to ports other than the intended destination. In this way, Ethernet switching can allow the full wire speed of Ethernet to be used by any given pair of ports on a single switch. Since packets are typically only delivered to the port they are intended for, traffic on a switched Ethernet is slightly less public than on shared-medium Ethernet. Despite this, switched Ethernet should still be regarded as an insecure network technology, because it is easy to subvert switched Ethernet systems by means such as ARP spoofing and MAC flooding, as well as for network administrators to use monitoring functions to copy traffic from the network. When only a single device (anything but a hub) is connected to a switch port, full-duplex Ethernet becomes possible. In full duplex mode both devices can transmit to each other at the same time and there is no collision domain. This doubles the agregate bandwidth of the link and was sometimes advertised as double the link speed (e.g. 200 Mbit/s) to account for this. However this is misleading as performance will only double if traffic patterns are symetrical (which in reality they rarely are). The elimination of the collision domain also means that all the links bandwidth can be used (collisions can occupy a lot of bandwidth as links get busy) and that segment length is not limited by the need for correct collision detection (this is most significiant with some of the fiber variants of ethernet). It is essential that both the switch port and the device connected to it use the same duplex setting. Most 100BASE-TX and 1000BASE-T devices support auto-negotiation, where they signal the speed and duplex to use. However, if auto-negotiation is disabled or not supported, the duplex must be set by auto-detection or manually on both the switch port and the device to prevent duplex mismatch, a common cause of problems with Ethernet (the device set to half-duplex will report late collisions and the device set to full-duplex will report runts). Many low-end switches lack the ability for manual speed and duplex setting, so ports always try to auto-negotiate. When auto-negotiation is enabled but does not succeed (e.g., because the other device does not support it), auto-detection sets the port to half-duplex. The speed can be automatically sensed, so connecting a 10BASE-T device to a 10/100 switch port with auto-negotiation enabled will correctly result in a half-duplex 10BASE-T connection. But connecting a device configured for full duplex 100 Mbit operation to a switch port configured to auto-negotiate (or vice versa) will result in a duplex mismatch. Even when both ends of a cable are capable of autosensing speed and duplex settings, it is very common for them to guess wrongly and fall back to 10 Mbit mode. Therefore, if performance is worse than expected, one should check whether a computer has put itself into 10 Mbit mode, and if one knows the other end is 100 Mbit capable, manually force it into the correct mode. Problems also occur when two nodes try to operate at speeds faster than the cable can support, such as attempting 100BASE-T on Category 3 cable or 1000BASE-T on Category 3 or Category 5 cable. Unlike ADSL and conventional dialup modems, which perform an elaborate "training" sequence to determine the maximum data rate supported by the link, Ethernet nodes merely exchange speed capability messages and choose the highest speed supported by both ends. No attempt is made to see if the link can actually run at that speed, so if it's beyond the cable's capability, then the link will fail. The solution is to force either or both ends down to a speed supported by the cable.

Dual speed hubs

In the early days of fast ethernet, fast ethernet switches were relatively expensive devices. However, hubs suffered from the problem that if there were any 10baseT devices connected then the whole system would have to run at 10 Mbit. Therefore a compromise between a hub and a switch appeared known as a dual speed hub. These effectively split the network into two sections, each acting like a hubbed network at its respective speed then acted as a two port switch between those two sections. This meant they allowed mixing of the two speeds without the cost of a fast ethernet switch.

Ethernet frame types and the EtherType field

Frames are the format of data packets on the wire. There are several types of Ethernet frame:
- Original Ethernet Version I (no longer used)
- The Ethernet Version 2 or Ethernet II frame, the so-called DIX frame (named after DEC, Intel, and Xerox), this is the most common today, as it is often used directly by the Internet Protocol.
- Novell's homegrown Variation of IEEE 802.3 ("raw 802.3 frame") without LLC
- IEEE 802.2 LLC frame
- IEEE 802.2 LLC/SNAP frame In addition, Ethernet frames may optionally contain a IEEE 802.1Q tag to identify what VLAN it belongs to and its IEEE 802.1p priority (quality of service). This doubles the potential number of frame types. The different frame types have different formats and MTU values, but can coexist on the same physical medium. Ethernet Type II Frame format The most common Ethernet Frame format, type II The original Xerox Version 1 Ethernet had a 16 bit payload length field, although the maximum length of the payload was 1500 bytes. This length field was soon re-used in DEC/Intel/Xerox's Ethernet II as a label field, with the convention that values equal to or lower than 1500 indicated the use of the original Ethernet format, while higher values indicated what became known as an EtherType, and the use of the new frame format. This is now supported in the IEEE 802 protocols using the SNAP header. IEEE 802.2 defined the 16 bit field after the MAC addresses as a length field again. As Ethernet I framing is no longer used, this allows software to determine whether a frame is an Ethernet II frame or an IEEE 802.2 frame, allowing the coexistence of both standards on the same physical medium. All 802.2 frames have a logical link control (LLC) header. By examining this header, it is possible to determine whether it is followed by a SNAP (subnetwork access protocol) header. (Some protocols, particularly those designed for the OSI networking stack, operate directly on top of 802.2 LLC, which provides both datagram and connection-oriented network services.) The LLC header includes two additional eight-bit address fields (called service access points or SAPs in OSI terminology); when both source and destination SAP are set to the value 0xAA, the SNAP service is requested. Novell's "raw" 802.3 frame format was based on early IEEE 802.3 work. Novell used this as a starting point to create the first implementation of its own IPX Network Protocol over Ethernet. They did not use any LLC header but started the IPX packet directly after the length field. In principle this is not interoperable with the other later variants of 802.x Ethernet, but since IPX has always FF at the first byte (while LLC has not), this mostly coexists on the wire with other Ethernet implementations (with the notable exception of some early forms of DECnet which got confused by this). Novell Netware used this frame type by default until the mid nineties, and since Netware was very widespread back then (while IP was not) at some point in time most of the world's Ethernet traffic ran over "raw" 802.3 carrying IPX. Since Netware 4.10 Netware now defaults to IEEE 802.2 with LLC (Netware Frame Type Ethernet_802.2) when using IPX. There is a [http://groups-beta.google.com/group/bit.listserv.novell/browse_thread/thread/d00a24530625714c classic series of Usenet postings] by Novell's Don Provan that have found their way into numerous FAQs and are widely considered the definitive answer to the Novell Frame Type jungle. Mac OS uses 802.2/SNAP framing for the AppleTalk protocol suite on Ethernet ("EtherTalk") and Ethernet 2 framing for TCP/IP. The 802.2 variants of Ethernet are not in widespread use on common networks currently, with the exception of large corporate Netware installations that have not yet migrated to Netware over IP. In the past, many corporate networks supported 802.2 Ethernet to support transparent translating bridges between Ethernet and IEEE 802.5 Token Ring or FDDI networks. The most common framing type used today is Ethernet Version 2, as it is used by most Internet Protocol-based networks, with its EtherType set to 0x0800. There exists an [http://www.ietf.org/rfc/rfc1042.txt Internet standard] for encapsulating IP version 4 traffic in IEEE 802.2 frames with LLC/SNAP headers. It is almost never implemented on Ethernet (although it is used on Token Ring and FDDI networks). IP traffic can not be encapsulated in IEEE 802.2 LLC frames without SNAP because, although there is an LLC protocol type for IP, there is no LLC protocol type for ARP. IP Version 6 over Ethernet is also standardized based on IEEE 802.2 with LLC/SNAP. The IEEE 802.1Q tag, if present, is placed between the Source Address and the EtherType or Length fields. The first two bytes of the tag are the Tag Protocol Identifier (TPID) value of 0x8100. This is located in the same place as the EtherType/Length field in untagged frames, so an EtherType value of 0x8100 means the frame is tagged, and the true EtherType/Length is located after the tag. The TPID is followed by two bytes containing the Tag Control Information (TCI) (the IEEE 802.1p priority (quality of service) and VLAN id). The tag is followed by the rest of the frame, using one of the types described above.

