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1 ICS 153 Introduction to Computer Networks Inst: Chris Davison

2 ICS 153 Introduction to Computer Networks MAC Sublayer Contents Fixed Assignment Protocols Demand Assignment Protocols Contention Access Protocols IEEE 802 LANs DQDB and FDDI Fast Ethernet and Gigabit Ethernet

3 ICS 153 Introduction to Computer Networks Chapter 4 Homework: 2, 3, 9, 15, 16, 17, 18, 24, 25, 42

4 ICS 153 Medium Access Sublayer Bottom part of the Data Link Layer Not important on point-to-point links MAC sublayer is only important on broadcast or shared channel networks Satellites Ethernet Cellular Wireless

5 ICS 153 Medium Access Sublayer Definitions Contention multiple users share a common channel in a way that can lead to conflicts ICS Network Collision two transmissions trying to occupy the channel simultaneously results in both being garbled

6 Fixed Assignment Protocols Static and predetermined allocation of channel access: independent of user activity Idle users may be assigned to the channel, in which case channel capacity is wasted Examples: TDM, FDM WDM

7 Fixed Assignment Advantages: simple Protocols efficient with a small number of highly active hosts Disadvantages: does not scale well to large numbers of burst transmissions

8 Demand Assignment Protocols Allocate channel capacity to hosts on a demand basis Only to active users Requires methods for measuring the demand for the channel Polling Reservation schemes

9 Demand Assignment Advantages: Protocols scales well with large numbers of highly active hosts Collision free (resource reservation) Disadvantages injects a contention period for bandwidth reservation does not scale well for burst traffic

10 Polling A central controller interrogates each host and allocates channel capacity to those who need it Good for systems with: Short propagation delay Small polling messages Non-bursty traffic

11 Reservation Schemes Hosts independently reserve the channel for a period of time Reservations are usually piggybacked on data messages passing along the channel Good for systems with: short propagation delay no central controller node non-bursty traffic

12 Reservation Schemes Reservation Scheme examples: Bit-map Protocol Binary Countdown Protocol

13 Bit-Map Protocol Contention and data transmission periods alternate The contention period is divided into slots, with 1 bitwide slot for each host on the network If a host wants to transmit a packet, it sets its contention slot equal to 1. Otherwise it sets it to zero. The slots pass all hosts in sequence so every host is aware of who will transmit

14 Bit-Map Protocol Contention Data Frame

15 Bit-Map Protocol What if there are a large number of hosts in the network? The contention period must grow to include them all With a large number of hosts, the contention period may be very long, leading to inefficiency

16 Binary Countdown Protocol Has a contention period and a broadcast period During the contention period each host broadcasts its binary address one bit at a time, starting with the most significant bit bits transmitted simultaneously are Boolean OR d Arbitration Rule: If a hosts sends a 0 bit but the OR results in a 1, the host gives up The last host remaining wins the

17 Binary Countdown: Fairness Stations with the highest addresses will always win This is good if you wish to implement priority, but bad if you want to give all hosts fair access to the channel

18 Binary Countdown: Station Address Permutation After host A successfully transmits, all hosts with addresses less than host A add one to their address, improving their priority. Host A changes its address to zero, giving it the lowest priority

19 Contention Access Protocols Single channel shared by a large number of hosts No coordination between hosts Control is completely distributed Examples: ALOHA, CSMA, CSMA/CD

20 Contention Access Advantages Short delay for bursty traffic Simple (due to distributed control) Flexible to fluctuations in the number of hosts Fairness

21 Contention Access Disadvantages Low channel efficiency with a large number of hosts Not good for continuous traffic (voice) Cannot support priority traffic High variance in transmission delays

22 Contention Access Pure ALOHA Methods Slotted ALOHA CSMA Carrier Sense Multiple Access 1-Persistent CSMA Non-Persistent CSMA P-Persistent CSMA CSMA/CD Carrier Sense Multiple Access with Collision Detection

23 Pure ALOHA Originally developed for ground-based packet radio communications in 1970 Abramson, University of Hawaii Channel Allocation: Let users transmit whenever they have something to send

24 Pure ALOHA Algorithm 1. Transmit whenever you have data to send 2. Listen to the broadcast Because broadcast is fed back, the sending host can always find out if its packet was destroyed just by listening to the downward broadcast one round-trip time after sending the packet 3. If the packet was destroyed, wait a random amount of time and send it again. The waiting time must be random to prevent the same packets from colliding over and over again.

25 Pure ALOHA Due to collisions and idle periods, pure ALOHA is limited to approximately 18% throughput in the best case. Can we improve this?

26 Slotted ALOHA Slotted ALOHA cuts the vulnerable period for packets in half. This doubles the best possible throughput from 18.4% to 36.8% How? Time is slotted. Packets must be transmitted within a slot

27 Slotted ALOHA Algorithm 1. If a host has a packet to transmit, it waits until the beginning of the next slot before sending. 2. Listen to the broadcast and check if the packet was destroyed. 3. If there was a collision, wait a random number of slots and try to send again.

28 CSMA We could achieve better throughput if we could listen to the channel before transmitting a packet This way, we would stop avoidable collisions This is Carrier Sense Multiple Access protocols

29 Assumptions with CSMA Networks Constant length packets No errors, except those caused by collisions Each host can sense the transmission of all other hosts Propagation delay is small compared to transmission time

30 CSMA There are several types of CSMA 1-Persistent CSMA Non-Persistent CSMA P-Persistent CSMA

31 1-Persistent CSMA Sense the channel If busy, keep listening to the channel and transmit immediately when the channel becomes idle. If idle, transmit immediately. If collision occurs Wait a random amount of time and start over.

32 1-Persistent CSMA The protocol is called 1- persistent because the host transmits with a probability of 1 whenever it finds the channel idle.

