1 Chapter 7 Introduction to Local Area Networks

2 Introduction A local area network is a communication network that interconnects a variety of data communicating devices within a small geographic area and broadcasts data at high data transfer rates with very low error rates. Since the local area network first appeared in the 1970s, its use has become widespread in commercial and academic environments. Chapter Seven - Introduction to Local Area Networks

3 Primary Function of a LAN To provide access to hardware and software resources that will allow users to perform one or more of the following activities: File serving - A large storage disk drive acts as a central storage repository. For example, suppose two or more users wish to share a data set. In this case, the data set, like the application software, would be stored on the file server, while the network provided access to those users who had the appropriate permissions.. Print serving - Providing the authorization to access a particular printer, accept and queue print jobs, and providing a user access to the print queue to perform administrative duties. Chapter Seven - Introduction to Local Area Networks

4 Primary Function of a LAN continued Video transfers - High speed LANs are capable of supporting video image and live video transfers. Manufacturing support - LANs can support manufacturing and industrial environments. Academic support – In classrooms, labs, and wireless support Interconnection between multiple systems Chapter Seven - Introduction to Local Area Networks

5 Advantages of Local Area Networks Ability to share hardware and software resources. Individual workstation might survive network failure. Component and system evolution are possible ( both evolutions can be independent of each other ). For example, if new workstations are desired, it is possible to replace older workstations with newer ones with few, if any, changes required to the network itself. Support for heterogeneous forms of hardware and software. Under some conditions, local area networks allow equipment from different manufacturers to be mixed on the same network. For example, IBM personal computers with Apple microcomputers. Private ownership. Secure transfers at high speeds with low error rates. Chapter Seven - Introduction to Local Area Networks

6 Access to other LANs and WANs (Figure 7-1). Chapter Seven - Introduction to Local Area Networks Advantages of Local Area Networks

7 Disadvantages of Local Area Networks Equipment and support can be costly. Level of maintenance continues to grow. Some types of hardware may not interoperate. Just because a LAN can support two different kinds of packages does not mean their data can interchange easily. For example, even if a local area network supports two different types of database systems, users may not be able to share data between the two database systems. A LAN is only as strong as it weakest link, and there are many links. For example, a network may suffer if the file server cannot adequately serve all the requests from users of the network. Chapter Seven - Introduction to Local Area Networks

8 Basic Network Topologies Local area networks are interconnected using one of four basic configurations: 1. Bus/tree 2. Star-wired bus 3. Star-wired ring 4. Wireless Chapter Seven - Introduction to Local Area Networks

9 Bus/Tree Topology The original topology Workstation has a network interface card (NIC) that attaches to the bus (a coaxial cable) via a tap. Data can be transferred using either baseband digital signals or broadband analog signals. Chapter Seven - Introduction to Local Area Networks

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12 Bus/Tree Topology Baseband signals are bidirectional and more outward in both directions from the workstation transmitting. Broadband signals are usually uni-directional and transmit in only one direction. Because of this, special wiring considerations are necessary. Buses can be split and joined, creating trees. Chapter Seven - Introduction to Local Area Networks

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15 Star-wired Bus Topology Logically operates as a bus, but physically looks like a star. Star design is based on hub. All workstations attach to a central device such as hub. Unshielded twisted pair usually used to connect workstation to hub. Hub takes incoming signal and immediately broadcasts it out to all other workstations (or devices) connected to the hub. Hubs can be interconnected to extend size of network. Chapter Seven - Introduction to Local Area Networks

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17 Chapter Seven - Introduction to Local Area Networks

18 Star-wired Bus Topology Modular connectors and twisted pair make installation and maintenance of star-wired bus better than standard bus. simple installation and maintenance, low-cost components Hubs can be interconnected with twisted pair, coaxial cable, or fiber optic cable. Biggest disadvantage: when one station talks, everyone hears it. This is called a shared network. All devices are sharing the network medium. Chapter Seven - Introduction to Local Area Networks