Varieties of Ethernet

Other than the framing types mentioned above, most of the other differences between Ethernet varieties have all been variations on speed and wiring. Therefore, in general, network protocol stack software will work identically on most of the following types. The following sections provide a brief summary of all the official Ethernet media types. In addition to these official standards, many vendors have implemented proprietary media types for various reasons—often to support longer distances over fiber optic cabling. Many Ethernet cards and switch ports support multiple speeds, using auto-negotiation to set the speed and duplex for the best values supported by both connected devices. If auto-negotiation fails, a multiple speed device will sense the speed used by its partner, but will assume half-duplex. A 10/100 Ethernet port supports 10BASE-T and 100BASE-TX. A 10/100/1000 Ethernet port supports 10BASE-T, 100BASE-TX, and 1000BASE-T.

Some early varieties of Ethernet

- Xerox Ethernet -- the original, 3-Mbit/s Ethernet implementation, which in turn had two versions, Version 1 and Version 2, during its development. The version 2 framing format is still in common use.
- 10BROAD36 -- Obsolete. An early standard supporting Ethernet over longer distances. It utilized broadband modulation techniques similar to those employed in cable modem systems, and operated over coaxial cable.
- 1BASE5 -- Also known as StarLAN, was the first implementation of Ethernet on twisted pair wiring. It operated at 1 Mbit/s and was a commercial failure.

10 Mbit/s Ethernet

- 10BASE5 (also called Thicknet, Thickwire or Yellow Cable) -- This is the original 10 Mbit/s implementation of Ethernet. The early IEEE standard uses a single 50-ohm coaxial cable of a type designated RG-8, of maximum length 500 metres. Transceivers could be connected by a so-called "vampire tap", which was attached by drilling into the cable to connect to the core and screen, or using N connectors at the end of a cable segment. An AUI cable then connected the transceiver to the Ethernet device. Largely obsolete, though due to its widespread deployment in the early days, some systems may still be in use. It requires precise termination at each end of the cable.
- 10BASE2 (also called Thinnet, Thinwire or Cheapernet) -- 50 ohm RG-58 coaxial cable, of maximum length 200 metres, connects machines together, each machine using a T-adaptor to connect to its NIC, which has a BNC connector. Requires termination at each end. For many years this was the dominant 10 Mbit/s Ethernet standard.
- StarLAN 10 -- First implementation of Ethernet on twisted pair wiring at 10 Mbit/s. Later evolved into 10BASE-T.
- 10BASE-T -- Runs over 4 wires (two twisted pairs) on a cat-3 or cat-5 cable up to 100 metres in length. A hub or switch sits in the middle and has a port for each node.
- FOIRL -- Fiber-optic inter-repeater link. The original standard for Ethernet over fiber.
- 10BASE-F (also called 10BASE-FX) -- A generic term for the family of 10 Mbit/s Ethernet standards using fiber optic cable: 10BASE-FL, 10BASE-FB and 10BASE-FP. Of these only 10BASE-FL is in widespread use.
  - 10BASE-FL -- An updated version of the FOIRL standard.
  - 10BASE-FB -- Intended for backbones connecting a number of hubs or switches, it is now obsolete.
  - 10BASE-FP -- A passive star network that required no repeater, it was never implemented

Fast Ethernet (100 Mbit/s)

- 100BASE-T -- A term for any of the three standards for 100 Mbit/s Ethernet over twisted pair cable up to 100 meters long. Includes 100BASE-TX, 100BASE-T4 and 100BASE-T2.
  - 100BASE-TX -- Similar star-shaped configuration to 10BASE-T. It also uses two pairs, but requires cat-5 cable to achieve 100Mbit/s.
  - 100BASE-T4 -- 100 Mbit/s Ethernet over cat-3 cabling (as used for 10BASE-T installations). Uses all four pairs in the cable. Now obsolete, as cat-5 cabling is the norm. Limited to half-duplex.
  - 100BASE-T2 -- No products exist. 100 Mbit/s Ethernet over cat-3 cabling. Supports full-duplex, and uses only two pairs. It is functionally equivalent to 100BASE-TX, but supports old telephone cable (cat-3).
- 100BASE-FX -- 100 Mbit/s Ethernet over multimode fibre. Maximum length is 400 meters for half-duplex connections (to ensure collisions are detected) or 2 kilometers for full-duplex.
- 100BASE-SX -- 100 Mbit/s Ethernet over multimode fibre. Maximum length is 300 meters. Unlike 100BASE-FX using laser as light sources, 100BASE-SX uses LED, so it is cheaper.
- 100Base-VG -- Not Ethernet. Standardized by a different IEEE 802 subgroup, 802.12, because it used a different, more centralized form of media access ("Demand Priority"). Championed by only HP, 100VG-AnyLAN (as was the marketing name) was the earliest in the market. It needed four pair of cat-3 cables. As of 2005, obsolete (802.12 has been "inactive" since 1997).

