Chapter 7 Local Area Networks : The Basics

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

2. Last time • Types of errors and their prevention • Error detection • Parity – 50% • CRC – can detect nearly all errors • Error correction • 1. Do nothing • 2. Return an error message to the transmitter • 3. Fix the error with no further help from the transmitter

3. Three Major Types of Networks • Local Area Network (LAN) • Serves users within a confined geographical area (usually within a mile). • Metropolitan Area Network (MAN) • Covers a geographic area the size of a city or suburb. The purpose of a MAN is often to bypass local telephone companies when accessing long-distance service. • Wide Area Network (WAN) • Covers a wide geographical area, such as a state or a country. Examples: Tyment, Telenet, Uninet, and Accunet.

4. Emphasis Definition of a Local Area Network (LAN) • Physical and Logical LAN Topologies • Different Medium Access Control Protocols • Common LAN Systems • Ethernet (1st commercially available LAN) • Token ring • FDDI (fiber data distributed interface) ring

5. 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 • very low error rates. • (WANs now do this too) • Since the local area network first appeared in the 1970s, its use has become widespread in commercial and academic environments.

6. Data Communications and Computer Networks Chapter 7 Functions of a Local Area Network File server - A large storage disk drive that acts as a central storage repository. Print server - Provides the authorization to access a particular printer, accept and queue print jobs, and provides a user access to the print queue to perform administrative duties. Interconnection - A LAN can provide an interconnection to other LANs and to wide area networks.

7. Data Communications and Computer Networks Chapter 7 Functions of a Local Area Network Video transfers - High speed LANs are capable of supporting video image and live video transfers. Manufacturing support - LANs can support manufacturing and industrial environments. Distributed processing - LANs can support network operating systems which perform the operations of distributed processing.

8. 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.

9. Data Communications and Computer Networks Chapter 7

10. 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.

11. Data Communications and Computer Networks Chapter 7 • Basic Network Topologies • Topology: the geometric configuration • Separate from protocols! • Local area networks are interconnected using one of four basic configurations: • 1. Bus/tree • 2. Star-wired bus • 3. Star-wired ring • 4. Wireless

12. Basic Network Topologies • Logical vs physical design (topologies) • 1. Logical design: how the data moves around the network from workstation to workstation • 2. Physical design: how the network physically appears if drawn on a sheet of paper (laid out) • Names of topologies usually refer to the physical design.

13. Data Communications and Computer Networks Chapter 7 • Bus/Tree Topology • The original topology – 1970’s • Workstation has a network interface card (NIC) provides a physical connection to a network • Data can be transferred using either • baseband digital signals • broadband analog signals.

14. Data Communications and Computer Networks Chapter 7 • Bus/Tree Topology • Workstation has a network interface card (NIC) provides a physical connection to a network • Attaches to the bus (a coaxial cable) via a tap. • NIC is an electronic device that performs the necessary signal conversions and protocols operations so that the workstation can send and receive data on the network. • Tap is a passive device • Does not alter the signal • Does not require electricity to operate

15. Data Communications and Computer Networks Chapter 7

16. Data Communications and Computer Networks Chapter 7

17. Bus/Tree Topology • Baseband signals • Digital signals – 10 Mbps • Bidirectional and more outward in both directions from the workstation transmitting. • Easy to install and maintain • Fewer than 100 workstations • Buses can be split and joined, creating trees.

18. Data Communications and Computer Networks Chapter 7

19. Bus/Tree Topology • Broadband signals • Usually uni-directional and transmit in only one direction. • Analogy and FDM for multiple channels (amplification necessary). Because of this, special wiring considerations are necessary. • 100 to 1000 workstations over larger distances due to easy amplification • Buses can be split and joined, creating trees.

20. Data Communications and Computer Networks Chapter 7 Twice the propagation delay of baseband

21. Data Communications and Computer Networks Chapter 7

22. Data Communications and Computer Networks Chapter 7 • Advantages/disadvantages of a bus • Difficult to add new devices if no tap exists. • No tap existing means cutting into the line • As such, this topology is loosing popularity • Plenty still around; Ethernet uses this.

