IEEE 802 LANs. LAN: Local Area Network What is a local area network?

Transcription

1 IEEE 802 LANs LAN: Local Area Network What is a local area network? A LAN is a network that resides in a geographically restricted area LANs usually span a building or a campus 1

2 Characteristics of LANs Short propagation delays Small number of users Single shared medium (usually) Inexpensive 2

3 Bus-based LANs Ethernet (*) Token Bus (*) Ring-based LANs Token Ring (*) Switch-based LANs Common LANs Switched Ethernet ATM LANs (*) IEEE 802 LANs 3

4 IEEE 802 Standards 802.1: Introduction 802.2: Logical Link Control (LLC) 802.3: CSMA/CD (Ethernet) 802.4: Token Bus 802.5: Token Ring 802.6: DQDB : CSMA/CA (Wireless LAN) 4

5 IEEE 802 Standards (cont d) 802 standards define: Physical layer protocol Data link layer protocol Medium Access (MAC) Sublayer Logical Link Control (LLC) Sublayer 5

6 OSI Layers and IEEE 802 OSI layers Higher Layers IEEE 802 LAN standards Higher Layers Data Link Layer Logical Link Control Medium Access Control Physical Layer CSMA/CD Token-passing Token-passing bus bus ring 6

7 Ethernet Token Ring IEEE 802 LANs (cont d) 7

8 Ethernet (CSMA/CD) IEEE defines Ethernet Layers specified by 802.3: Ethernet Physical Layer Ethernet Medium Access (MAC) Sublayer 8

9 Ethernet (cont d) Possible Topologies: 1. Bus 2. Branching non-rooted tree for large Ethernets 9

10 Ethernet: MAC Layer Data encapsulation Frame Format Addressing Error Detection Link Management CSMA/CD Backoff Algorithm 10

11 Ethernet Frame Format Bytes: Preamble Preamble SFD SFD DA DA SA SA Type Type Data Pad Pad CRC CRC 1. Preamble: trains clock-recovery circuits 2. Start of Frame Delimiter: indicates start of frame 3. Destination Address: 48-bit globally unique address assigned by manufacturer. 1b: unicast/multicast 1b: local/global address 4. Type: Indicates protocol of encapsulated data (e.g. IP = 0x0800) 5. Pad: Zeroes used to ensure minimum frame length 6. Cyclic Redundancy Check: check sequence to detect bit errors. 11

12 Ethernet MAC Frame Address Field Destination and Source Addresses: 6 bytes each Two types of destination addresses Physical address: Unique for each user Multicast address: Group of users First bit of address determines which type of address is being used 0 = physical address 1 = multicast address 12

13 Length Field Ethernet MAC Frame Other Fields 2 bytes in length determines length of data payload Data Field: between 0 and 1500 bytes Pad: Filled when Length < 46 Frame Check Sequence Field 4 bytes Cyclic Redundancy Check (CRC-32) 13

14 Recall: CSMA/CD CSMA/CD is a carrier sense protocol. If channel is idle, transmit immediately If busy, wait until the channel becomes idle CSMA/CD can detect collections. Abort transmission immediately if there is a collision Try again later according to a backoff algorithm 14

15 CSMA/CD (cont d) Carrier sense reduces the number of collisions Collision detection reduces the impact of collisions 15

16 Ethernet: CSMA/CD and Ethernet Short end-to-end propagation delay Broadcast channel Ethernet access protocol: 1-Persistent CSMA/CD with Binary Exponential Backoff Algorithm 16

17 Ethernet Backoff Algorithm: Binary Exponential Backoff If collision, Choose one slot randomly from 2 k slots, where k is the number of collisions the frame has suffered. One contention slot length = 2 x end-to-end propagation delay This algorithm can adapt to changes in network load. 17

18 Binary Exponential Backoff (cont d) slot length = 2 x end-to-end delay = 15 µs A B t=0µs: Assume A and B collide (k A = k B = 1) A, B choose randomly from 2 1 slots: [0,1] Assume A chooses 1, B chooses 1 t=30µs: A and B collide (k A = k B = 2) A, B choose randomly from 2 2 slots: [0,3] Assume A chooses 2, B chooses 0 t=45µs: B transmits successfully t=75µs: A transmits successfully 18

19 Binary Exponential Backoff (cont d) In Ethernet, Binary exponential backoff will allow a maximum of 15 retransmission attempts If 16 backoffs occur, the transmission of the frame is considered a failure. 19

20 Ethernet Performance 20

21 Ethernet Features and Advantages 1. Passive interface: No active element 2. Broadcast: All users can listen 3. Distributed control: Each user makes own decision Simple Reliable Easy to reconfigure 21