Gigabit Ethernet

- 1000BASE-T -- 1 Gbit/s over cat-5e or cat-6 copper cabling.
- 1000BASE-TX -- 1 Gbit/s over only cat-6 copper cabling.
- 1000BASE-SX -- 1 Gbit/s over multi-mode fiber (up to 550 m).
- 1000BASE-LX -- 1 Gbit/s over multi-mode fiber (up to 550 m). Optimized for longer distances (up to 10 km) over single-mode fiber.
- 1000BASE-LH -- 1 Gbit/s over single-mode fiber (up to 100 km). A long-haul solution.
- 1000BASE-CX -- A short-haul solution (up to 25 m) for running 1 Gbit/s Ethernet over special copper cable. Predates 1000BASE-T, and now obsolete.
- 1000BASE-PX10-D -- 1Gbit/s over single-mode fiber using point-to-multipoint topology (supports at least 10 km). This PMD specifies downstream direction only (from head-end to tail-ends). Standardized in IEEE Std. 802.3ah in 2004.
- 1000BASE-PX10-U -- 1Gbit/s over single-mode fiber using point-to-multipoint topology (supports at least 10 km). This PMD specifies upstream direction only (from a tail-end to the head-end). Standardized in IEEE Std. 802.3ah in 2004.
- 1000BASE-PX20-D -- 1Gbit/s over single-mode fiber using point-to-multipoint topology (supports at least 20 km). This PMD specifies downstream direction only (from head-end to tail-ends). Standardized in IEEE Std. 802.3ah in 2004.
- 1000BASE-PX20-U -- 1Gbit/s over single-mode fiber using point-to-multipoint topology (supports at least 20 km). This PMD specifies upstream direction only (from a tail-end to the head-end). Standardized in IEEE Std. 802.3ah in 2004.

10 Gigabit Ethernet

The new 10 gigabit Ethernet standard encompasses seven different media types for LAN, MAN and WAN. It is currently specified by a supplementary standard, IEEE 802.3ae, and will be incorporated into a future revision of the IEEE 802.3 standard.
- 10GBASE-CX4 - designed to support short distances over copper cabling, it uses InfiniBand 4x connectors and CX4 cabling and allows a cable length of up to 15 m.
- 10GBASE-SR -- designed to support short distances over deployed multi-mode fiber cabling, it has a range of between 26 m and 82 m depending on cable type. It also supports 300 m operation over a new 2000 MHz.km multi-mode fiber.
- 10GBASE-LX4 -- uses wavelength division multiplexing to support ranges of between 240 m and 300 m over deployed multi-mode cabling. Also supports 10 km over single-mode fiber.
- 10GBASE-LR and 10GBASE-ER -- these standards support 10 km and 40 km respectively over single-mode fiber.
- 10GBASE-SW, 10GBASE-LW and 10GBASE-EW. These varieties use the WAN PHY, designed to interoperate with OC-192 / STM-64 SONET/SDH equipment. They correspond at the physical layer to 10GBASE-SR, 10GBASE-LR and 10GBASE-ER respectively, and hence use the same types of fiber and support the same distances. (There is no WAN PHY standard corresponding to 10GBASE-LX4.)
- 10GBASE-T -- Uses unshielded twisted-pair wiring. 10GBASE-T should be ready by August 2006. 10 gigabit Ethernet is very new, and it remains to be seen which of the standards will gain commercial acceptance.

Related standards

These networking standards are not part of the IEEE 802.3 Ethernet standard, but support the Ethernet frame format, and are capable of interoperating with it.
- LattisNet -- A SynOptics pre-standard twisted-pair 10 Mbit/s variant.
- 100BaseVG -- An early contender for 100 Mbit/s Ethernet. It runs over Category 3 cabling. Uses four pairs. Commercial failure.
- TIA 100BASE-SX -- Promoted by the Telecommunications Industry Association. 100BASE-SX is an alternative implementation of 100 Mbit/s Ethernet over fiber; it is incompatible with the official 100BASE-FX standard. Its main feature is interoperability with 10BASE-FL, supporting autonegotiation between 10 Mbit/s and 100 Mbit/s operation -- a feature lacking in the official standards due to the use of differing LED wavelengths. It is targeted at the installed base of 10 Mbit/s fiber network installations.
- TIA 1000BASE-TX -- Promoted by the Telecommunications Industry Association, it was a commercial failure, and no products exist. 1000BASE-TX uses a simpler protocol than the official 1000BASE-T standard so the electronics can be cheaper, but requires Category 6 cabling.

See also

- IEEE 802.3
- CHAOSnet
- Attachment Unit Interface
- Virtual LAN
- Spanning Tree Protocol
- Telecommunication
- Internet
- Category 5 cable
- RJ45 and extension cable
- Crossover cable
- Fragment free cut-through
- Power over Ethernet
- MII and PHY
- Wake-on-LAN
- List of device bandwidths
- Power line communication

External links

- [http://standards.ieee.org/getieee802/download/802.3-2002.pdf IEEE 802.3 2002 standard]
- [http://www.10gea.org/ 10 Gigabit Ethernet Alliance website]
- [http://www.wildpackets.com/support/compendium/ethernet/frame_formats Ethernet frame formats]
- [http://www.siemon.com/us/white_papers/ 10 Gigabit Ethernet over IP White Papers]
- [http://www.windowsnetworking.com/articles_tutorials/thistedg.html Gigabit-Ethernet (1000BaseT)]
- [http://whatis.techtarget.com/definition/0,,sid9_gci214198,00.html The speed of ...] Category:Ethernet ko:이더넷 ja:イーサネット

