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Introduction• LAN, WAN, MAN Characteristics• LAN Topologies - Ring - Bus - Star• Wan Architectures - Point-to-Point- Circuit Switching - Packet Switching - Cell Switching

• Simple and Fully meshed WAN

• WAN selection criteria

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LAN Characteristics

A local-area network (LAN) is a high-speed, fault tolerant data network that covers a relatively small geographic area. It typically connects workstations, personal computers, printers, and other devices.

LANs offer computer users many advantages, including shared access to devices and applications, file exchange between connected users, and communication between users via electronic mail and other applications.

LAN protocols function at the lowest two layers of the OSI reference model: the physical layer and the data link layer.

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LAN topologies define the manner in which network devices are organized. There are three commonly used LAN topologies: bus, ring, and star.

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LAN protocols typically use one of two methods to access the physical network medium: Carrier sense multiple access collision detect (CSMA/CD) and Token Passing. These methods or algorithms determine the order in which the connected devices can use the LAN.

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A wide-area network (WAN) is a data communications network covering a relatively broad geographic area and often using transmission facilities provided by the common carriers (telephone companies). Very often a WAN consists of several bridged (connected) LANS. In a WAN, you may see hybrid networks; that means that there is a combination of LANs of different topologies, connected by using a WAN technology. A WAN can span the globe.

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WAN technologies function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer.

There are four general classifications of WAN technologies:1. WAN point-to-point links2. Circuit switched WANs3. Packet switched WANs4. Cell switched WANs

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1. A point-to-point link provides a single, pre-established WAN communications path from the customer premises, through a carrier network (the telephone company), to a remote network. Point-to-point links are also known as leased lines. The established path is permanent and is fixed for each remote network reached through the carrier facilities. Point-to-point links are reserved by the carrier company for the private use of the customer.

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2. Circuit switching is a WAN switching method in which a dedicated physical circuit through a carrier network is established, maintained, and terminated for each communication session. Circuit switching, used extensively in telephone company networks, operates much like a normal telephone call. Integrated Services Digital Network (ISDN) is an example of a circuit-switched WAN technology.

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3. Packet switching is a WAN switching method in which network devices share a single point-to-point link to transport packets from a source to a destination across a carrier network. For this purpose, data to be transmitted needs to he broken into smaller data packets. Statistical multiplexing is used to allow devices to share these circuits.

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4. Cell switching is sometimes considered as a special packet switching method. Like with packet switching, cell switching requires that the data to be transmitted is broken into data packets, but these packets are of a fixed, small size, the cells. Because cells are fixed- length, they can be processed and switched in hardware at high speeds. Cell relay is the basis for many high-speed network protocols including ATM. ATM (Asynchronous Transfer Mode) is a cell switching protocol.

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A metropolitan-area network (MAN) is a communications network that serves an urban area An example of a MAN would be the connection of several LANs in several buildings of an. enterprise. There are no MAN-specific protocols or definitions.

Nevertheless FDDI (Fiber Distributed Data Interface) could be considered as a typical MAN protocol. FDDI specifies a high-speed token-passing ring LAN, using fiber optic media. FDDI was created to fill the need for a high-bandwidth, secure, local-to-medium area network. See module 5 for further specifications. On a campus site, connecting buildings may not be possible using LAN technologies due to the distance involved. Very often a FDDI network is installed to provide a high-speed backbone.

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The terms LAN, WAN and MAN are used to distinguish the size of a network in terms of the distance it spans. As there are different requirements needed by the different media and devices depending on the distances to be served, there are some protocols that have been especially designed for short distance networks, others for medium or long distance networks. Consequently, they are usually considered as typical LAN protocols or as typical for a WAN. In this sense the terms LAN, WAN and MAN are used to categorize the networks and the used protocols.

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The word LAN is used when we are talking about a limited geographic area, as well as when we talk about the typical protocols like Ethernet and Token Ring.

Generally, a MAN spans a larger geographic area than a LAN, but a smaller geographic area than a WAN. FDDI would be a typical MAN protocol.

A data communications network that serves users across a broad geographic area and often uses transmission devices provided by common carriers is called a WAN. Frame Relay and X.25 are examples of WANS.