23. Data Communications and Computer Networks Chapter 7 • Star-wired Topologies • Stars versus a single line • Two types: • Star-wired bus • (often call the star topology) • Star-wired ring

24. 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. • Hub is an unintelligent device that immediately transmits whatever data it receives to all connections • 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. • Very popular!

25. Star-wired bus physical topology for a LAN

26. Interconnection of two hubs in a star-wired bus LAN

27. 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.

28. 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 in one direction. Like hubs, MAUs can be interconnected to increase network size.

29. Ring topology viewed logically

30. Data Communications and Computer Networks Chapter 7 • NIC Repeaters for Star-wired Ring • All data must pass through a NIC repeater • Performs basic functions: • Bypass: data does not copy to the workstation, used for inactive devices • Copy: data from ring copied to workstation • Write: data from the workstation copied to the ring • Regenerates: recopies data back onto ring

31. Three possible operations of the workstation repeater on a ring topology

32. Physical organization of a ring topology Looks like a star!

33. Multi-station access unit on a ring topology The MAU replaces the hub for this topology

34. 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. • Range varies from 50 to 800 ft with speeds of 2 to 11 Mbps • Most wireless LANs include a wired LAN backbone • Usually requires a wireless NIC

35. Stanford’s wireless network

36. Data Communications and Computer Networks Chapter 7 Wireless Topology – different forms Newer IEEE 802.11 and 802.11b (Wi-Fi) standard defines various forms of wireless LAN connections. Speeds up to 11 Mbps with 802.11b standard. Workstations reside within a basic service set, while multiple basic service sets create an extended service set.

37. Data Communications and Computer Networks Chapter 7 • Wireless Topology • Acceptable transmission ranges broken up into areas: • Basic service set – that surrounding an access point • Extended service set – collection of basic service sets • Workstations reside within a basic service set, while multiple basic service sets create an extended service set.

38. Data Communications and Computer Networks Chapter 7 Wireless Topology Two basic components necessary: the client radio, usually a PC card with an integrated antenna, 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.

39. Single-cell wireless LAN configuration

40. Multiple-cell wireless LAN configuration

41. Ad-hoc configuration for a wireless LAN

42. Data Communications and Computer Networks Chapter 7 Wireless Topology With directional antennae designed for point-to-point transmission (rare), 802.11b can work for more than 10 miles. With an omni-directional antenna on a typical AP, range may drop to as little as 100 feet. Distance is inversely proportional to transmission speed - as speed goes up, distance goes down.

43. Data Communications and Computer Networks Chapter 7 Wireless Topology In actual tests, 11 Mbps 802.11b devices managed 5.5 Mbps To provide security, most systems use Wired Equivalent Privacy (WEP), which provides either 40- or 128-bit key protection. (how good?) What will Bluetooth’s impact be on 802.11b (Wi-Fi)?

44. Other Wireless Standards • IEEE 802.11 (older 2 Mbps) • IEEE 802.11b (11 Mbps, 2.4 GHz) • Also called Wi-Fi (wireless fidelity) • IEEE 802.11a (54 Mbps, 5 GHz, in 2002) • IEEE 802.11g (54 Mbps, 2.4 GHz, in 2002) • HiperLAN/2 (European standard, 54 Mbps in 5 GHz band)

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46. Summary of topologies • Logical vs physical topologies • Bus and star-rings - old technologies • Still some around • You’ll probably use • Star-wired bus (star) With bus or routers (now much easier to use) • Wireless network (Wi-fi)

47. Protocols • Last time topologies • Now how the data actually flows

48. Data Communications and Computer Networks Chapter 7 Medium Access Control (MAC) 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. Since a LAN is a broadcast network, it’s imperative that only one workstation at a time be allowed to transmit data onto the network. (for multiple channels, one channel per workstation) Switches have changed that, but we’ll take about that in Ch8.

49. Data Communications and Computer Networks Chapter 7 Medium Access Control Protocols Three basic categories: 1. Contention-based protocols (1st come; 1st serve) 2. Round robin protocols (take your turn!!) 3. Reservation protocols (requests please!)

50. 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. Half duplex protocol.