TDMA

Time Division Multiple Access (TDMA) is a technology for shared medium (usually radio) networks. It allows several users to share the same frequency by dividing it into different time slots. The users transmit in rapid succession, one after the other, each using their own timeslot. This allows multiple users to share the same transmission medium (e.g. radio frequency) whilst using only the part of its bandwidth they require. Used in the GSM, PDC and iDEN digital cellular standards, among others. TDMA is also used extensively in satellite systems, local area networks, physical security systems, and combat-net radio systems. :The name "TDMA" is also commonly used in America to refer to a specific second generation (2G) mobile phone standard, more properly referred to as IS-136 or D-AMPS, which uses the TDMA technique to timeshare the bandwidth of the carrier wave. :The two different uses of this term can be confusing. TDMA (the technique) is used in the GSM standard. However, TDMA (the standard, i.e. IS-136) has been competing against GSM and systems based on CDMA modulation for adoption by the carriers, although it is now being phased out in favor of GSM technology. carrier wave TDMA is a type of Time-division multiplexing, with the special point that instead of having one transmitter connected to one receiver, there are multiple transmitters. In the case of the uplink from a mobile phone to a base station this becomes particularly difficult because the mobile phone can move around and vary the timing offset required to make its transmission match the gap in transmission from its peers. In the GSM system, the synchronisation of the mobile phones is achieved by sending timing offset commands from the base station which instructs the mobile phone to transmit earlier or later. The mobile phone is not allowed to transmit for its entire timeslot, but there is a guard period at the beginning and end of the timeslot. As the transmission moves into the guard period, the mobile network adjusts the timing offset to re-center the transmission. Initial synchronisation of a phone requires even more care. Before a mobile transmits there is no way to actually know the offset required. For this reason, an entire timeslot has to be dedicated to mobiles attempting to contact the network (known as the RACH in GSM). The mobile attempts to broadcast at the beginning of the timeslot, as received from the network. If the mobile is located next to the base station, there will be no time delay and this will succeed. If, however, the mobile phone is at just less than 35km from the base station, the time delay will mean the mobile's broadcast arrives at the very end of the timeslot. In that case, the mobile will be instructed to broadcast its messages starting a whole timeslot earlier than would be expected otherwise. Finally, if the mobile is beyond the 35 km cell range in GSM, then the RACH will arrive in a neighboring time slot and be ignored. It is this feature, rather than limitations of power which limits the range of a GSM cell to 35 kilometers when no special tricks are used. By changing the synchronization between the uplink and downlink at the base station, however, this limitation can be overcome. In radio systems, TDMA is almost always used alongside FDMA (Frequency division multiple access) and FDD (Frequency division duplex); the combination is referred to as FDMA/TDMA/FDD. This is the case in both GSM and IS-136 for example. The exceptions to this rule include WCDMA-TDD which combines FDMA/CDMA/TDMA and TDD instead. A major advantage of TDMA is that the radio part of the mobile only needs to listen and broadcast for its own timeslot. For the rest of the time, the mobile can carry out measurements on the network, detecting surrounding transmitters on different frequencies. This allows safe inter frequency handovers, something which is difficult in CDMA systems, not supported at all in IS-95 and supported through complex system additions in UMTS. This in turn allows for co-existence of microcell layers with macrocell layers. But, CDMA supports "soft hand-off" which allows a mobile phone to be in communication with up to 6 base stations simultaneously, a type of "same-frequency handover". The incoming packets are compared for quality, and the best one is selected. This enables CDMA to perform in areas where TDMA calls would be dropped. A disadvantage of TDMA systems is that they create interference at a frequency which is directly connected to the time slot length. This is the irritating buzz which can sometimes be heard if a GSM phone is left next to a radio. Another disadvantage is that the "dead time" between time slots limits the potential bandwidth of a TDMA channel. This is why early efforts to incorporate timeslots into UMTS failed, leaving UMTS as a purely CDMA technology. The only country to continue pursuing TD-SCDMA (time division synchronous CDMA) is mainland China, because the government does not want to pay patent royalties to Qualcomm of the USA or licensing fees to the mainly European UMTS consortium.

TDMA features

- Shares single carrier frequency with multiple users
- Non-continuous transmission makes handoff simpler
- Slots can be assigned on demand ⇒ BW on demand
- Less stringent power control due to reduced interuser interference
- Higher synchronization overhead
- Equalization is necessary for high data rates
- Frequency/slot allocation complexity
- Pulsating power envelop: Interference with other devices

See also:

- FDMA (Frequency-division multiple access)
- CDMA (Code division multiple access) Category:Channel access methods Category:Multiplexing ja:Time Division Multiple Access

FDMA

FDMA, or frequency-division multiple access, is the oldest and most important of the three main ways for multiple radio transmitters to share the radio spectrum. The other two methods are Time Division Multiple Access (TDMA), and Code Division Multiple Access (CDMA). FDMA is similar in concept to the 100 channels that share the FM broadcast spectrum in the USA. Each station transmits on its own assigned frequency. This allows multiple users to transmit simultaneously within the same spectrum (in this case, 88MHz-108MHz).

Spectrum usage

CDMA
Category:Channel access methods Category:Multiplexing

CDMA

Code division multiple access (CDMA) is (not a modulation scheme, but a form of multiplexing) a method of multiple access that does not divide up the channel by time (as in TDMA), or frequency (as in FDMA), but instead encodes data with a certain code associated with a channel and uses the constructive interference properties of the signal medium to perform the multiplexing. CDMA also refers to digital cellular telephony systems that makes use of this multiple access scheme, such as those pioneered by Qualcomm, or W-CDMA.

History of CDMA

see: direct-sequence spread spectrum (DSSS)

Usage in mobile telephony

A number of different terms are used to refer to CDMA implementations. The original standard spearheaded by QUALCOMM was known as IS-95, the IS referring to an Interim Standard of the Telecommunications Industry Association (TIA). IS-95 is often referred to as 2G or second generation cellular. The QUALCOMM brand name cdmaOne may also be used to refer to the 2G CDMA standard. After several revisions, IS-95 was superseded by the IS-2000 standard. This standard was introduced to meet some of the criteria laid out in the IMT-2000 specification for 3G, or third generation, cellular. It is also sometimes referred to as 1xRTT which simply means "1 times radio transmission technology" and indicates that IS-2000 uses the same 1.25 MHz shared channel as the original IS-95 standard. More recently, QUALCOMM has led the creation of a new CDMA-based technology called 1xEVDO which provides the higher packet data transmission rates required by IMT-2000 and desired by wireless network operators. The QUALCOMM CDMA system includes highly accurate time signals (usually referenced to a GPS receiver in the cell base station), so cellphone CDMA-based clocks are an increasingly popular type of Radio clock for use in computer networks. The main advantage of using CDMA cell-phone signals for reference clock purposes is that they work better inside buildings, thus often eliminating the need to mount the GPS antenna on the outside of a building. Also frequently confused with CDMA is W-CDMA. The CDMA technique is used as the principle of the W-CDMA air interface, and the W-CDMA air interface is used in the global 3G standard, UMTS, and Japanese 3G standards, FOMA by NTT DoCoMo and Vodafone, however, the CDMA family of standards (including cdmaOne and CDMA2000) are not compatible with the W-CDMA family of standards. Another important application of CDMA — predating and entirely distinct from CDMA cellular — is the Global Positioning System, GPS.