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We will now look at the three common LAN topologies. When using the terms Ring, Bus and Star, we are referring to logical topologies (i.e. logically how the topology functions). The physical topology may be different.For an Ethernet environment, where a single coaxial cable is connecting all systems on the network, the physical and logical topology is identical. However, there are other examples. The typical Token Ring cabling looks like a star configuration, since one cable is connecting each system to an access point of the ring. The same is true for another type of Ethernet bus topology cabling, Ethertwist, which uses the same configuration, a single cable from the system to an access point of the bus.

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In a ring, the network cable passes from one system to another, until they are interconnected to form a ring. Between each neighboring system, there is a direct point-to-point link. Sharing of the ring between the systems is ensured by appropriate medium access control algorithms. A typical network using ring topology is Token Ring. In the Token Ring example, the physical connection is made using an MSAU (Multi-Station Access Unit) and appears as a physical STAR connection.

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In the case of FDDI which uses a Logical Ring (as for Token Ring) the physical representation may be as:

• Physical Star (when using a FDDI concentrator)

or

• Physical Ring (in the case of a dual attached FDDI Ring).

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Mostly, a single network cable is passed along all locations that require systems to be connected to the network. Each system has a physical connection to the cable so that the systems can access the network in parallel. The available transmission bandwidth is shared between the systems by the use of appropriate medium access control algorithms.

A typical network using bus topology is Ethernet.

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In a star, a networking device in the center of the network is connected to all systems via a direct point-to-point link. Sharing of the star between the systems is controlled by the networking device in the center. A typical network using star topology is 100VG AnyLAN.

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Token Ring networks are used mostly for technical and office environments and for IBM Mainframe systems. In a ring network, data flows from system to system, always in one direction, preceded by a token, giving the network its name. The characteristics of a Token Ring network are illustrated in the slide.

Token Ring is deterministic, each station on the ring is guaranteed an opportunity to transmit data at regular intervals. It is a non-contention access control method, and looks similar to a polling method as stations can only transmit data when given authority to do so. As only the station holding the token can transmit data, token ring networks never experience collisions.

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The token (a special sequence of three bytes) constantly circulates around the network from one station to the next. Only the station holding the token is allowed to transmit data onto the ring. The token circulates around the ring, in an idle state, until a station wants to transmit data. The station waits to receive the token, removes it from the network, and then transmits its data. The recipient copies the data from the network and allows the frame to carry on to the sender. If a node MAC address matches the source address of the packet, it removes the packet from the network and regenerates a new token.

The Token Ring Network uses a totally different access method & protocol than the rest special type of bridge is needed to connect it to the rest of the network.

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Note: Hubs are also called multi-port repeaters. Ethernet, Token Ring, etc., describe how the data is framed

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Ethernet is the most widespread bus network. It can be implemented as a coaxial cable (ThinLAN) or using telephone-style twisted pair cabling (Ethertwist). The characteristics of an Ethernet network are illustrated in the slide.Ethernet allows multiple stations to access the transmission medium without prior coordination, using a carrier sense access method (CSMA/CD) to govern access to the network.Ethernet is a half-duplex, non-deterministic technology.The non-deterministic operation of Ethernet networks can be fast and efficient, but as the amount of traffic increases, collisions increase, performance drops, and throughput decreases.When a node is accessing the network, all other nodes must be in receive mode. When two or more signals exist on the LAN (nodes transmitting at the same time), a collision results; after a collision, each node involved in the collision waits a random amount of time, and then retransmits its information onto the LAN.

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100VG AnyLAN is a networking technology similar to standard Ethernet running at much higher speed. It is implemented using the same type of twisted pair cabling as in the Ethernet network example. In a 100VG AnyLAN network, access to the network is controlled by intelligent network devices that allot transmission time to the connected devices instead of having the systems compete. AnyLAN brings increased overall bandwidth (many users need to exchange a lot of information), brings individual increased bandwidth (an application exchanging very large amounts of information, e.g. database, imaging, desktop publishing applications or network printing) and allows time-sensitive applications (real-time video requires continuous transfer of packets with minimal delay).

