# Chapter 8 LAN Topologies

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1 Chapter 8 LAN Topologies Point-to-Point Networks In a Point-to-Point network, each wire connects exactly two computers Point To Point Link Machine A Machine B Figure 1: Each line connects two machines Advantages Univ. of Kentucky CS 471, Spring Jim Griffioen 1

2 P-t-P Networks: (continued) Disadvantages Univ. of Kentucky CS 471, Spring Jim Griffioen 2

3 Multiaccess Networks Problem: P-t-P networks have many lines => high cost Solution: IDEA: the computers must take turns we need an allocation strategy to determine who s turn it is (TDM, FDM, Statistical Multiplexing) Shared lines reduce cost But it means you only get a fraction of the line s bandwidth Univ. of Kentucky CS 471, Spring Jim Griffioen 3

4 LAN Topologies Networks are typically classified by their shape There are three popular shapes: Univ. of Kentucky CS 471, Spring Jim Griffioen 4

5 Bus Topologies Shared Bus Connector Wire Figure 2: Example Bus Topology Must solve the Channel Allocation Problem. Univ. of Kentucky CS 471, Spring Jim Griffioen 5

6 Ring Topologies Computers are connected in a closed loop/ring Packets Travel Around The Ring in Some Direction Figure 3: Example Ring Topology Univ. of Kentucky CS 471, Spring Jim Griffioen 6

7 All computers connect to Star Topologies If it is a multiaccess star, hosts must synchronize their sending. If hub does store-n-forwarding of messages, then hosts do not need to synchronize. Hub Figure 4: Example Star Topology Univ. of Kentucky CS 471, Spring Jim Griffioen 7

8 Why Multiple Topologies Each topology has advantages and disadvantages Advantages Bus: Ring: Star: Ring: Disadvantages Bus: Ring: Ring: Star: Univ. of Kentucky CS 471, Spring Jim Griffioen 8

9 Ethernet: A Bus Network is probably most popular LAN technology developed at Xerox PARC by R. Metcalf and D. Boggs 1976 DEC, Intel, and Xerox developed the DIX ethernet standard later IEEE defined the ethernet standard first versions ran at 3 Mbps current versions run at 10 Mbps and 100 Mbps is a multiaccess network technology is a broadcast-based technology is inexpensive because it uses a passive cable only provides best-effort service higher level protocols do all the real work comes in three varietys: thick net, thin net, twisted pair Univ. of Kentucky CS 471, Spring Jim Griffioen 9

10 Ethernet Basics Sending Animation Univ. of Kentucky CS 471, Spring Jim Griffioen 10

11 CSMA/CD Carrier Sense, Multiple Access with Collision Detection Ethernet is just one example of a CSMA/CD protocol The goal is to There is no THE BASIC IDEA of CSMA/CD protocols is: CSMA/CD Animation Univ. of Kentucky CS 471, Spring Jim Griffioen 11

12 Ethernet Collision Detection Note that CSMA does not prohibit two machines from sending at the same time! Consequently, their signals may collide How do you detect collisions? (Short Packet Animation) Univ. of Kentucky CS 471, Spring Jim Griffioen 12

13 Two Short Packets that will collide When Packets Collide, Garbage Results (see green line below) Red host sends: Blue host sends: Wire sees: Figure 5: Short Packets might not detect collision Univ. of Kentucky CS 471, Spring Jim Griffioen 13

14 Collision Detection: (continued) What should the minimum Length Packet be? (See min-length animation) What is the Round Trip Delay of and Ethernet? if we transmit for 51.2 usecs, we will have seen the first few bits of a colliding host before we get done transmitting thus the minimum transmission time = 51.2 usec minimum packet size = Univ. of Kentucky CS 471, Spring Jim Griffioen 14

15 Ethernet Transmission Algorithm wait till idle line, send immediately, and monitor you may only send 1500 bytes. Why? you must wait 9.6 usec before sending again if you detect a collision, continue sending a jam signal until the end of the 51.2 usec interval (the minimum packet length) to make sure everyone sees the collision. wait a certain (with slight randomness) amount of time and try again if you collide again, double the wait time, then try again, etc. called exponential backoff Univ. of Kentucky CS 471, Spring Jim Griffioen 15

16 Receiever Algorithm the ethernet card sees all packets sent over the ether the ethernet card runs in one of two modes: Univ. of Kentucky CS 471, Spring Jim Griffioen 16

17 LocalTalk LAN technology that uses bus topology Interface included with all Macintosh computers Relatively low speed (230.4 Kbps) Low cost ( free with a Macintosh); easy to install and connect Univ. of Kentucky CS 471, Spring Jim Griffioen 17

18 CSMA/CA LocalTalk uses collision avoidance rather than collision detection CA = Collision Avoidance Transmitting computer sends very short message to reserve bus If reservation succeeds, transmitter can use bus without conflict Univ. of Kentucky CS 471, Spring Jim Griffioen 18

19 Two popular examples: IBM Token Ring FDDI Example Ring Networks Univ. of Kentucky CS 471, Spring Jim Griffioen 19

20 IBM Token Ring network manages access to the shared ring via a token Univ. of Kentucky CS 471, Spring Jim Griffioen 20

21 forward packet s Token Ring Access Points Receiver Sender copy packet and forward Figure 6: (Left) Abstract Ring (Right) Ring in Operation Token Ring Animation Univ. of Kentucky CS 471, Spring Jim Griffioen 21