Technical details

Mathematical foundation

CDMA exploits at its core mathematical properties of orthogonality. Suppose we represent data signals as vectors. For example, the binary string "1011" would be represented by the vector (1, 0, 1, 1). We may wish to give a vector a name, we may do so by using boldface letters, eg a. We also use an operation on vectors, known as the dot product, to "multiply" vectors, by summing the product of the components. For example, the dot product of (1, 0, 1, 1) and (1, -1, -1, 0) would be (1)(1)+(0)(-1)+(1)(-1)+(1)(0)=1+-1=0. Where the dot product of vectors a and b is 0, we say that the two vectors are orthogonal. The dot product has a number of properties, and one will aid us in understanding why CDMA works. For vectors a, b, c: :$\backslash mathbf\backslash cdot(\backslash mathbf+\backslash mathbf)=\backslash mathbf\backslash cdot\backslash mathbf+\backslash mathbf\backslash cdot\backslash mathbf,\backslash quad\backslash mathrm$ :$\backslash mathbf\backslash cdot\; k\backslash mathbf=k(\backslash mathbf\backslash cdot\backslash mathbf).$ The square root of a.a is a real number, and is important. We write :$||\backslash mathbf||=\backslash sqrt.$ Suppose vectors a and b are orthogonal. Then: :$\backslash mathbf\backslash cdot(\backslash mathbf+\backslash mathbf)=||\backslash mathbf||^2\backslash quad\backslash mathrm\backslash quad\backslash mathbf\backslash cdot\backslash mathbf+\backslash mathbf\backslash cdot\backslash mathbf=\; ||a||^2+0,$ :$\backslash mathbf\backslash cdot(-\backslash mathbf+\backslash mathbf)=-||\backslash mathbf||^2\backslash quad\backslash mathrm\backslash quad-\backslash mathbf\backslash cdot\backslash mathbf+\backslash mathbf\backslash cdot\backslash mathbf=\; -||a||^2+0,$ :$\backslash mathbf\backslash cdot(\backslash mathbf+\backslash mathbf)=||\backslash mathbf||^2\backslash quad\backslash mathrm\backslash quad\backslash mathbf\backslash cdot\backslash mathbf+\backslash mathbf\backslash cdot\backslash mathbf=\; 0+||b||^2,$ :$\backslash mathbf\backslash cdot(\backslash mathbf-\backslash mathbf)=-||\backslash mathbf||^2\backslash quad\backslash mathrm\backslash quad\backslash mathbf\backslash cdot\backslash mathbf-\backslash mathbf\backslash cdot\backslash mathbf=0\; -||b||^2.$

Implementation

dot product Suppose now we have a set of vectors that are mutually orthogonal to each other. Usually these vectors are specially constructed for ease of decoding -- they are columns or rows from Walsh matrices that are constructed from Walsh functions -- but strictly mathematically the only restriction on these vectors is that they are orthogonal. An example of orthogonal functions is shown in the picture on the right. Now, associate with one sender a vector from this set, say v, which is called the chip code. Associate a zero digit with the vector -v, and a one digit with the vector v. For example, if v=(1,-1), then the binary vector (1, 0, 1, 1) would correspond to (1,-1,-1,1,1,-1,1,-1). For the purposes of this article, we call this constructed vector the transmitted vector. Each sender has a different, unique vector chosen from that set, but the construction of the transmitted vector is identical. Now, the physical properties of interference say that if two signals at a point are in phase, they will "add up" to give twice the amplitude of each signal, but if they are out of phase, they will "subtract" and give a signal that is the difference of the amplitudes. Digitally, this behaviour can be modelled simply by the addition of the transmission vectors, componentwise. So, if we have two senders, both sending simultaneously, one with the chip code (1, -1) and data vector (1, 0, 1, 1), and another with the chip code (1, 1), and data vector (0,0,1,1), the raw signal received would be the sum of the transmission vectors (1,-1,-1,1,1,-1,1,-1)+(-1,-1,-1,-1,1,1,1,1)=(0,-2,-2,0,2,0,2,0). Suppose a receiver gets such a signal, and wants to detect what the transmitter with chip code (1, -1) is sending. The receiver will make use of the property described in the above foundation section, and take the dot product to the received vector in parts. Take the first two components of the received vector, that is, (0, -2). Now, (0, -2).(1, -1) = (0)(1)+(-2)(-1) = 2. Since this is positive, we can deduce that a one digit was sent. Taking the next two components, (-2, 0), (-2, 0).(1,-1)=(-2)(1)+(0)(-1)=-2. Since this is negative, we can deduce that a zero digit was sent. Continuing in this fashion, we can successfully decode what the transmitter with chip code (1, -1) was sending: (1, 0, 1, 1). Likewise, applying the same process with chip code (1, 1): (1, 1).(0,-2) = -2 gives digit 0, (1, 1).(-2,0)=(1)(-2)+(1)(0)=-2 gives digit 0, and so on, to give us the data vector sent by the transmitter with chip code (1, 1): (0, 0, 1, 1). Now, there are certain issues where this mathematical process can be disrupted. Suppose that one sender transmits at a higher signal strength than another. Then the critical orthogonality property can be disrupted, and thus the system can fail. Thus controlling power strength is an important issue with CDMA transmitters. A TDMA or FDMA receiver can in theory completely reject arbitrarily strong signals on other time slots or frequency channels. This is not true for CDMA; rejection of unwanted signals is only partial. If any or all of the unwanted signals are much stronger than the desired signal, they will overwhelm it. This leads to a general requirement in any CDMA system to approximately match the various signal power levels as seen at the receiver. In CDMA cellular, the base station uses a fast closed-loop power control scheme to tightly control each mobile's transmit power. Suppose that noise present in a channel takes a zero bit to some other value. Then this will also disrupt the orthogonality property, and thus adding an extra level of forward error correction (FEC) coding is also vital. So far, we have assumed that CDMA timing is absolutely exact, that is, transmitters exactly transmit at points in multiples of the length of the chip sequence. Of course, in reality, this is impractical to achieve, so all forms of CDMA use spread spectrum process gain to allow receivers to partially discriminate against unwanted signals. Signals with the desired chip code and timing are received, while signals with different chip codes (or the same spreading code but a different timing offset) appear as wideband noise reduced by the process gain. CDMA's main advantage over TDMA and FDMA is that the number of available CDMA codes is essentially infinite. This makes CDMA ideally suited to large numbers of transmitters each generating a relatively small amount of traffic at irregular intervals, as it avoids the overhead of continually allocating and deallocating a limited number of orthogonal time slots or frequency channels to individual transmitters. CDMA transmitters simply send when they have something to say, and go off the air when they don't.