19 Star-wired Ring Topology Logically operates as a ring but physically appears as a star. Star-wired ring topology is based on MAU (multi-station access unit) which functions similarly to a hub. Where a hub immediately broadcasts all incoming signals onto all connected links, the MAU passes the signal around in a ring fashion. Like hubs, MAUs can be interconnected to increase network size. Chapter Seven - Introduction to Local Area Networks

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21 Chapter Seven - Introduction to Local Area Networks

22 Wireless Topology Not really a specific topology since a workstation in a wireless LAN can be anywhere as long as it is within transmitting distance to an access point. Several versions of IEEE standard defines various forms of wireless LAN connections. Workstations reside within a basic service set, while multiple basic service sets create an extended service set. Chapter Seven - Introduction to Local Area Networks

23 Wireless Topology Two basic components necessary: the client radio, usually a PC card with an integrated antenna installed in a laptop or workstation, and the access point (AP), which is an Ethernet port plus a transceiver(جهاز الإرسال والاستقبال ). The AP acts as a bridge between the wired and wireless networks and can perform basic routing functions. Workstations with client radio cards reside within a basic service set, while multiple basic service sets create an extended service set. Chapter Seven - Introduction to Local Area Networks

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26 Wireless Topology IEEE – The original wireless standard, capable of transmitting data at 2 Mbps IEEE b – The second wireless standard, capable of transmitting data at 11 Mbps In actual tests, 11 Mbps b devices managed 5.5 Mbps (from a July 2000 test by Network Computing). Chapter Seven - Introduction to Local Area Networks

27 Wireless Topology With directional antennae designed for point-to-point transmission, b can transmit for more than 10 miles. With an omni-directional(متعددة الاتجاهات ) antenna on a typical AP, range may drop to as little as 100 feet. Chapter Seven - Introduction to Local Area Networks

28 Wireless Topology IEEE a – One of the more recent standards, capable of transmitting data at 54 Mbps (theoretical) using the 5 GHz frequency range. IEEE g – The other recent standard, also capable of transmitting data at 54 Mbps (theoretical) but using the same frequencies as b (2.4 GHz) and is backwards compatible with b. Chapter Seven - Introduction to Local Area Networks

29 Wireless Topology HiperLAN/2 (European standard, 54 Mbps in 5 GHz band) To provide security, most systems use either Wired Equivalent Privacy (WEP), which provides either 40- or 128- bit key protection, or a more advanced standard such as WPA ( Wi-Fi Protected Access ). Wireless LANs may also be configured without an access point. These configurations are called “ad-hoc”. Chapter Seven - Introduction to Local Area Networks

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32 Medium Access Control Protocols How does a workstation get its data onto the LAN medium? A medium access control protocol is the software that allows workstations to “take turns” at transmitting data. Two basic categories: 1. Contention-based protocols 2. Round robin protocols Chapter Seven - Introduction to Local Area Networks

33 Contention-Based Protocols - CSMA/CD Essentially first come first served. Most common example is carrier sense multiple access with collision detection (CSMA/CD). If no one is transmitting, a workstation can transmit. If someone else is transmitting, the workstation “backs off” and waits. Chapter Seven - Introduction to Local Area Networks

34 Contention-Based Protocols - CSMA/CD If two workstations transmit at the same time, a collision occurs. When the two workstations hear the collision, they stop transmitting immediately. Each workstation backs off a random amount of time and tries again. Hopefully, both workstations do not try again at the exact same time. CSMA/CD is half-duplex protocol. Chapter Seven - Introduction to Local Area Networks

35 Chapter Seven - Introduction to Local Area Networks Because the number of times a workstation will have to wait is unknown, it is not possible to determine exactly when a workstation will be allowed to transmit its data without collision. Thus, CSMA/CD is a nondeterministic protocol.