22 Ethernet Disadvantages Lack of priority levels Cannot perform real-time communication Security issues 22

23 Ethernet Switching Recent development: Connect many Ethernet segments or subnets through an Ethernet switch to segment 4 to segment 1 to segment 3 to segment 2 23

24 Why Ethernet switching? LANs may grow very large The switch has a very fast backplane It can forward frames very quickly from one segment to another Cheaper than upgrading all host interfaces to use a faster network 24

25 Token Ring IEEE Standard Layers specified by 802.5: Token Ring Physical Layer Token Ring MAC Sublayer 25

26 Token Ring (cont d) Token Ring, unlike Ethernet, requires an active interface Ring interface Host 26

27 Token Ring Configuration 27

28 Token Ring Configuration 28

29 Token Ring MAC Sublayer Token passing protocol Frame format Token format 29

30 Token Passing Protocol A token (8 bit pattern) circulates around the ring Token state: Busy: Idle:

31 Token Passing Protocol (cont d) General Procedure: Sending host waits for and captures an idle token Sending host changes the token to a frame and circulates it Receiving host accepts the frame and continues to circulate it Sending host receives its frame, removes it from the ring, and generates an idle token which it then circulates on the ring 31

32 Token Ring Frame and Token Formats Bytes SD AC ED Token Format /6 2/6 unlimited SD AC FC Destination Address Frame Format Source Address Data Checksum ED FS 32

33 Token Ring Delimiters SD AC ED SD AC FC Destination Address Source Address Data Checksum ED FS SD = Starting Delimiter ED = Ending Delimiter They contains invalid differential Manchester codes 33

34 Token Ring Access Control Field SD AC ED (Note: The AC field is also used in frames) P = Priority bits provides up to 8 levels of priority when accessing the ring T = Token bit T=0: Token T=1: Frame P P P T M R R R 34

35 Token Ring Access Control Field (cont d) SD AC ED M = Monitor Bit P P P T M R R R Prevents tokens and frames from circulating indefinitely All frames and tokens are issued with M=0 On passing through the monitor station, M is set to 1 All other stations repeat this bit as set A token or frame that reaches the monitor station with M=1 is considered invalid and is purged 35

36 Token Ring Access Control Fields (cont d) SD AC ED P P P T M R R R R = Reservation Bits Allows stations with high priority data to request (in frames and tokens as they are repeated) that the next token be issued at the requested priority 36

37 Token Ring Frame Control Field SD AC FC Destination Address Source Address Data Checksum ED FS FC = Frame Control Field Defines the type of frame being sent Frames may be either data frames or some type of control frame. Example control frames: Beacon: Used to locate breaks in the ring Duplicate address test: Used to test if two stations have the same address 37

38 Token Ring Address & Data Fields Address Fields: Indicate the source and destination hosts Broadcast: Data SD AC FC Destination Address Set all destination address bits to 1s. Source Address Data Checksum ED FS No fixed limit on length Caveat: Hosts may only hold the token for a limited amount of time (10 msec) 38

39 Token Ring Checksum and Frame Status SD AC FC Destination Address Checksum: 32-bit CRC FS = Frame Status Source Address Data Checksum ED FS Contains two bits, A and C When the message arrives at the destination, it sets A=1 When the destination copies the data in the message, it sets C=1 39

40 The Token Ring Monitor Station One station on the ring is designated as the monitor station The monitor station: marks the M bit in frames and tokens removes marked frames and tokens from the ring watches for missing tokens and generates new ones after a timeout period 40

41 Using Priority in Token Ring If a host wants to send data of priority n, it may only grab a token with priority value n or lower. A host may reserve a token of priority n by marking setting the reservation bits in the AC field of a passing token or frame Caveat: The host may not make the reservation if the token or frame s AC field already indicates a higher priority reservation The next token generated will have a priority equal to the reserved priority 41

42 When a new token is generated (i.e., when a sender finishes sending and releases an idle token), or when a sender sends a data frame, RRR is set to the lowest priority. 42

43 Priority Transmission: Example A B C D Host B has 1 frame of priority 3 to send to A Host C has 1 frame of priority 2 to send to A Host D has 1 frame of priority 4 to send to A Token starts at host A with priority 0 and circulates clockwise Host C is the monitor station (priority 0: lowest priority in this example) 43

44 Example (cont d) Event Token/Frame AC Field A generates a token P=0, M=0, T=0, R=0 B grabs the token and sets the message destination to A P=3, M=0, T=1, R=0 Frame arrives at C, and C reserves priority level 2. Monitor bit set. P=3, M=1, T=1, R=2 Frame arrives at D, and D attempts to reserve priority level 4: P=3, M=1, T=1, R=4 Frame arrives at A, and A copies it P=3, M=1, T=1, R=4 Frame returns to B, so B removes it, and generates a new token P=4, M=0, T=0, R=0 Token arrives at C, but its priority is too high. C reserves priority 2. M bit. P=4, M=1, T=0, R=2 44