Soft Handoff

Soft handoff (or soft handover) is an innovation in mobility which was only possible with CDMA technology. It refers to the technique of moving from one cell to another without dropping the radio link at any time. In TDMA and analog systems, each cell transmits on its own frequency, different from those of its neighbouring cells. If a mobile device reaches the edge of the cell currently serving its call, it is told to break its radio link and quickly tune to the frequency of one of the neighbouring cells where the call has been moved by the network due to the mobile's movement. If the mobile is unable to tune to the new frequency in time the call is dropped. In CDMA, a set of neighbouring cells all use the same frequency for transmission and distinguish cells (or base stations) by means of a number called the "PN offset", a time offset from the beginning of the well-known pseudo-random noise sequence that is used to spread the signal from the base station. Because all of the cells are on the same frequency, listening to different base stations is now an exercise in digital signal processing based on offsets from the PN sequence, not RF transmission and reception based on separate frequencies. As the CDMA phone roams through the network, it detects the PN offsets of the neighbouring cells and reports the strength of each signal back to the reference cell of the call (usually the strongest cell). If the signal from a neighbouring cell is strong enough, the mobile will be directed to "add a leg" to its call and start transmitting and receiving to and from the new cell in addition to the cell (or cells) already hosting the call. Likewise, if a cell's signal becomes too weak the mobile is directed to drop that leg. In this way, the mobile can move from cell to cell and add and drop legs as necessary in order to keep the call up without ever dropping the link. In practice there are frequency boundaries, often between different carriers or sub-networks. In this situation, the CDMA phone behaves in the same way as TDMA or analog and performs a hard handoff in which it breaks the existing connection and tries to pick up on the new frequency where it left off.

CDMA features

- Narrowband message signal multiplied by wideband spreading signal or codeword
- Each user has its own pseudo-codeword
- Soft capacity limit: system performance degrades for all users as number of users increases
- Cell frequency reuse one: no frequency planning
- Soft handover increases capacity
- Near-far problem
- Interference limit: power control is required
- Wide bandwidth induces diversity: RAKE receiver is used

See also

- Near-far problem
- GSM
- Frequency-division multiplexing
- Time-division multiple access

External links

- [http://www.3gpp2.org/ The Third Generation Partnership Project 2 (3GPP2)]
- [http://www.3gpp.org/ The Third Generation Partnership Project (3GPP) ]
- [http://www.cdg.org/ CDMA Development Group (CDG)]
- [http://www.radio-electronics.com/info/cellulartelecomms/ Radio-Electronics.Com]
- [http://www.cdmatech.com/ Qualcomm CDMA Technologies (QCT)]
- [http://www.mobileafrica.net/cdma.php CDMA in Africa ]
- PN Sequences

Further reading

- Andrew J. Viterbi. (1995) CDMA : Principles of Spread Spectrum Communication (1st edition) Prentice Hall PTR ISBN 0201633744 Category:Channel access methods Category:Multiplexing ko:CDMA ja:符号分割多元接続

T-carrier

In telecommunications, T-carrier is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America and Japan. The basic unit of the T-carrier system is the DS0, which has a transmission rate of 64 kbit/s, and is commonly used for one voice circuit. The E-carrier system, where 'E' stands for European, is incompatible with the T-carrier and is used just about everywhere else in the world besides North America and Japan. It typically uses the E1 line rate and the E3 line rate. The E2 line rate is less commonly used. See the table below for bandwidth comparisons.

T1

The most common legacy of this whole system is the line rate designations. A "T1" now seems to mean any data circuit that runs at the original 1.544 Mbit/s line rate. Originally the T1 format carried 24 pulse-code modulated, time-division multiplexed speech signals each encoded in 64 kbit/s streams, leaving 8 kbit/s of framing information which facilitates the synchronization and demultiplexing at the receiver. T2 and T3 circuit channels carry multiple T1 channels multiplexed, resulting in transmission rates of up to 44.736 Mbit/s. Supposedly, the 1.544 Mbit/s rate was chosen because tests done by AT&T Long Lines in Chicago were conducted underground, and cable vault manholes were physically 6600 feet apart, and so the optimum rate was chosen empirically--the capacity was increased until the failure rate was unacceptable, then reduced. A more common understanding of how the rate of 1.544 Mbit/s was achieved is as follows. (This explanation glosses over T1 voice communications, and deals mainly with the numbers involved.) Given that the highest frequency at which voice communications occurs is at 4000 Hz, one needs, when converting analog voice to digital data, at least double that frequency for the sample rate. This yields the number 8000. Since each T1 frame contains 1 byte of voice data for each of the 24 channels, that system needs then 8000 frames per second to maintain those 24 simultaneous voice channels. Since each frame of a T1 is 193 bits in length (24 channels X 8 bits per channel + 1 control bit = 193 bits), 8000 frames per second is multiplied by 193 bits to yield a transfer rate of 1.544 Mbit/s (8000 X 193 = 1544000).

Digital signal crossconnect

DS1 signals are frequently used to connect equipment within a facility. In this case, a low-level signal (6 volts peak-to-peak differential) called the DSX1 is used. DSX refers to a digital signal crossconnect, and it is essentially a patch panel allowing easy interconnection. When a DS1 leaves the building, it becomes a T1 and is referred to as a span. The signal is boosted to a higher level and superimposed on a DC voltage, enabling repeaters in the field to be powered from the span itself. Repeaters are placed every few thousand feet, to clean up and strengthen the signal. DS3 signals are almost exclusively used within buildings, for interconnections and as an intermediate step before being muxed onto a SONET circuit. This is because a T3 circuit can only go about 600 feet between repeaters. When a customer orders a DS3, they usually get a (much faster) SONET circuit run into the building and a multiplexer mounted in a big cabinet. The DS3 is delivered in its familiar form, two coax cables with BNC connectors on the ends.

