Chapter Seven Local Area Networks The Basics Data

Chapter Seven Local Area Networks: The Basics Data Communications and Computer Networks: A Business User’s Approach Fifth Edition

Objectives • 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 layouts (topologies) of local area networks Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 2

Objectives (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 Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 3

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 1970 s, its use has become widespread in commercial and academic environments Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 4

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 Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 5

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 Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 6

Primary Function of Local Area Networks (continued) Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 7

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 Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 8

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 Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 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 Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 10

Bus/Tree Topology (continued) Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 11

Bus/Tree Topology (continued) Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 12

Bus/Tree Topology (continued) • Baseband signals are bidirectional and more outward in both directions from the workstation transmitting • Broadband signals are usually unidirectional and transmit in only one direction – Because of this, special wiring considerations are necessary • Buses can be split and joined, creating trees Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 13

Bus/Tree Topology (continued) Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 14

Bus/Tree Topology (continued) Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 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 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 Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 16

Star-Wired Bus Topology (continued) Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 17

Star-Wired Bus Topology (continued) Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 18

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 Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 19

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 Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 20

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 Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 21

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 Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 22

Contention-Based Protocols (continued) Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 23

Hubs • Hub interconnects two or more workstations into a local area network • When a workstation transmits to a hub, hub immediately resends the data frame out to all connecting links • Can be managed or unmanaged – A managed hub possesses enough processing power that it can be managed from a remote location Data Communications and Computer Networks: A Business User's Approach, Fourth Edition 24

Bridges • A bridge (or bridge-like device) can be used to connect two similar LANs, such as two CSMA/CD LANs • Can also be used to connect two closely similar LANs, such as a CSMA/CD LAN and a token ring LAN Data Communications and Computer Networks: A Business User's Approach, Fourth Edition 25

Bridges (continued) • Examines destination address in a frame and either forwards this frame onto next LAN or does not • Examines source address in a frame and places this address in a routing table, to be used for future routing decisions Data Communications and Computer Networks: A Business User's Approach, Fourth Edition 26

Bridges (continued) Data Communications and Computer Networks: A Business User's Approach, Fourth Edition 27

Transparent Bridge • A bridge observes each frame that arrives at a port, extracts the source address from the frame, and places that address in the port’s routing table • A transparent bridge is found with CSMA/CD LANs Data Communications and Computer Networks: A Business User's Approach, Fourth Edition 28

Transparent Bridge (continued) Data Communications and Computer Networks: A Business User's Approach, Fourth Edition 29

Transparent Bridge (continued) Data Communications and Computer Networks: A Business User's Approach, Fourth Edition 30

Transparent Bridge (continued) • Can also convert one frame format to another – This does not happen too often anymore since most networks are CSMA/CD – Note that some people / manufacturers call a bridge such as this a gateway or sometimes a router • Removes headers and trailers from one frame format and inserts (encapsulates) headers and trailers for the second frame format Data Communications and Computer Networks: A Business User's Approach, Fourth Edition 31

Switches • A combination of hub and bridge • Can interconnect two or more workstations, but like a bridge, it observes traffic flow and learns • When a frame arrives at a switch, switch examines destination address and forwards frame out the one necessary connection • Workstations that connect to a hub are on a shared segment • Workstations that connect to a switch are on a switched segment Data Communications and Computer Networks: A Business User's Approach, Fourth Edition 32

Switches (continued) • The backplane of a switch is fast enough to support multiple data transfers at one time • A switch that employs cut-through architecture is passing on frame before entire frame has arrived at switch • Multiple workstations connected to a switch use dedicated segments – This is a very efficient way to isolate heavy users from the network • Can allow simultaneous access to multiple servers or multiple simultaneous connections to a single server Data Communications and Computer Networks: A Business User's Approach, Fourth Edition 33

Switches (continued) Data Communications and Computer Networks: A Business User's Approach, Fourth Edition 34

Switches (continued) Data Communications and Computer Networks: A Business User's Approach, Fourth Edition 35

Virtual LANs • Virtual LAN (VLAN) – logical subgroup within a LAN that is created via switches and software rather than by manually moving wiring from one network device to another • Even though employees and their actual computer workstations may be scattered throughout the building, LAN switches and VLAN software can be used to create a “network within a network” Data Communications and Computer Networks: A Business User's Approach, Fourth Edition 36

