Local Area Network (LAN) is restricted to a small geographical area, usually to a relatively small number of stations. Wide Area Network (WAN) refers to networks across cities and countries. Between LAN and WAN, the term Metropolitan Area Network (MAN) is sometimes also used.

Table History of Digital Subscriber Lines

History of Digital Subscriber Lines

Local Area Networks (LANs)

A local area network (LAN) is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building using network media. The defining characteristics of LANs, in contrast to wide area networks (WANs), include their usually higher data - transfer rates, smaller geographic area, and lack of a need for leased telecommunication lines.

ARCNET, Token Ring and other technology standards have been used in the past, but Ethernet over twisted pair cabling, and Wi - Fi are the two most common technologies currently used to build LANs.

Following are some of the active IEEE 802 subcommittees and the areas they define:

  • 802.1(Higher Layer LAN Protocols). The relationship between the 802.X standards and the OSI reference model, the interconnection and management of the LANs

  • 802.2 (LLC). The general standard for logical link control (LLC)

  • 802.3 (Ethernet). Medium access control (CSMA / CD) and physical layer specifications for Ethernet

  • 802.5 (Token Ring). Medium access control and physical layer specifications for token ring

  • 802.9. LAN interfaces at the medium access control and physical layers for integrated services

  • 802.10(Security). Interoperable LAN / MAN security for other IEEE 802 standards

  • 802.11(Wireless LAN). Medium access method and physical layer specifications for wireless LAN (WLAN)

  • 802.14 (Cable - TV based broadband communication network). Standard protocol about two - way transmission of multimedia services over cable TV; e.g., HybridFiber - Coax (HFC) cable modem and cable network

  • 802.15 (Wireless PAN).Access method and physical layer specifications for Wire ­ less Persona! Area Network (WPAN). A Personal Area Network (PAN) supports coverages on the order of 10 meters

  • 802.16 (Broadband wireless). Access method and physical layer specifications for broadband wireless networks

Ethernet. Ethernet is a family of computer networking technologies for local area networks (LANs). Ethernet was commercially introduced in 1980 and standardized in 1985 as IEEE 802.3. Ethernet has largely replaced competing wired LAN technologies.

The Ethernet standards comprise several wiring and signaling variants of the OSI physical layer in use with Ethernet. The original 10BASE5 Ethernet used coaxial cable as a shared medium. Later the coaxial cables were replaced by twisted pair and fiber optic links in conjunction with hubs or switches. Data rates were periodically increased from the original 10 megabits per second, to 100 gigabits per second.

Systems communicating over Ethernet divide a stream of data into shorter pieces called frames. Each frame contains source and destination addresses and error - checking data so that damaged data can be detected and retransmitted. As per the OSI model Ethernet provides services up to and including the data link layer. Since its commercial release, Ethernet has retained a good degree of compatibility. Features such as the 48 - bit MAC address and Ethernet frame format have influenced other networking protocols.

Token Ring

Token ringlocal area network (LAN) technology is a protocol which resides at the data link layer (DLL) of the OSI model. It uses a special three - byte frame called a token that travels around the ring. Token - possession grants the possessor permission to transmit on the medium. Token ring frames travel completely around the loop. Initially used only in IBM computers, it was eventually standardized with protocol IEEE 802.5.

The data transmission process goes as follows:

  1. Empty information frames are continuously circulated on the ring.

  2. When a computer has a message to send, it inserts a token in an empty frame (this may consist of simply changing a 0 to a 1 in the token bit part of the frame) and inserts a message and a destination identifier in the frame.

  3. The frame is then examined by each successive workstation. The workstation that identifies itself to be the destination for the message copies it from the frame and changes the token back to 0.

  4. When the frame gets back to the originator, it sees that the token has been changed to 0 and that the message has been copied and received. It removes the message from the frame.

  5. The frame continues to circulate as an "empty" frame, ready to be taken by a workstation when it has a message to send.

The token scheme can also be used with bus topology LANs.

