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1 Topic 7 Local Area Networks (LAN) Overview of LANs LAN Bridges LAN Standards Reference A. Leon-Garcia and I. Widjaja, Communication Networks, pp. 421-479.

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Presentation on theme: "1 Topic 7 Local Area Networks (LAN) Overview of LANs LAN Bridges LAN Standards Reference A. Leon-Garcia and I. Widjaja, Communication Networks, pp. 421-479."— Presentation transcript:

1 1 Topic 7 Local Area Networks (LAN) Overview of LANs LAN Bridges LAN Standards Reference A. Leon-Garcia and I. Widjaja, Communication Networks, pp (Reserved in the DC library. Call No. TK5105. L )

2 2 What is a LAN? LAN Topology LAN Protocol Architecture 7.1 Overview of LANs

3 3 What is a LAN? Private ownership freedom from regulatory constraints of WANs Short distance (~1km) between computers imply low cost very high-speed, relatively error-free communication complex error control unnecessary Machines are constantly moved Simply give each machine a unique address Broadcast all messages to all machines in the LAN Need a medium access control protocol

4 4 LAN Topology A number of nodes (computers and network devices) are interconnected by a shared transmission medium Nodes are connected to the cabling system through a network interface card (NIC) or LAN adapter card Nodes can be arranged in bus, ring, or star topology Bus Ring Star

5 5 LAN Protocol Architecture Data link layer CSMA-CD Token Ring Logical link control Physical layer MAC LLC Wireless LAN Network layer Physical layer OSI IEEE 802 Fiber, twisted pairs, coax, wireless Other LANs

6 6 Medium Access Control Coordinate access to medium Connectionless frame transfer service Machines identified by MAC/physical address Broadcast frames with MAC addresses

7 7 7.2 LAN Bridges

8 8 Hub Station Two Twisted Pairs Interconnecting Networks Types of devices Repeater (physical layer): Signal regeneration - extend the range Bridge (data link layer): MAC address filtering - reduce LAN saturation Routers (network layer): Internet routing Gateway (higher layers): Protocol conversion and security Hub Station Two Twisted Pairs ?

9 9 Operation at data link level Implies capability to work with multiple network layers However, need deal with Difference in MAC formats, data rates, maximum frame length Security Broadcast storm Types: transparent, source routing Bridges Bridge Network Physical Network LLC Physical LLC MAC

10 10 Interconnection of IEEE LANs with complete transparency Use table lookup, and discard frame, if source & destination in same LAN forward frame, if source & destination in different LAN use flooding, if destination unknown Use backward learning to build table observe source address of arriving LANs handle topology changes by removing old entries Prevents loops in the topology Transparent Bridges Bridge S1S2 S4 S3 S5 S6 LAN1 LAN2

11 11 B1 S1S2 B2 S3S4 S5 Port 1Port 2Port 1Port 2 LAN1LAN2LAN3 Address Port

12 12 B1 S1S2 B2 S3S4 S5 Port 1Port 2Port 1Port 2 LAN1LAN2LAN3 Address Port S11 Address Port S11 S1→S5 S1 to S5

13 13 B1 S1S2 B2 S3S4 S5 Port 1Port 2Port 1Port 2 LAN1LAN2LAN3 Address Port S11 S31 Address Port S11 S32 S3→S2 S3  S2

14 14 B1 S1S2 B2 S3S4 S5 Port 1Port 2Port 1Port 2 LAN1LAN2LAN3 S4 S3 Address Port S11 S32 S4 2 Address Port S11 S31 S4 2 S4  S3

15 15 B1 S1S2 B2 S3S4 S5 Port 1Port 2Port 1Port 2 LAN1LAN2LAN3 Address Port S11 S32 S4 2 S2 1 Address Port S11 S31 S4 2 S2  S1

16 16 Adaptive Learning In a static network, tables eventually store all addresses & learning stops In practice, stations are added & moved all the time Introduce timer (minutes) to age each entry & force it to be relearned periodically If frame arrives on port that differs from frame address & port in table, update immediately

