1	Chapter Seven Local Area Networks: The Basics
2	After reading this chapter, you should be able to: State the definition of a local area network List the primary function, activities, and application areas of a local area network Cite the advantages and disadvantages of local area networks Identify the physical and logical topologies of local area networks
3	After reading this chapter, you should be able to (continued): Cite the characteristics of wireless local area networks and their medium access control protocols Specify the different medium access control techniques Recognize the different IEEE 802 frame formats Describe the common local area network systems
4	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
5	Primary Function of Local Area Networks 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 Video transfers High speed LANs are capable of supporting video image and live video transfers
6	Primary Function of Local Area Networks (continued) To provide access to hardware and software resources that will allow users to perform one or more of the following activities (continued): Manufacturing support LANs can support manufacturing and industrial environments Academic support In classrooms, labs, and wireless E-mail support Interconnection between multiple systems
7	Primary Function of Local Area Networks (continued)
8	Advantages and Disadvantages of Local Area Networks Advantages 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	Advantages and Disadvantages of Local Area Networks (continued) Disadvantages 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 LAN is only as strong as its weakest link, and there are many links
10	Basic Local Area Network Topologies Local area networks are interconnected using one of four basic configurations: Bus/tree Star-wired bus Star-wired ring Wireless
11	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
12	Bus/Tree Topology (continued)
13	Bus/Tree Topology (continued)
14	Bus/Tree Topology (continued) 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
15	Bus/Tree Topology (continued)
16	Bus/Tree Topology (continued)
17	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
18	Star-Wired Bus Topology (continued)
19	Star-Wired Bus Topology (continued)
20	Star-Wired Bus Topology (continued) 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
21	Star-Wired Ring Topology Logically operates as a ring but physically appears as a star Based on MAU (Multistation 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
22	Star-Wired Ring Topology (continued)
23	Star-Wired Ring Topology (continued)
24	Wireless LANS Not really a specific topology Workstation in a wireless LAN can be anywhere as long as it is within transmitting distance to an access point Several versions of IEEE 802.11 standard define various forms of wireless LAN connections Workstations reside within Basic Service Set, while multiple basic service sets create an Extended Service Set
25	Wireless LANS (continued) Two basic components necessary: Client radio Usually a PC card with an integrated antenna installed in a laptop or workstation Access point (AP) An Ethernet port plus a transceiver AP acts as a bridge between the wired and wireless networks and can perform basic routing functions Workstations with client radio cards reside within Basic Service Set, while multiple basic service sets create an Extended Service Set
26	Wireless LANS (continued)
27	Wireless LANS (continued)
28	Wireless LANS (continued) IEEE 802.11 Original wireless standard, capable of transmitting data at 2 Mbps IEEE 802.11b Second wireless standard, capable of transmitting data at 11 Mbps In actual tests, 11 Mbps 802.11b devices managed 5.5 Mbps (from a July 2000 test by Network Computing)
29	Wireless LANS (continued) With directional antennae designed for point-to-point transmission (rare), 802.11b can transmit for more than 10 miles With an omni-directional antenna on typical AP, range may drop to as little as 100 feet
30	Wireless LANS (continued) IEEE 802.11a One of the more recent standards Capable of transmitting data at 54 Mbps (theoretical) using the 5-GHz frequency range IEEE 802.11g The other recent standard Also capable of transmitting data at 54 Mbps (theoretical) but using the same frequencies as 802.11b (2.4-GHz) Is backwards compatible with 802.11b
31	Wireless LANS (continued) HiperLAN/2 (European standard, 54 Mbps in 5-GHz band) To provide security, most systems use either: Wired Equivalent Privacy (WEP) – provides either 40- or 128-bit key protection WPA or some other more advanced standard Wireless LANs may also be configured without an access point These configurations are called “ad-hoc”
32	Wireless LANS (continued)
33	Comparison of Bus, Star-Wired Bus, Star-Wired Ring, and Wireless Topologies
34	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: Contention-based protocols Round-robin protocols
35	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, workstation can transmit If someone else is transmitting, workstation “backs off” and waits
36	Contention-Based Protocols (continued) If two workstations transmit at same time, collision occurs When two workstations hear 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 exact same time CSMA/CD is an example of a nondeterministic protocol
37	Contention-Based Protocols (continued)
38	Contention-Based Protocols (continued) CA (Collision avoidance) Protocol does not listen and detect collisions Instead, 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 interframe space (IFS) Some applications have a short IFS, while others have a long IFS If two applications want to transmit at same time, the application with shorter IFS will go first
39	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
40	Round Robin Protocols (continued)
41	IEEE 802 To better support local area networks, the data link layer of the OSI model was broken into two sublayers: Logical link control sublayer Medium access control sublayer Medium access control sublayer defines 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
42	IEEE 802 (continued)
43	IEEE 802.3 Frame Format IEEE 802 suite of protocols defines frame formats for CSMA/CD (IEEE 802.3) and token ring (IEEE 802.5) Each frame format describes how 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
44	IEEE 802.3 Frame Format (continued)
45	Wired Ethernet Most common form of LAN today Star-wired bus is most common topology but bus topology still not totally dead yet Comes in many forms depending upon medium used and transmission speed and technology
46	Wired Ethernet (continued) 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
47	Wired Ethernet (continued) 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)
48	Wired Ethernet (continued)
49	Wired Ethernet (continued) One of the latest features is 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 Power to drive Ethernet NIC is sent over wiring along with usual Ethernet signals
50	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
51	Fiber Distributed Data Interface (FDDI) Based on 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 outgoing packet, rather than waiting for outgoing packet to circle entire ring
52	Wireless Ethernet As we have already seen, IEEE has created the 802.11b, 802.11a, and 802.11g wireless standards IEEE 802.11n (100 Mbps) will be ratified soon and should start appearing in product form in 2006 (maybe?) Latest wireless Ethernet is using MIMO technology (multiple input multiple output) Sender and receiver have multiple antennas for optimum reception
53	LANs In Action: A Small Office Solution What type of system will interconnect 20 workstations in one room and 15 workstations in another room to a central server, which offers: Internal e-mail A database that contains all customer information High-quality printer access
54	LANs In Action: A Small Office Solution (continued)
55	LANs In Action: A Small Office Solution (continued)
56	LANs In Action: A Home Office 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
57	LANs In Action: A Home Office Solution (continued)
58	Summary Primary functions of a LAN are to enable sharing of data, software, and peripherals and to provide common services such as file serving, print serving, support for electronic mail, and process control and monitoring in office, academic, and manufacturing environments Local area networks have numerous advantages and disadvantages A LAN can be configured as a bus/tree topology, a star-wired bus topology, a star-wired ring topology, or a wireless network
59	Summary (continued) With both baseband and broadband buses, expansion in the form of adding a new workstation is difficult when a tap is not available For a workstation to place data onto a LAN, network must have a medium access control protocol Two basic forms of medium access control protocols are: Contention-based Round-robin CSMA/CD works on first-come, first-served basis, supports half-duplex and full-duplex connections, and is clearly the most popular access protocol
60	Summary (continued) Round-robin protocols are good under heavy loads but require more software support To standardize the medium access control protocols, IEEE created the 802 series of network standards The most popular types of LAN systems are Ethernet (CSMA/CD) and wireless Ethernet