Bit robbing

The T-carrier system traditionally uses in-band signalling or bit robbing, resulting in lower transmission rates than the E-carrier system. This resulted in many US ISDN installations only having an effective data rate of 56 kbit/s over a nominal 64 kbit/s channel. See also A&B. This depends on the framing format used, and almost all systems are now capable of transmitting a "clear" 64 kbit/s channel, despite the failure of providers to sell such services.

Notes

Note 1: The designators for T-carrier in the North American digital hierarchy correspond to the designators for the digital signal (DS) level hierarchy. Note 2: T-carrier systems were originally designed to transmit digitized voice signals. Current applications also include digital data transmission. Note 3: Historically, if an "F" precedes the "T", optical fiber cables are utilized at the same rates. Note 4: The North American and Japanese hierarchies are based on multiplexing 24 voice-frequency channels and multiples thereof, whereas the European hierarchy is based on multiplexing 32 voice-frequency channels and multiples thereof. See table below.

T-Carrier Systems	North American	Japanese	European (CEPT)
Level zero (Channel data rate)	64 kbit/s (DS0)	64 kbit/s	64 kbit/s
First level	1.544 Mbit/s (DS1) (24 user channels) (T1)	1.544 Mbit/s (24 user channels)	2.048 Mbit/s (32 user channels) (E1)
(Intermediate level, US. hierarchy only)	3.152 Mbit/s (DS1C) (48 Ch.)	-	-
Second level	6.312 Mbit/s (DS2) (96 Ch.)	6.312 Mbit/s (96 Ch.), or 7.786 Mbit/s (120 Ch.)	8.448 Mbit/s (128 Ch.) (E2)
Third level	44.736 Mbit/s (DS3) (672 Ch.) (T3)	32.064 Mbit/s (480 Ch.)	34.368 Mbit/s (512 Ch.) (E3)
Fourth level	274.176 Mbit/s (DS4) (4032 Ch.)	97.728 Mbit/s (1440 Ch.)	139.268 Mbit/s (2048 Ch.) (E4)
Fifth level	400.352 Mbit/s (5760 Ch.)	565.148 Mbit/s (8192 Ch.)	565.148 Mbit/s (8192 Ch.) (E5)

Note 1: The DS designations are used in connection with the North American hierarchy only. Technically a DS1 is the data carried on a T1 circuit, and likewise for a DS3 and a T3, but the terms are almost always used interchangeably. Note 2: There are other data rates in use, e.g., military systems that operate at six and eight times the DS1 rate. At least one manufacturer has a commercial system that operates at 90 Mbit/s, twice the DS3 rate. New systems, which take advantage of the high data rates offered by optical communications links, are also deployed or are under development. Higher data rates are now often achieved by using Synchronous optical networking, SONET or Synchronous digital hierarchy, SDH.

See also

- Digital Signal 0 (DS0)
- Digital Signal 1 (DS1)
- DS1 Encoding schemes: B8ZS, HDB3, AMI
- Optical Carrier (OC-n)
- Time-division multiplexing
- Multiplexing
- Plesiochronous Digital Hierarchy

References

- ANSI T1.403-1999 - Network and Customer Installation Interfaces
- Federal Standard 1037C
- [http://www.oreilly.com/catalog/t1survival/chapter/ch05.html T1: A Survival Guide Chapter 5] Category:Computer_and_telecommunication_standards

10BASE-T

10BASE-T is an implementation of Ethernet which allows stations to be attached via twisted pair cable. The name 10BASE-T is derived from several aspects of the physical medium. The 10 refers to the transmission speed of 10 Mbit/s. The BASE is short for baseband. This means only one Ethernet signal is present on the send and/or receive pair. In other words there is no multiplexing as with broadband transmissions. The T comes from twisted pair, which is the type of cable that is used. The nominal segment length for a 10Base-T cable is 100 meters, not 10, as with 100BASE-T and 1000BASE-T. Unlike earlier Ethernet standards such as 10BASE5 and 10BASE2, 10BASE-T does not specify the exact type of wiring to be used. This was done in anticipation of using 10BASE-T in existing twisted pair wiring systems that may not conform to any specified wiring standard. Instead, 10BASE-T wiring is specified using a set of characteristics that a 10BASE-T link segment must conform to. These include attenuation, characteristic impedance, timing jitter, propagation delay, and several types of noise. Cable testers are widely available to check these parameters to determine if a cable can be used with 10BASE-T. These characteristics are expected to be met by 100 meters of 24 gauge unshielded twisted pair cable. 10BASE-T uses RJ-45 jacks wired to either the TIA-568A or TIA-568B standard. Only the second and third pairs are used (orange and green); though these are wired opposite in the two standards - TIA-568A puts pair two (orange) on pins 3 and 6, pair three (green) on pins 1 and 2; TIA-568B is the reverse. A 10BASE-T hub/switch transmits on pins 1 and 2, and receives on pins 3 and 6, while a 10BASE-T node transmits on pins 3 and 6 and receives on pins 1 and 2. If the wiring standard is identical on both ends the segment is a patch cable suitable for transmission between a hub/switch/patch panel and a node. If the wiring standards are opposite on either end the segment is a crossover cable suitable for connecting a node to a node or a hub/switch to another hub/switch. The EIA/TIA 568 standards are as follows: TIA-568B 10BASE-T was the first vendor-independent standard implementation of Ethernet on twisted pair wiring. However, it was in fact an evolutionary development from AT&T StarLAN which had both 1 Mbit/s and 10 Mbit/s versions. 10BASE-T is essentially StarLAN-10 with the addition of the link-beat. In the OSI model, 10BASE-T is at the physical layer. Ethernet encompasses both addressing at the data link layer and a number of physical-layer implementations. In this model, 10BASE-T is one of the possible physical layer standards for ethernet-- some others include 10BASE2, 10BASE5, and 100BASE-TX. Network layer protocols, such as IP, do not generally need to know whether they are being hosted on 10BASE-T or not, provided they know that they are being hosted on Ethernet.