17 17 Avoiding Loops

18 18 Spanning Tree Algorithm 1. Select a root bridge among all the bridges. root bridge = the lowest bridge ID. 2. Determine the root port for each bridge except the root bridge root port = port with the least-cost path to the root bridge 3. Select a designated bridge for each LAN designated bridge = bridge has least-cost path from the LAN to the root bridge. designated port connects the LAN and the designated bridge 4. All root ports and all designated ports are placed into a “forwarding” state. These are the only ports that are allowed to forward frames. The other ports are placed into a “blocking” state.

19 19 LAN1 LAN2 LAN3 B1 B2 B3 B4 B5 LAN4 (1) (2) (1) (2) (3)

20 20 LAN1 LAN2 LAN3 B1 B2 B3 B4 B5 LAN4 (1) (2) (1) (2) (3) Bridge 1 selected as root bridge

21 21 LAN1 LAN2 LAN3 B1 B2 B3 B4 B5 LAN4 (1) (2) (1) (2) (3) Root port selected for every bridge except root port R R R R

22 22 LAN1 LAN2 LAN3 B1 B2 B3 B4 B5 LAN4 (1) (2) (1) (2) (3) Select designated bridge for each LAN R R R R D D D D

23 23 LAN1 LAN2 LAN3 B1 B2 B3 B4 B5 LAN4 (1) (2) (1) (2) (3) All root ports & designated ports put in forwarding state R R R R D D D D

24 24 Ethernet Token Ring FDDI Wireless LANs 7.3 LAN Standards

25 25 A bit of history… 1970 ALOHAnet radio network deployed in Hawaiian islands 1973 Metcalf and Boggs invent Ethernet, random access in wired net 1979 DIX Ethernet II Standard 1985 IEEE LAN Standard (10 Mbps) 1995 Fast Ethernet (100 Mbps) 1998 Gigabit Ethernet Gigabit Ethernet Ethernet is the dominant LAN standard Metcalf’s Sketch Ethernet: IEEE 802.3

26 26 IEEE MAC Protocol CSMA/CD Minislot Time is the critical system parameter upper bound on time to detect collision upper bound on time to acquire channel upper bound on length of frame segment generated by collision quantum for retransmission scheduling max{round-trip propagation, MAC jam time} Truncated binary exponential backoff algorthm For retransmission n, select an integer r equally likely btw 0 and 2 k -1 (0 <= r <= 2 k -1), where k=min(n,10) Retransmission time = (minislots time)x(r) Give up after 16 retransmissions

27 27 IEEE Physical Layer (a) transceivers (b) 10base510base210baseT10baseFX MediumThick coaxThin coaxTwisted pairOptical fiber Max. Segment Length500 m200 m100 m2 km TopologyBus Star Point-to- point link IEEE Mbps medium alternatives Thick Coax: Stiff, hard to work with T connectors Hubs & Switches!

28 28 Ethernet Repeaters & Bridges (a) Single collision domain (b) High-Speed backplane or interconnection fabric Twisted Pair Cheap Easy to work with Reliable Star-topology CSMA-CD Twisted Pair Cheap Bridging increases scalability Separate collision domains Full duplex operation

29 29 Fast Ethernet 100baseT4100baseTX100baseFX Medium Twisted pair category 3 UTP 4 pairs Twisted pair category 5 UTP two pairs Optical fiber multimode Two strands Max. Segment Length 100 m 2 km TopologyStar IEEE Mbps Ethernet medium alternatives To preserve compatibility with 10 Mbps Ethernet: Same frame format, same interfaces, same protocols Hub topology only with twisted pair & fiber Bus topology & coaxial cable abandoned Category 3 twisted pair (ordinary telephone grade) requires 4 pairs Category 5 twisted pair requires 2 pairs (most popular) Most prevalent LAN today