Fiber Distributed Data Interface (FDDI)

Fiber Distributed Data Interface (FDDI) provides a 100 Mbit/s optical standard for data transmission in a local area network that can extend in range up to 200 kilometers (120 mi). Although FDDI logical topology is a ring - based token network, it does not use the IEEE 802.5 token ring protocol as its basis; instead, its protocol is derived from the IEEE 802.4 token bus timed token protocol. In addition to covering large geographical areas, FDDI local area networks can support thousands of users.

As a standard underlying medium it uses optical fiber, although it can use copper cable, in which case it may be referred to as CDDI(Copper Distributed Data Interface). FDDI offers both a Dual - Attached Station (DAS), counter - rotating token ring topology and a Single - Attached Station (SAS), token bus passing ring topology.

FDDI was considered an attractive campus backbone technology in the early to mid 1990s since existing Ethernet networks only offered 10 Mbit/s transfer speeds and Token Ring networks only offered 4 Mbit/s or 16 Mbit/s speeds. Thus it was the preferred choice of that era for a high-speed backbone, but FDDI has since been effectively obsolesced by fast Ethernet which offered the same 100 Mbit/s speeds, but at a much lower cost and, since 1998, by Gigabit Ethernet due to its speed, and even lower cost, and ubiquity.

FDDI, as a product of American National Standards Institute X3T9.5 (now X3T12), conforms to the Open Systems Interconnection (OSI) model of functional layering of LANs using other protocols. FDDI - II, a version of FDDI, adds the capability to add circuit - switched service to the network so that it can also handle voice and video signals. Work has started to connect FDDI networks to the developing Synchronous Optical Network (SONET).

A FDDI network contains two rings, one as a secondary backup in case the primary ring fails. The primary ring offers up to 100 Mbit/s capacity. When a network has no requirement for the secondary ring to do backup, it can also carry data, extending capacity to 200 Mbit/s. The single ring can extend the maximum distance; a dual ring can extend 100 km (62 mi). FDDI has a larger maximum - frame size (4,352 bytes) than standard 100 Mbit/s Ethernet which only supports a maximum - frame size of 1,500 bytes, allowing better throughput.

Designers normally construct FDDI rings in the form of a "dual ring of trees" (see network topology). A small number of devices (typically infrastructure devices such as routers and concentrators rather than host computers) connect to both rings - hence the term "dual - attached". Host computers then connect as single - attached devices to the routers or concentrators. The dual ring in its most degenerate form simply collapses into a single device. Typically, a computer - room contains the whole dual ring, although some implementations have deployed FDDI as a Metropolitan area network.

Wide Area Networks (WANs)

A Wide Area Network (WAN) is a network that covers a broad area (i.e., any network that links across metropolitan, regional, or national boundaries). The Internet is the most popular WAN, and is used by businesses, governments, non - profit organizations, individual consumers, artists, entertainers, and numerous others for almost any purpose imaginable. Related terms for other types of networks are personal area networks (PANs), local area networks (LANs), campus area networks (CANs), or metropolitan area networks (MANs) which are usually limited to a room, building, campus or specific metropolitan area (e.g., a city) respectively.

Switching Technologies. The common types of switching technologies are circuit switching and packet switching. The latter also has its modern variants of frame relay and cell relay.

  • Circuit Switching. The public switched telephone network (PSTN) is a good example of circuit switching, in which an end - to - end circuit (duplex, in this case) must be established that is dedicated for the duration of the connection at a guaranteed band ­ width. Although initially designed for voice communications, it can also be used for data transmission. Indeed, it is still the basis for narrowband ISDN. To cope with multi - users and variable data rates, it adopts FDM or synchronous TDM multiplexing.

    Circuit switching is preferable if the user demands a connection and / or more or less constant data rates, as in certain constant - bitrate video communications. It is inefficient for general multimedia communication, especially for variable (sometimes bursty) data rates.

  • Packet Switching. Packet switching is used for almost all data networks in which data rates tend to be variable and sometimes bursty. Before transmission, data is broken into small packets, usually 1,000 bytes or less. The header of each packet carries necessary control information, such as destination address, routing, and so on. X.25 was the most commonly used protocol for packet switching.

    Generally, two approaches are available to switch and route the packets: datagram and virtual circuits. In the former, each packet is treated independently as a datagram. No transfer route is predetermined prior to the transmission; hence, packets may be unknowingly lost or arrive in the wrong order. It is up to the receiving station to detect and recover the errors, as is the case with TCP / IP.