See also

- 25-pair color code
- Computer network
- Ethernet
- RJ-45 Category:Ethernet cables ja:10BASE-T

ITU-T

The ITU Telecommunication Standardization Sector (ITU-T) coordinates standards for telecommunications on behalf of the International Telecommunication Union (ITU) and is based in Geneva, Switzerland. Prior to 1992, it was known as the International Telegraph and Telephone Consultative Committee (CCITT, from the French name "Comité consultatif international téléphonique et télégraphique").

Primary function

The international standards that are produced by the ITU-T are referred to as "Recommendations" (with the word ordinarily capitalized to distinguish its meaning from the ordinary sense of the word "recommendation"). Since the ITU-T is part of the ITU, which is a United Nations Organization (UNO), its standards carry more formal international recognition than those of most other organizations that publish technical specifications of a similar form. The sector divides its work into categories that are each identified by a single letter, referred to as the "series" (see below), and Recommendations are numbered within each series, for example "V.90".

History

Historically from 1960 until the formation of ITU-T in 1992, the Recommendations of the CCITT were presented to four-yearly "plenary assemblies" for endorsement, and the full set of Recommendations were published after each plenary assembly, in a set of volumes titled collectively for the colour of their covers. For example the publication after the 1980 plenary session was the Yellow Book while that after 1984 was the Red Book. These publications were divided into "fascicles" of several hundred pages that could be bought separately. The four-year approval cycle made the CCITT a rather slow and deliberate organization.

ITU reorganization 1970s-1990s

The rise of the personal computer industry in the early 1980s created a new common practice among both consumers and businesses of adopting "bleeding edge" communications technology even if it was not yet standardized. Thus, standards organizations had to put forth standards much faster, or find themselves ratifying de facto standards after the fact. Unfortunately, like the International Organization for Standardization (ISO), CCITT was slow to adapt. In some cases, a hopeless hodgepodge of proprietary standards resulted, with no clear winner; this was and still is the case with color fax technology. Another phenomenon was that the general public sought standards from organizations which it perceived as more responsive or inclusive; these included informal non-governmental organizations like the Internet Engineering Task Force (IETF) or private consortia like the World Wide Web Consortium (W3C).

ITU's "real time" standardization: 2000-Present

In response to the mess that previous ITU practices had created, the ITU-T now operates under much more streamlined processes. The time between an initial proposal of a draft document by a member company and the final approval of a full-status ITU-T Recommendation can now be as short as a few months (or less in some cases). This makes the standardization approval process in the ITU-T much more responsive to the needs of rapid technology development than in the ITU's historical past.

Changes in ITU-T compliance practices

A standard that has been amended can (if desired) retain its designation so that, for example, in the mid-1980s, terminal equipment for connection to an X.25 (packet switched) network might need alternative modes of operation depending on whether the network implemented the 1980 (Yellow Book) or the 1984 (Red Book) version of the standard. However, it is now more common for older versions of a standard to simply be marked as "superseded" when a standard is revised, and features of prior versions are ordinarily kept unchanged within the specification as new enhancements are added in new versions. A standard can be developed that extends or is complementary to an existing one rather than replacing it. Such a standard is sometimes designated by the suffix "bis" or "ter" added to the base standard name, for example "V.26bis" and "V.26ter".

Series and Recommendations

ITU-T issues Recommendations that have names like X.500, where X is the series and 500 is a serial number. See :Category:ITU-T recommendations. Significant ITU-T series and Recommendations are:
- A - Organization of the work of ITU-T
- B - Means of expression: definitions, symbols, classification,
- C - General telecommunication statistics
- D - General tariff principles
- E - Overall network operation, telephone service, service operation and human factors
  - E.123 Notation for national and international telephone numbers
  - E.163 Numbering plan for the international telephone service
  - E.164 The international public telecommunication numbering plan
    - Supplement 2 - Number Portability
- F - Non-telephone telecommunication services
- G - Transmission systems and media, digital systems and networks
  - G.711 Audio compression (mu-law)
  - G.722 Audio compression (wideband)
  - G.722.1 Audio compression (wideband, lower bit rates)
  - G.722.2 Speech compression AMR-WB (wideband, lower bit rates)
  - G.723.1 Speech compression CELP
  - G.726 Audio compression ADPCM
  - G.728 Speech compression LD-CELP
  - G.729 Speech compression ACELP
  - G.992.1 ADSL (G.DMT)
  - G.992.2 ADSL (G.Lite)
  - G.992.3/4 ADSL2
  - G.992.5 ADSL2+
- H - Audiovisual and multimedia systems
  - H.223 Multiplexing protocol for low bit rate multimedia communication
  - H.225.0 Also known as RTP
  - H.261 Video compression standard, circa 1991
  - H.262 Video compression standard (common text with part 2 of MPEG-2), circa 1994
  - H.263 Video compression standard, circa 1995
  - H.263v2 (a.k.a. H.263+) Video compression standard, circa 1998
  - H.264 Video compression standard (technically aligned with MPEG-4 part 10), circa 2003
  - H.320 Narrow-band visual telephone systems and terminal equipment
  - H.323 Packet-based multimedia communications systems
    - Annex D - Real-time facsimile over H.323 systems
    - Annex G - Text conversation and Text SET
    - Annex J - Security for H.323 Annex F
    - Annex K - HTTP based service control transport channel in H.323
    - Annex M.1 - Tunnelling of signalling protocol (Qsig) in H.323
    - Annex M.2 - Tunnelling of signalling protocol (Isup) in H.323
  - H.324 Terminal for low bit-rate multimedia communication
  - H.332 H.323 extended for loosely coupled conferences
- I - Integrated services digital network (ISDN)
- J - Transmission of television, sound programme and other multimedia signals
- K - Protection against interference
- L - Construction, installation and protection of cables and other elements of outside plant
- M - TMN and network maintenance: international transmission systems, telephone circuits, telegraphy, facsimile and leased circuits
- N - Maintenance: international sound programme and television transmission circuits
- O - Specifications of measuring equipment
- P - Telephone transmission quality, telephone installations, local line networks
- Q - Switching and signalling
  - Q.931 Is the layer 3 standard for ISDN signaling
- R - Telegraph transmission
- S - Telegraph services terminal equipment
- T - Terminals for telematic services
  - T.31 and T.32 Provide an interface between fax machines and data terminals.
  - T.411 - T.424 Comprise the