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A Wide Area Network, WAN, is a communications network that can support data communications over a large area such as across a country, or even across the globe. A WAN is usually made up of a combination of several LANs and/or other types of data communication environments and, when properly implemented, should appear to work in the same way as a LAN.

A LAN may be expanded to a WAN using components such as Routers, Switches and Bridges.

Exception: In the case of Frame Relay, Frame Relay Switches are used instead of Routers.

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The various data communication components are linked together using communication links called WAN links such as:

• Packet-switching networks

• Fiber-optic cable

• Microwave transmitters

• Satellite links

• Cable television coaxial systems

Wide area telephone networks are prohibitively expensive for most private companies to utilize and maintain and are often leased from service providers.

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A point-to-point link provides a single, pre-established WAN communications path from the customer premises, through a carrier network (the telephone company), to a remote network. Point-to-point links are also known as leased lines. The established path is permanent and is fixed for each remote network reached through the carrier facilities. Point-to-point links are dedicated transmission links reserved by the carrier company for the private use of the customer.Leased lines can make use of two different types of transmission facilities: - Analog transmission - Digital transmission.The picture shows point-to-point links using these types of transmission. further definitions of Analog transmission and Digital transmission are listed below.

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Analog transmission

One method by which LANs may be linked together is by using modems across the telephone system (PSTN), using voice-grade telephone lines. Voice-grade communications transmission methods are slow, with modems reducing the transmission speed even further.

Digital transmission

If the volume of WAN transmissions within an Organization is high, analog transmission will become inefficient and expensive.

For digital transmissions, the device that connects the customer equipment to the network is no longer a modem, but a device called a CSU/DSU (Channel Service Unit/Data Service Unit).

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Digital lines are available in several forms including:

• T1 (USA and Japan)• El (Europe)• T3• Fractional T1/El

T1 is a method of point-to-point transmission that uses two-wire pairs (one pair to send and one to receive) to transmit a full-duplex signal at a rate of 1.544 Mbps. T1 is a very costly WAN link, utilizing a very large bandwidth; if this bandwidth is not required, it is possible to subscribe to one or more T1 channels in 64Kbps increments known as Fractional T-1 (FT-1).

El is a Wide area digital transmission scheme used in Europe that carries data at the rate of 2.048Mbps. E1 lines can be leased for private use from commercial carriers.

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T3 and Fractional T-3 provide voice and data-grade services from 6Mbps to 45Mbps and are designed to transmit large amounts of data, at high speed, between two fixed points. A T3 line may be used to replace several T1 lines.

Fractional T1/E1: Wide Area Service Providers offer incremented digital services for leased lines. Speeds of 56Kbps (USA and Japan) or 64Kbps (Europe) are supported by Digital Services level 0 (DS0). You may sometimes hear services referred to as fractional T1/E1. Fractional T1 service is available in increments of 64, 128, etc.

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Circuit switching is a WAN switching method in which a dedicated physical circuit through a carrier network is established, maintained, and terminated for each communication session.

Circuit switching as opposed to point-to-point provides multiple access to a network. In this particular case, we do not need to deal with a pre-established dedicated link between two physical locations, but we talk about a circuit established on demand from one physical location to any other location on the network whenever a communication takes place.

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The public switched telephone network (PSTN) is a circuit-switched network. With this type of network, a communication line is re-opened each time that it is used and, therefore, the exact route of the transmitted data cannot be guaranteed. This type of communication is also referred to as Dial-Up as you will dial into the network to establish a communication between two locations.

The quality of the data transmission is governed by the quality of each of the links involved invariably introducing inconsistencies in transmission quality from one session to the next.

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Circuit switching facilities as previously stated for point-to-point can also rely on analog transmission or digital transmission.

An example of Circuit switching relying on digital transmission facilities is ISDN (Integrated Services Digital Network) which is covered in more detail on the next page.

Another example of WAN technology using Circuit switching is Frame Relay.