    In virtual circuits, a route is predetermined through request and accept by all nodes along the route. It is a "circuit" because the route is fixed (once negotiated) and used for the duration of the connection; nonetheless, it is "virtual" because the "circuit" is only logical and not dedicated, and packets from the same source to the same destination can be transferred through different "circuits".

    Sequencing (ordering the packets) is much easier in virtual circuits. Retransmission is usually requested upon detection of an error. Packet switching becomes ineffective when the network is congested and becomes unreliable by severely delaying or losing a large number of packets.

  • Frame Relay.Modern high - speed links have low error rate; in optical fiber, it can be down to the order of 10 - 12. Many bits added to each packet for excessive error checking in ordinary packet switching (X.25) thus become unnecessary.

    As X.25, frame relay works at the data link control layer. Frame relay made the following major changes to X.25:

    • Reduction of error checking.No more acknowledgment, no more hop - to - hop flow control and error control. Optionally, end - to - end flow control and error control can be performed at a higher layer.

    • Reduction of layers.The multiplexing and switching virtual circuits are changed from layer 3 in X.25 to layer 2. Layer 3 of X.25 is eliminated.

    Frame relay is basically a cheaper version of packet switching, with minimal services. Frames have a length up to1,600 bytes. When a bad frame is received, it will simply be discarded. The data rate for frame relay is thus much higher, in the range of T1 (1.5 Mbps) to T3 (44.7 Mbps).

  • Cell Relay (ATM). Asynchronous transfer mode adopts small and fixed - length (53 bytes) packets referred to as cells. Hence, ATM is also known as cell relay.As the following figure shows, the small packet size is beneficial in reducing latency in ATM networks. When the darkened packet arrives slightly behind another packet of a normal size (e.g., 1 kB), it must wait for the completion of the other's transmission, causing serialization delay. When the packet (cell) size is small, much less waiting time is needed for the darkened cell to be sent.

    This turns out to significantly increase network throughput, which is especially beneficial for real - time multimedia applications. ATM is known to have the potential to deliver high data rates at hundreds (and thousands) of Mbps.

Asynchronous Transfer Mode (ATM)

Asynchronous Transfer Mode (ATM) is, according to the ATM Forum, "a telecommunications concept defined by ANSI and ITU (formerly CCITT) standards for carriage of a complete range of user traffic, including voice, data, and video signals," and is designed to unify telecommunication and computer networks. It uses asynchronous time - division multiplexing, and it encodes data into small, fixed - sized cells. This differs from approaches such as the Interne Protocol or Ethernet that use variable sized packets or frames.

ATM provides data link layer services that run over a wide range of OSI physical Layer links. ATM has functional similarity with both circuit switched networking and small packet switched networking. It was designed for a network that must handle both traditional high - throughput data traffic (e.g., file transfers), and real - time, low - latency content such as voice and video. ATM uses a connection - oriented model in which a virtual circuit must be established between two endpoints before the actual data exchange begins. ATM is a core protocol used over the SONET / SDH backbone of the public switched telephone network (PSTN) and Integrated Services Digital Network (ISDN), but its use is declining in favour of All IP.

Latency: (a) serialization delay in a normal packet switching network; (b) lower latency in a cell network

Latency: (a) serialization delay in a normal packet switching network; (b) lower latency in a cell network

Comparison of different switching techniques

Comparison of different switching techniques

Initially, ATM was used for WANs, especially serving as backbones. Nowadays, it is also used in LAN applications.

The ATM Cell Structure. ATM cells have a fixed format: their size is 53 bytes, of which the first 5 bytes are for the cell header, followed by 48 bytes of payload. The ATM layer has two types of interfaces: User - Network Interface (UNI) is local, between a user and an ATM network, and Network - Network Interface (NNI) is between ATM switches.

The following figure illustrates the structure of an ATM UNI cell header. The header starts with a 4 - bit general flow control (GFC) which controls traffic entering the network at the local user - network level. It is followed by an 8 - bit Virtual Path Identifier (VPI) and 16 - bit Virtual Channel Identifier (VCI) for selecting a particular virtual path and virtual circuit, respectively. The combination of VPI (8 bits) and VCI (16 bits) provides a unique routing indicator for the cell. As an analogy, VPI is like an area code (604), and VCI is like the following digits (555 - 1212) in a phone number.