22 The Token The token is There is only A computer must hold the token Univ. of Kentucky CS 471, Spring Jim Griffioen 22

23 Token Passing Algorithm Univ. of Kentucky CS 471, Spring Jim Griffioen 23

24 Token Ring Failures What kinds of problems/failurs can occur? Univ. of Kentucky CS 471, Spring Jim Griffioen 24

25 FDDI (Fiber Distributed Data Interface) FDDI was a very popular ring technology Ran at 100 Mbps Still used in some places, but has been largely replaced by other technologies. Gigabit ring networks (follow-ons to FDDI) have also been developed (CSMA/RN) Univ. of Kentucky CS 471, Spring Jim Griffioen 25

26 Physical Characteristics runs over fiber an alternative version called CDDI runs over copper the attachment stations form a Dual, Counter-Rotating Ring Concentrator DAS SAS SAS Dual Attachment Station (Concentrator) 2 fibers in / 2 fibers out SAS hang off it expensive/complex To a To a Figure 7: Rings, DAS, and SAS only use the outer ring, the inner ring is idle until a failure occurs Univ. of Kentucky CS 471, Spring Jim Griffioen 26

27 Physical Characteristics: (continued) two types of attachments Dual Attachment Stations (DAS) Single Attachment Stations (SAS) the network is said to be self-healing a DAS has an optical by-pass that fixes SAS failures each DAS monitors connectivity and reverses the ring if a failure occurs X X X Figure 8: Self-Healing Capability Univ. of Kentucky CS 471, Spring Jim Griffioen 27

28 Physical Characteristics: (continued) has a bandwidth of 100 Mbps, latency depends on ring size max ring size is 100 Kilometers (really 200 Km / 2 rings) max dist between stations is 2 Km max of 500 stations each station adds a 50 ns delay because of an elasticity buffer. uses 4B/5B encoding max packet length is approx 4500 bytes Univ. of Kentucky CS 471, Spring Jim Griffioen 28

29 Token Rotation Algorithm you are allowed to send data as long as you hold the token How long can you hold the token? Univ. of Kentucky CS 471, Spring Jim Griffioen 29

30 Setting THT if (THT is small) if (THT is large) TTRT (Target Token Rotation Time) want a TTRT Target Token Rotation Time for the ring that is as big as possible to get the best efficiency, but small enough so that everyone is happy FDDI selects the minimum of all hosts desired TRT s as the ring s TTRT (vie a bidding algorithm) each machine tries to keep the token rotating at the TTRT speed Univ. of Kentucky CS 471, Spring Jim Griffioen 30

31 Setting THT: (continued) each hosts measures the last rotation time MTRT (Measured Token Rotation Time) if (MTRT < TTRT) the token is moving faster than it needs to you are allowed to hold the token for a maximum of THT = (TTRT - MTRT) if (MTRT >= TTRT) the token is behind schedule you must forfeit your turn and send the token on immediately Univ. of Kentucky CS 471, Spring Jim Griffioen 31

32 When do you send the token? Token Token Packet Packet Figure 9: When to send the Token? Univ. of Kentucky CS 471, Spring Jim Griffioen 32

33 Univ. of Kentucky CS 471, Spring Jim Griffioen 33

34 ATM (Asynchronous Transfer Mode) does not stand for Automated Teller Machine is a packet switching system spans the physical, data link, and network levels standards are still in development (CCITT and ATM forum) highly influenced by the Telephone industry a goal of ATM was to be able to send voice, data, and video over the same network (merge computer and voice networks) also need to carry control information (things needed to support new telephone services like call forwarding, call-waiting, discount dialing plans, etc) In the past this information ran over a parallel network to the voice network. Univ. of Kentucky CS 471, Spring Jim Griffioen 34

35 Goals and Assumptions 1. ATM networks will be organized as a two-level hierarchy consisting of user and network level machines/switches we need a network-network interface (NNI) and we need a user-network interface (UNI) 2. should provide connection-oriented service; set up a channel to communicate 3. should be run over fiber and have very low error rates subgoal is to provide Quality-of-Service such as guaranteed bandwidth, latency, interpacket arrival times, etc. can only do this is the communication links do not introduce errors 4. should allow low cost attachements (ATM telephone devices) standards body decided to prohibit cell reordering cells must arrive at the receiver in order across the link Univ. of Kentucky CS 471, Spring Jim Griffioen 35

36 this means end-point devices can be made with much simpler bufferring schemes in particular, they can use FIFO memory instead of RAM memory ATM telephones can just play the samples as they arrive ATM typically carried over some fiber-based data link layer like SONET uses cells (really just another term for packet) Univ. of Kentucky CS 471, Spring Jim Griffioen 36

37 What is a Cell? Recall that Packets can be any size (i.e., they adjust their size to fit the data) A Cell is just a fixed length packet (usually a small packet) Cells are always the same size (even if you want to send a single bit or byte) ATM uses 53 byte cells : 48 bytes of payload and 5 bytes of header Univ. of Kentucky CS 471, Spring Jim Griffioen 37

38 small packet caught behind big packet big packet divided into cells Time Packets can cause wasted bandwidth Figure 10: Cells have small queueing delays Univ. of Kentucky CS 471, Spring Jim Griffioen 38

39 Characteristics of ATM each switch is a star topology switches can by hooked together uses connection-oriented communication between machines destination identified by a connection id ATM ATM Figure 11: Example ATM Network Univ. of Kentucky CS 471, Spring Jim Griffioen 39

40 Pictures of Real ATM Switches Each machine uses two fiber optic lines to connect to the switch Typically multimode fiber is used running at 155 Mbps Figure 12: Fibers into back of a Sparc IPC Univ. of Kentucky CS 471, Spring Jim Griffioen 40

41 Pictures of Real ATM Switches Univ. of Kentucky CS 471, Spring Jim Griffioen 41

42 Univ. of Kentucky CS 471, Spring Jim Griffioen 42

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