36 Contention-Based Protocols - CSMA/CA CA stands for Collision Avoidance - protocol does not listen and detect collisions. Instead, it tries to avoid collisions before they happen How does CSMA/CA do this? All devices, before they transmit, must wait an amount of time called an intra-frame space (IFS) Some applications have a short IFS, while others have a long IFS. If two applications want to transmit at the same time, the application with the shorter IFS will go first. Chapter Seven - Introduction to Local Area Networks

37 Round Robin Protocols Each workstation takes a turn transmitting and the turn is passed around the network from workstation to workstation. Most common example is token ring LAN in which a software token is passed from workstation to workstation. Token ring is an example of a deterministic protocol. Token ring more complex than CSMA/CD. What happens if token is lost? Duplicated? Token ring LANs are losing the battle with CSMA/CD LANs. Chapter Seven - Introduction to Local Area Networks

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39 Medium Access Control Sublayer To better support local area networks, the data link layer of the OSI model was broken into two sublayers: 1. Logical link control sublayer 2. Medium access control sublayer Medium access control sublayer defines the frame layout and is more closely tied to a specific medium at the physical layer. Thus, when people refer to LANs they often refer to its MAC sublayer name, such as 10BaseT. Chapter Seven - Introduction to Local Area Networks

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41 IEEE 802 Frame Formats The IEEE 802 suite of protocols defines the frame formats for CSMA/CD (IEEE 802.3) and token ring (IEEE 802.5). Each frame format describes how the data package is formed. The two frames do not have the same layout. If a CSMA/CD network connects to a token ring network, the frames have to be converted from one to another. Chapter Seven - Introduction to Local Area Networks

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43 Local Area Network Systems Wired Ethernet or CSMA/CD Most common form of LAN today. Star-wired bus is most common topology but bus topology still not totally dead yet. Wired Ethernet comes in many forms depending upon medium used and transmission speed and technology. Chapter Seven - Introduction to Local Area Networks

44 Wired Ethernet Originally, CSMA/CD was 10 Mbps Then 100 Mbps was introduced. Most NICs sold today are 10/100 Mbps Then 1000 Mbps (1 Gbps) was introduced 10 Gbps is now being installed in high-end applications Chapter Seven - Introduction to Local Area Networks

45 Wired Ethernet 1000 Mbps introduces a few interesting things: Transmission is full duplex (separate transmit and receive), thus no collisions. Prioritization is possible using 802.1p protocol. Topology can be star or mesh. Chapter Seven - Introduction to Local Area Networks

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47 Wired Ethernet One of the latest features: Power over Ethernet (PoE) What if you have a remote device that has an Ethernet connection? It will require a power connection. What if you don’t have an electrical outlet nearby? Use PoE. The power to drive the Ethernet NIC is sent over the wiring along with the usual Ethernet signals. Chapter Seven - Introduction to Local Area Networks

48 Wireless Ethernet As we have already seen, IEEE has created the b, a, and g wireless standards IEEE n (100 Mbps) will be ratified soon and should start appearing in product form in 2007 (maybe?) Latest wireless Ethernet is using MIMO technology – multiple input multiple output – sender and receiver have multiple antennas for optimum reception Chapter Seven - Introduction to Local Area Networks

49 Local Area Network Systems IBM Token Ring Deterministic LAN offered at speeds of 4, 16 and 100 Mbps. Very good throughput under heavy loads. More expensive components than CSMA/CD. Losing ground quickly to CSMA/CD. May be extinct(تنقرض) soon. Chapter Seven - Introduction to Local Area Networks

50 Local Area Network Systems FDDI (Fiber Distributed Data Interface) Based on the token ring design using 100 Mbps fiber connections. Allows for two concentric (متحد المركز) rings - inner ring can support data travel in opposite direction or work as backup. Token is attached to the outgoing packet, rather than waiting for the outgoing packet to circle the entire ring. Chapter Seven - Introduction to Local Area Networks

51 LANs In Action : A Small Office Solution What type of system will interconnect twenty workstations in one room and fifteen workstations in another room to a central server, which offers: Internal A database that contains all customer information High quality printer access Chapter Seven - Introduction to Local Area Networks

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53 Chapter Seven - Introduction to Local Area Networks

54 LANs In Action : A Small Office Home Office (SOHO) LAN Solution What if you have two computers at home and want both to share a printer and a connection to the Internet. Some type of SOHO solution might solve this problem. Essentially a LAN with a 2- or 3-port hub, connecting cables, and software. In some models the hub also acts as a router to the Internet. Chapter Seven - Introduction to Local Area Networks

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