Introduction • LAN, WAN, MAN Characteristics • LAN Topologies - Ring - Bus - Star

2. 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

4. 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.

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

7. 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.

9. 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.

10. 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 links 2. Circuit switched WANs 3. Packet switched WANs 4. Cell switched WANs

11. 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.

12. 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.

13. 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.

14. 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.

16. 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.

18. 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.

19. 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 carriersis called a WAN. Frame Relay and X.25 are examples of WANS.

21. 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.

23. 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.

25. 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).

27. 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.

29. 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.

32. 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.

33. 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.

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

35. 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.

37. 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).

39. 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.

40. 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.

42. 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.

43. 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).

44. 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.

45. 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.

47. 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.

48. 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.

49. 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.