45 Example (cont d) Event Token/Frame AC Field Token arrives at D, and D grabs it, sending a message to A P=4, M=0, T=1, R=0 Frame arrives at A, and A copies it P=4, M=0, T=1, R=0 Frame arrives at B, which does nothing to it P=4, M=0, T=1, R=0 Frame arrives at C, which sets the monitor bit P=4, M=1, T=1, R=2 Frame returns to D, so D removes it and generates a new token with P=2 P=2, M=0, T=0, R=0 etc Attempt to complete this scenario on your own. 45

46 TOKEN RING Performance Ring Topology A bit pattern token ( ) floats on the ring Station captures token, converts to connector ( ), transmits frame Intermediate stations relay message/token. Token is released when (a) Leading edge of frame is received, and (b) Frame is transmitted. 46

47 Throughput : Simple Analysis Time required by a bit to traverse the whole ring a = Frame transmission time Number of active stations : N Average time to pass token to the next station: a/n 47

48 t 0 t 0 +a t 0 +1 t 0 +a+1 (a) (b) (c) (d) Fig for case 1 48

49 Case 1 ( a<1) (a) Frame transmission begins (b) Leading edge received. Total frame is transmitted and token is released. (d) Total frame is received. Average time to transmit frame S = (Time elapsed between a token is transmitted + Average Token Passing Time) S = 1+ 1 a N 49

50 t 0 (a) t 0 +1 t 0 +a (b) (c) Fig for case 2 ( a > 1) t 0 +a+1 (d) 50

51 Case 2 (a > 1) (a) Frame transmission begins (b) Frame transmission completed. Leading edge received and token is released. (d) Total frame is received. 1 S = a for a > 1 a + N 51

52 Delay and stability No. of stations (equally spaced) = N Mean time for token to travel round the ring = R Mean Token Cycle Time = T Mean Packet Transmission Time = During T All N queues are served. Mean number of packets are transmitted = Q Token rotates (with mean value R) X T= R + Q X 52

53 For stable system Departures = Arrivals Q = N λ T ρ = λ (for one station) X ( 1 Nρ) R T = - Token is free with probability (1-Nρ) - Token is in use with probability Nρ Nρ<1 ρ<1/n 53

54 Average number of packets transmitted from a queue in T = Q / N In limited service (IEEE has THT) λt < m m packets served per token visit Tagged job methodology and residual service time analysis gives Average waiting delay (excluding service delay),w as W = 2 + NρΕ [ x ] 2 ( 1 Nρ λr) + ( 1+ ρ ) R ( Nρ λr)

55 Flavor #1: Release After Reception (RAR) Computer captures token, transmits data, waits for data to successfully travel around ring, then releases token again. Allows computer to detect errored frames and retransmit them. Example time evolution in which host 1 and host 3 have packets to transmit: Token arrives at host 1 TRANSP Data PROP TRANST Token l 1 /c l 2 /c l N /c l 1 /c TRANST Token l 2 /c Token departs Token arrives from host 1 at host 3 Token arrives at host 2 TRANSP Data l 3 /c time 55

56 Efficiency of RAR Recall: Efficiency, η, is the fraction of time spent sending useful data Define: T i,j to be the time from when the token arrives at host i until it next arrives at host j. T 1,2 T TRANSP + PROP + TRANST + l 1,1 / c N( TRANSP + PROP + TRANST ) + = N( TRANSP + PROP + TRANST ) + PROP 1 l i i / c η RAR 1, 1+ a N( TRANSP) N( TRANSP + PROP + TRANST ) + PROP a = PROP TRANSP, TRANSP >> TRANST 56

57 Flavor #2: Release After Transmission (RAT) Computer captures token, transmits data, then releases token again. Example time evolution in which host 1 and host 3 have packets to transmit: Token arrives at host 1 TRANSP Data TRANST Token l 1 /c TRANST Token l 2 /c Token departs Token arrives from host 1 at host 3 Token arrives at host 2 TRANSP Data Token time 57

58 58 Efficiency of RAT TRANST TRANSP TRANSP PROP a N a PROP TRANST TRANSP N TRANSP N PROP TRANST TRANSP N c l TRANST TRANSP N T c l TRANST TRANSP T RAT i i >> = = ,, / 1 1 ) ( ) ( ) ( / ) ( / 1,1 1 1,2 η