The 3 - bit payload type (PT) specifies whether the cell is for user data or management and maintenance, network congestion, and so on. For example, 000 indicates user data cell type 0, no congestion; 010 indicates user data cell type 0, congestion experienced. PT may be altered by the network, say from 000 to 010, to indicate that the network has become congested.

The 1 - bit cell loss priority (CLP) allows the specification of a low - priority cell when CLP is set to 1. This provides a hint to the ATM switches about which cells to drop when the network is congested.

The 8 - bit header error detection (HEC) checks errors only in the header (not in the payload). Since the rest of the header is only 32 bits long, this is a relatively long 8 - bit field; it is used for both error checking and correction.

The NNI header is similar to the UNI header, except it does not have the 4 - bit GFC. Instead, its VPI is increased to 12 bits.

ATM UNI cell header

ATM UNI cell header

ATM Layers and Sublayers. The following figure illustrates the comparison between OSI layers and ATM layers and sublayers at ATM Adaptation Layer (AAL) and below. As shown, AAL corresponds to the OSI Transport layer and part of the Network layer. It consists of two sublayers: convergence sublayer (CS) and segmentation and reassembly (SAR). CS provides interface (convergence) to user applications. SAR is in charge of cell segmentation and reassembly.

The ATM layer corresponds to parts of the OSI Network and Data Link layers. Its main functions are flow control, management of virtual circuit and path, and cell multiplexing and demultiplexing. The ATM Physical layer consists of two sublayers: Transmission Conver­gence (TC) and Physical Medium Dependent (PMD). PMD corresponds to the OSI Physical layer, whereas TC does header error checking and packing / unpacking frames (cells). This makes the ATM Physical layer very different from the OSI Physical layer, where framing is left for the OSI Data Link layer.

Comparison of OSI (layer 4 and below) and ATM layers

Comparison of OSI (layer 4 and below) and ATM layers

Gigabit and 10 - Gigabit Ethernet

Gigabit Ethernet became a standard (IEEE 802.3z) in 1998, It employs the same frame format and size as the previous Ethernets and is backward compatible with 10BASE - T and 100BASE - T. It is generally known as 1000BASE - T although it can be further classified as 1000BASE - LX, 1000BASE - SX, 1000BASE - CX, and 1000BASE - T when it uses various fiber or copper media. The maximum link distance under 1000BASE - LX is 5 kilometers for single - mode optical fiber (SM fiber), 550 meters for multi - mode fiber {MMfiber), and merely 25 meters for shielded twisted pair.

Table Comparison of Fast, Gigabit, and 10 - Gigabit Ethernets

Comparison of Fast, Gigabit, and 10 - Gigabit Ethernets

Gigabit Ethernet adopts full - duplex modes for connections to and from switches and half - duplex modes for shared connections that use repeaters. Since collisions do occur frequently in half - duplex modes, Gigabit Ethernet uses standard Ethernet access method Carrier Sense Multiple Access with Collision Detection (CSMA / CD), as in its predecessors. Gigabit Ethernet has been rapidly replacing Fast Ethernet and FDDI, especially in network backbones. It has gone beyond LAN and found use in MANs.

10 - Gigabit Ethernet was completed in 2002. It retains the main characteristics of Ethernet (bus, packet switching) and the same packet format as before. At a data rate of 10 Gbps, it functions only over optical fiber. Since it operates only under full duplex (switches and buffered distributors), it does not need CSMA/CD for collision detection.

10 - Gigabit Ethernet is expected to finally enable the convergence of voice and data networks. It can be substantially cheaper than ATM. Its design encompasses all LAN, MAN, and WAN, and its carrying capacity is equivalent or superior to Fiber Channel, High Performance Parallel Interface (HIPPI), Ultra 320 or 640 SCSI, and ATM / SONET OC - 192. The maximum link distance is increased to 40 kilometers for SM fiber. In fact, special care is taken for interoperability with SONET / SDH, so Ethernet packets can readily travel across SONET / SDH links.

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