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ISDN was designed by CCITT as a set of digital transmission protocols to allow users to send multiple channels (voice, video, and data) across a single link. ISDN was originally developed as the digital replacement for the Public Switched Telephone Network (PSTN), to link Businesses and Homes and produces a dial-up service only. There are several reasons why ISDN is an attractive communication medium: ISDN is a circuit switched communication medium which means that you only pay for the service when you use it. ISDN offers moderate to high speed communication line rates which makes it attractive for LAN-to-LAN applications. ISDN is generally available throughout the world.

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Data TransmissionISDN transfers data using time division multiplexed channels of which there are two types:1. B Channels 2. D channels It is possible for the two B channels to be combined to provide a combined 128 Kbps data stream. If both end stations also support compression, a much higher data throughput may be achieved.

ISDN divides its available bandwidth into the two types of data channels, as detailed in the following table:ChannelType

TransmissionRate

Data Types Supported

B 64 Kbps VoiceDataImages

D 16 Kbps SignalingLink Management Data

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ISDN Channel Types

There are two types of ISDN interfaces available: 1. Primary Rate Interface 2. Basic Rate Interface

Primary Rate Interface: Primary Rate ISDN divides the total TI/El bandwidth to provide either: 23 B channels and 1 D channel (America and Japan), or 30 B channels and 1 D channel (Europe).

Basic Rate Interface: Basic Rate ISDN divides its available bandwidth to provide 2 B channels and 1 D channel.

Note: Cable Modem and DSL are better now than ISDN

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Other WAN technologies have been developed over the last few years in order to face the ever increasing demand for more speed and bandwidth which is imposed by applications like multimedia, backup and software distribution over the network.

Synchronous Optical Network (SONET) is another circuit switching example. It employs fiber-optic technology and is capable of transmitting voice, data and video at speeds of greater than one gigabit per second.

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SONET is a standard for optical transmission that has been formulated by the Exchange Carriers Standards Association (ECSA) for the American National Standards Institute (ANSI), and has been incorporated into the Synchronous Digital Hierarchy recommendations of the CCITT (or International Telecommunications Union-ITU), which sets the standards for international telecommunications.

SONET defines optical carrier (OC) levels and electrical equivalent synchronous transport signals (STSs) for the fiber-optic based transmission hierarchy.

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SONET uses a basic transmission rate of STS- 1, which is equivalent to 51.84 Mbps. However higher level signals are achievable and are of integer multiples of the base rate. For example, STS-3 is three times the rate of STS-1 (3 X 51.84 = 155.52 Mbps). An STS-12 would be at a rate of 12 X 51.84 = 622.08 Mbps.

SONET is flexible enough to be used as the underlying transport layer for BISDN ATM cells. BISDN (Broadband Integrated Services Digital Network) is a single ISDN network that can support voice, data and video services. ATM is a CCITT standard that supports cell-based voice, data, video and multimedia communication in a public network under BISDN. The ATM forum is aligning with SONET as the transport layer for cell-based traffic.

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Synchronous Digital Hierarchy (SDH) is the European version of SONET. It is a European standard that defines a set of rate and format standards which are transmitted using optical signals over fiber. SDH is similar to SONET with a basic rate of 155.52Mbps (up to 688 Mbps).

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Packet-switching involves transmitting packets of data from many different users over many different possible paths. This method of networking is fast, efficient and reliable, and may be used to transmit data globally.

The data to be transmitted is divided into small packets of data, each of which is uniquely identified with information including the destination address and details of its position in the original data message.

The individual, labeled data packages are then relayed through stations in a computer network, via the best route currently available between the source and destination. These networks are sometimes called any-to-any connections.

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The data packets may be routed through different paths, arriving at the destination at different times, and possibly out of order. The additional information linked to the data packets ensures that the original message is reassembled correctly at the destination.

Packet-switching networks will require the computers and software that control the data transmission to have the intelligence to manage the routing, assembling and disassembling of the data packets.

As the data packets are kept small, if a transmission error should occur, only the one packet will need to be re-sent, saving on transmission time and call-costs.