Virtual LANs (continued) • A relatively new standard, IEEE 802. 1 Q, was designed to allow multiple devices to intercommunicate and work together to create a virtual LAN • Instead of sending technician to a wiring closet to move a workstation cable from one switch to another, an 802. 1 Q-compliant switch can be remotely configured by a network administrator Data Communications and Computer Networks: A Business User's Approach, Fourth Edition 37

Isolating Traffic Patterns and Providing Multiple Access • Whether shared or dedicated segments are involved, the primary goal of a switch is to isolate a particular pattern of traffic from other patterns of traffic or from the remainder of the network • Switches, because of their backplane, can also allow multiple paths of communications to simultaneously occur Data Communications and Computer Networks: A Business User's Approach, Fourth Edition 38

Isolating Traffic Patterns and Providing Multiple Access (continued) Data Communications and Computer Networks: A Business User's Approach, Fourth Edition 39

Isolating Traffic Patterns and Providing Multiple Access (continued) Data Communications and Computer Networks: A Business User's Approach, Fourth Edition 40

Isolating Traffic Patterns and Providing Multiple Access (continued) • Using a pair of routers, it is possible to interconnect to switched segments, essentially creating one large local area network Data Communications and Computer Networks: A Business User's Approach, Fourth Edition 41

Isolating Traffic Patterns and Providing Multiple Access (continued) Data Communications and Computer Networks: A Business User's Approach, Fourth Edition 42

Full-Duplex Switches • Allow for simultaneous transmission and reception of data to and from a workstation • This full-duplex connection helps to eliminate collisions • To support a full-duplex connection to a switch, at least two pairs of wires are necessary – One for the receive operation – One for the transmit operation – Most people install four pairs today, so wiring is not the problem Data Communications and Computer Networks: A Business User's Approach, Fourth Edition 43

Full-Duplex Switches (continued) Data Communications and Computer Networks: A Business User's Approach, Fourth Edition 44

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 Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 45

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 Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 46

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. 1 p protocol • Topology can be star or mesh (for trunks) Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 47

Wired Ethernet (continued) Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 48

Wired Ethernet (continued) • One of the latest features is power over Ethernet (Po. E) • 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 Po. E • Power to drive Ethernet NIC is sent over wiring along with usual Ethernet signals Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 49

Wireless Ethernet • 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 Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 50

Wireless Ethernet (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 Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 51

Wireless Ethernet (continued) Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 52

Wireless Ethernet (continued) Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 53

Wireless Ethernet (continued) • IEEE 802. 11 – Original wireless standard, capable of transmitting data at 2 Mbps • IEEE 802. 11 b – Second wireless standard, capable of transmitting data at 11 Mbps – In actual tests, 11 Mbps 802. 11 b devices managed 5. 5 Mbps (from a July 2000 test by Network Computing) Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 54

Wireless Ethernet (continued) • With directional antennae designed for point-topoint transmission (rare), 802. 11 b can transmit for more than 10 miles • With an omni-directional antenna on typical AP, range may drop to as little as 100 feet Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 55

Wireless Ethernet (continued) • IEEE 802. 11 a – One of the more recent standards – Capable of transmitting data at 54 Mbps (theoretical) using the 5 -GHz frequency range • IEEE 802. 11 g – The other recent standard – Also capable of transmitting data at 54 Mbps (theoretical) but using the same frequencies as 802. 11 b (2. 4 -GHz) – Is backwards compatible with 802. 11 b Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 56

Wireless Ethernet (continued) • Hiper. LAN/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” Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 57

Wireless Ethernet (continued) Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 58

Wireless Ethernet (continued) • As we have already seen, IEEE has created the 802. 11 b, 802. 11 a, and 802. 11 g wireless standards • IEEE 802. 11 n (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 Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 59

Wireless CSMA/CA • 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 Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 60

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 their MAC sublayer name, such as 10 Base. T Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 61

IEEE 802 (continued) Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 62

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 Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 63

IEEE 802. 3 Frame Format (continued) Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 64

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 Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 65

LANs In Action: A Small Office Solution (continued) Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 66

LANs In Action: A Small Office Solution (continued) Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 67

LANs In Action: A Small Office Solution (continued) Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 68

LANs In Action: A Small Office Solution (continued) Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 69

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 Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 70

LANs In Action: A Home Office Solution (continued) Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 71

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, or a wireless network Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 72

Summary (continued) • With both baseband 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 Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 73

Summary (continued) • 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 Data Communications and Computer Networks: A Business User's Approach, Fifth Edition 74