30 30 Gigabit Ethernet IEEE Gbps Fast Ethernet medium alternatives 1000baseSX1000baseLX1000baseCX1000baseT Medium Optical fiber multimode Two strands Optical fiber single mode Two strands Shielded copper cable Twisted pair category 5 UTP Max. Segment Length 550 m5 km25 m100 m TopologyStar Slot time increased to 512 bytes Small frames need to be extended to 512 B Frame bursting to allow stations to transmit burst of short frames Frame structure preserved but CSMA-CD essentially abandoned Extensive deployment in backbone of enterprise data networks and in server farms

31 31 10 Gigabit Ethernet IEEE Gbps Ethernet medium alternatives 10GbaseSR10GBaseLR10GbaseEW10GbaseLX4 Medium Two optical fibers Multimode at 850 nm 64B66B code Two optical fibers Single-mode at 1310 nm 64B66B Two optical fibers Single-mode at 1550 nm SONET compatibility Two optical fibers multimode/single- mode with four wavelengths at 1310 nm band 8B10B code Max. Segment Length 300 m10 km40 km300 m – 10 km Frame structure preserved CSMA-CD protocol officially abandoned LAN PHY for local network applications WAN PHY for wide area interconnection using SONET OC-192c Extensive deployment in metro networks anticipated

32 32 Token Ring LAN: IEEE Unidirectional ring network 4 Mbps and 16 Mbps on twisted pair Token passing protocol provides access Fairness Access priorities  Breaks in ring bring entire network down Reliability can be improved by using star topology  Relatively low speed

33 33 Star Topology Ring LAN Stations connected in star fashion to wiring closet Use existing telephone wiring Ring implemented inside equipment box Relays can bypass failed links or stations

34 34 Fiber Distributed Data Interface (FDDI) Token ring protocol for LAN/MAN 100 Mbps on optical fiber Up to 200 km diameter, up to 500 stations FDDI has option to operate in multitoken mode Counter-rotating dual ring topology A E D C B X Dual ring becomes a single ring

35 35 Wireless LAN (+) Easy, low-cost deployment Mobility & roaming: Access information anywhere Supports personal devices PDAs, laptops, data-cell-phones Supports communicating devices Cameras, location devices, wireless identification (-)  Signal strength varies in space & time  Signal can be captured by snoopers  Spectrum is limited & usually regulated

36 36 B D C A Wireless LAN Communications Ad hoc mode

37 37 A2 B2 B1 A1 AP1 AP2 Distribution System Server Gateway to the Internet Portal Basic Service Set (BSS) A BSS B Infrastructure mode

38 38 IEEE Wireless LAN Stimulated by availability of unlicensed spectrum U.S. Industrial, Scientific, Medical (ISM) bands MHz, GHz, GHz Ad Hoc & Infrastructure networks Based on CSMA with Collision Avoidance (CA) Why not CSMA/CD? Cost: requires a full duplex radio Wireless media: not all stations hear each other

39 39 IEEE MAC MAC sublayer responsibilities Channel access PDU addressing, formatting, error checking Fragmentation & reassembly of MAC SDUs MAC security service options Authentication & privacy MAC management services Power management

40 40 MAC Services Contention Service: Best effort Contention-Free Service: time-bounded transfer Physical Distribution coordination function (CSMA-CA) Point coordination function Contention- free service Contention service MAC MSDUs

41 41 IEEE Physical Layer Options Frequency Band Bit RateModulation Scheme GHz1-2 MbpsFrequency-Hopping Spread Spectrum, Direct Sequence Spread Spectrum b2.4 GHz11 MbpsComplementary Code Keying & QPSK g2.4 GHz54 MbpsOrthogonal Frequency Division Multiplexing (OFDM) & CCK for backward compatibility with b a5-6 GHz54 MbpsOrthogonal Frequency Division Multiplexing n2.4GHz 5-6GHz 600+MbpsOFDM, Multiple input mulitple output (MIMO)


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