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Virtual circuits are used by many packet-switching networks. These circuits are composed of a series of logical, as opposed to physical, connections between the originating and destination computer. These connections may last either as long as the conversation (temporary connection) or as long as both computers are up and running (permanent connection). The circuit is bandwidth allocated on demand and is established and maintained once the two computers exchange the relevant information to define the communication parameters. These parameters are required to ensure reliability and include:

• Maximum message size• Data transmission path• Acknowledgements• Flow control• Error control

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Switched virtual circuits (SVCs)These circuits are also known as point-to-many-point connections. Network resources are dedicated to the route defined between the end computers and the connection is maintained until either one of the computers is switched off (Needs destination address, establishes connection, sends data, then disconnects [releases line]).

3 types of SVCs exist:• Incoming SVC [analogy: phone that can be used only to answer calls]• Outgoing SVC [analogy: phone that can be used only to dial out]• 2-way SVC

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Permanent virtual circuits (PVCs)PVCs are in effect similar to leased lines that are permanent or virtual, except that the customer is only charged for the time that the line is in use (permanent connection no need to send an address).

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X.25 is a set of protocols employed by a packet-switching network to transport data over which other protocols may be routed or bridged. The packet-switching network provides the best transmission route available at any time using switches, circuits and routes.

The original X.25 networks transmitted data over unreliable telephone lines, resulting in many errors, therefore, extensive error-checking facilities were incorporated into the X.25 protocol, which appears to slow the transmission of data further.

X.25 protocol now defines the interface between synchronous packet-mode host and the public data network (PDN) over a dedicated (leased) line. This interface is referred to as a Data Terminated Equipment/Data Communications Equipment interface (DTE/DCE) interface.

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Examples of DTEs (each having the PDN as the DCE part of the interface) include the following:

• A host computer with an X.25 interface

• A packet assembler/disassembler (PAD)PADs receive asynchronous characters from a low-speed terminal and assembles them into packets to be transmitted over the network.PADs also disassemble packets received from the network and delivers them as characters to the terminals.

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Asynchronous transfer mode is a method of packet switching where fixed-size packets of data are transmitted over broadband and baseband LANs or WANS. ATM uses fixed size cells of 53 bytes.

ATM was defined by the CCITT (Consultative Committee for International Telephony & Telegraphy) as part of the broadband integrated services digital network (BISDN). It is capable of transmitting data at very high speeds, typically between 155 Mbps and 622 Mbps. ATM will support the transmission of the following data types:• Data• Voice• Video

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Packet switching involves breaking the original data into smaller, variable-length data packets; ATM also divides the original data, with the size of the data packets being fixed at 53 bytes. These 53 byte cells consist of 48 bytes of application data and 5bytes of ATM header data.

Network equipment can switch, route and transmit these uniform cells much more quickly than random-sized data packets. The uniform cells use the buffers more efficiently and, therefore, improve the efficiency in the processing of the incoming data.

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Most ATM boards currently transmit data at about 155 Mbps, but has the theoretical capability of transmitting at 1.2 gigabits per second.

AT'M may be used in both LANs and WANs at approximately the same speed.

This is layer 2 switching (frames). ATM switches constantly send out 53 byte cells. No sequence numbers are assigned. Its job is to deliver the data (and deliver them fast). It is up to the above layers to notify the sender if something wrong (error) happens.

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In a simple mesh network (every switch in a forward sense is connected to at least one other switch), the nodes are organized in a mesh topology with some nodes having either a physical or virtual circuit connecting them to each node in the network, while other nodes may be connected, (either physically or virtually) to only one or two nodes.

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In a full mesh network (every switch is connected to every other switch in the network), the nodes are organized in a mesh topology with each node having either a physical or virtual circuit connecting them to each other node in the network as illustrated in the slide.

• Full mesh networks provide a great deal of redundancy, however they can be expensive to establish and maintain. Some WAN backbone networks implement this type of topology.

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WAN Selection Criteria* Point-to-point versus multi-point communications* Speed* Reliability* CostNote: ISDN 64K to 128K is for BRI (Basic Rate Interface - Defines an ISD network service consisting two B channels and one D channel. The PRI ISDN gives us basically T1 capability (Primary Rate Interface - defines an ISDN network service consisting of 23 or 30 channels and one D channel).

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