1 Chapter 7 Local Area Networks : The Basics Data Communications and Computer Networks: A Business User’s Approach

2 Data Communications and Computer Networks Chapter 7 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.

3 Data Communications and Computer Networks Chapter 7 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. 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.

4 Data Communications and Computer Networks Chapter 7 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

5 Data Communications and Computer Networks Chapter 7 Advantages of Local Area Networks Ability to share hardware and software resources. Individual workstation might survive network failure. Component and system evolution are possible. Support for heterogeneous forms of hardware and software. Access to other LANs and WANs (Figure 7-1). Private ownership. Secure transfers at high speeds with low error rates.

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7 Data Communications and Computer Networks Chapter 7 Disadvantages of Local Area Networks Equipment and support can be costly. Level of maintenance continues to grow. Private ownership? 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. A LAN is only as strong as it weakest link, and there are many links.

8 Data Communications and Computer Networks Chapter 7 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

9 Data Communications and Computer Networks Chapter 7 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.

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12 Data Communications and Computer Networks Chapter 7 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.

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15 Data Communications and Computer Networks Chapter 7 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 hub. Unshielded twisted pair usually used to connect workstation to hub. Hub takes incoming signal and immediately broadcasts it out all connected links. Hubs can be interconnected to extend size of network.

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18 Data Communications and Computer Networks Chapter 7 Star-wired Bus Topology Modular connectors and twisted pair make installation and maintenance of star-wired bus better than standard bus. 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.

19 Data Communications and Computer Networks Chapter 7 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.

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22 Data Communications and Computer Networks Chapter 7 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.

23 Data Communications and Computer Networks Chapter 7 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. 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.

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26 Data Communications and Computer Networks Chapter 7 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).

27 Data Communications and Computer Networks Chapter 7 Wireless Topology With directional antennae designed for point-to-point transmission (rare), b can transmit for more than 10 miles. With an omni-directional antenna on a typical AP, range may drop to as little as 100 feet.

28 Data Communications and Computer Networks Chapter 7 Wireless Topology IEEE a – One of the more recent standards, capable of transmitting data at 54 Mbps using the 5 GHz frequency range. IEEE g – The other recent standard, also capable of transmitting data at 54 Mbps but using the same frequencies as b (2.4 GHz) and is backwards compatible with b.

29 Data Communications and Computer Networks Chapter 7 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 (more on security in Chapter Thirteen). Wireless LANs may also be configured without an access point. These configurations are called “ad-hoc”.

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32 Data Communications and Computer Networks Chapter 7 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

33 Data Communications and Computer Networks Chapter 7 Contention-Based Protocols 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.

34 Data Communications and Computer Networks Chapter 7 Contention-Based Protocols 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 an example of a non-deterministic protocol.

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36 Data Communications and Computer Networks Chapter 7 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? Hogged? Token ring LANs are losing the battle with CSMA/CD LANs.

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38 Data Communications and Computer Networks Chapter 7 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.

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40 Data Communications and Computer Networks Chapter 7 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. Note how the two frames are different. If a CSMA/CD network connects to a token ring network, the frames have to be converted from one to another.

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43 Data Communications and Computer Networks Chapter 7 Local Area Network Systems Ethernet or CSMA/CD Most common form of LAN today. Star-wired bus is most common topology but bus topology also available. Ethernet comes in many forms depending upon medium used and transmission speed and technology.

44 Data Communications and Computer Networks Chapter 7 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 beginning to appear.

45 Data Communications and Computer Networks Chapter 7 Ethernet 1000 Mbps introduces a few interesting wrinkles: Transmission is full duplex (separate transmit and receive), thus no collisions. Prioritization is possible using 802.1p protocol. Topology can be star or mesh (for trunks).

46 Data Communications and Computer Networks Chapter 7 Ethernet A few more interesting wrinkles: Cabling can be either UTP or optical (but 10 Gbps Ethernet may not work over UTP due to radio frequency interference). Where 10 Mbps Ethernet has less than 30% utilization due to collisions, 1000 Mbps is limited only by traffic queueing. Distance with 10 Mbps is limited by CSMA/CD propagation time, whereas 1000 Mbps limited only by media.

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48 Data Communications and Computer Networks Chapter 7 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 soon.

49 Data Communications and Computer Networks Chapter 7 Local Area Network Systems FDDI (Fiber Distributed Data Interface) Based on the token ring design using 100 Mbps fiber connections. Allows for two concentric 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.

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51 Data Communications and Computer Networks Chapter 7 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

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54 Data Communications and Computer Networks Chapter 7 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.

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