33 Performance CSMA > Slotted ALOHA > Pure ALOHA

34 Performance

35 1-Persistent CSMA with Satellite Systems Satellite system: long propagation delay (270 msec) Carrier sense makes no sense It takes 270 msecs to sense the channel, which is a long time

36 1-Persistent CSMA Even if propagation delay is zero, there will be collisions Example If stations B and C become ready in the middle of A s transmission, B and C will wait until the end of A s transmission and then both will begin transmitting simultaneously. Result: collision If B and C were not so greedy there would be fewer collisions.

37 Non-Persistent CSMA Sense the channel If busy, wait a random amount of time and sense the channel again If idle, transmit a packet immediately If collision occurs Wait a random amount of time and start all over again

38 Tradeoff between 1- and Non-Persistent CSMA If B and C become ready in the middle of A s transmission, 1-Persistent: B and C collide Non-Persistent: B and C do not collide If only B becomes ready in the middle of A s transmission, 1-Persistent: B succeeds as soon as A ends. Non-Persistent: B may have to wait

39 P-Persistent CSMA Optimal strategy: use P- Persistent CSMA Assume channels are slotted One slot = contention period (one round-trip propagation delay)

40 P-Persistent CSMA Sense the channel. If busy, wait until the next slot. If idle transmit (with probability P) or defer (with probability 1- P) until next slot. If a collision occurs wait a random amount of time and start over again.

41 CSMA/CD In CSMA Protocols If two stations begin transmitting at the same time, each will transmit its complete packet, thus wasting the channel for an entire packet time. In CSMA/CD protocols The transmission is terminated immediately upon the detection of a collision CD = Collision Detection

42 CSMA/CD Sense the channel If idle, transmit immediately If busy, wait until the channel becomes idle Collision detection Abort a transmission immediately if a collision is detected Try again later after waiting a random amount of time

43 CSMA/CD Carrier sense reduces the number of collisions Collision detection reduces the effect of collisions, making the channel ready to use sooner

44 Collision Detection Time How long does it take to realize there has been a collision? Worst case : 2 X end-to-end propagation delay

45 CSMA/CA Wireless LANs Utilizes carrier sensing and collision avoidance Problems: Hidden Station Exposed Station

46 CSMA/CA Wireless LANs Distributed coordination function (DCF) The fundamental access method of the IEEE MAC is a DCF known as carrier sense multiple access with collision avoidance (CSMA/CA). The DCF shall be implemented in all STAs, for use within both IBSS and infrastructure network configurations. For a STA to transmit, it shall sense the medium to determine if another STA is transmitting. If the medium is not determined to be busy (see 9.2.1), the transmission may proceed. The CSMA/CA distributed algorithm mandates that a gap of a minimum specified duration exist between contiguous frame sequences. A transmitting STA shall ensure that the medium is idle for this required duration before attempting to transmit. If the medium is determined to be busy, the STA shall defer until the end of the current transmission. After deferral, or prior to attempting to transmit again immediately after a successful transmission, the STA shall select a random backoff interval and shall decrement the backoff interval counter while the medium is idle. A refinement of the method may be used under various circumstances to further minimize collisions here the transmitting and receiving STA exchange short control frames [request to send (RTS) and clear to send(cts) frames] after determining that the medium is idle and after any deferrals or backoffs, prior to data transmission. The details of CSMA/CA, deferrals, and backoffs are described in 9.2. RTS/CTS exchanges are also presented in 9.2.

47 Channel Allocation Methods

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

49 Characteristics of LANS Short propagation delays Small number of users Single shared medium (usually) Inexpensive

50 Common LANs Bus-based LANs Ethernet Token Bus Ring-based LANs Token Ring Switched-based LANs Switched Ethernet ATM LANs Wireless LANs uses RF transmission

51 IEEE 802 Standards Introduction Logical Link Control (LLC) CSMA/CD (Ethernet) Token Bus Token Ring DQDB ISDN Security Wireless LAN Cable TV

52 IEEE 802 Standards 802 Standards define: Physical layer protocol Data link layer protocol LLC sublayer (802.2) MAC sublayer

53 IEEE 802 LANs CSMA/CD (Ethernet) (802.3) Token Bus (802.4) Token Ring (802.5) Wireless LAN (802.11)

54 IEEE Ethernet Layers specified by Ethernet Physical Layer Ethernet Medium Access (MAC sublayer)

55 IEEE Ethernet Possible Topologies Bus Branching non-rooted tree for large Ethernet networks

56 Ethernet Physical Layer Transceiver Transceiver Cable twisted 4-pair 15 Pin connector Channel Logic Manchester Phase Encoding 64-bit preamble for synchronization

57 Ethernet Physical Configuration Thick Co-Ax (10Base5) Segments of 500 meters maximum Maximum of 100 nodes per segment Throughput is 10Mbs

58 Ethernet Physical Configuration Thin Co-Ax (10Base2) Segments of up to 200 meters maximum Maximum of 30 nodes per segment Throughput is 10Mbs

59 Ethernet Physical Configuration Twisted Pair (10/100/1000Base-T) Segments of up to 100 meters maximum Maximum of 1024 nodes per segment Throughput is 1000Mbps IEEE 802.3ae is in draft standard for 10000Mbps

60 Ethernet Physical Configuration Fiber Optics (10Base-F) Segments of up to 2000 meters maximum Maximum of 1024 nodes per segment Throughput of 1000Mbps

61 Manchester Encoding Each bit period is divided into two equal intervals. Binary 1 = High(+.85 volts) then Low(-.85 volts) Binary 0 = Low(-.85 volts) then High(+.85 volts)

62 Differential Manchester Encoding Each bit period is divided into two equal intervals. Binary 1 = Lack of voltage transition from pervious bit Binary 0 = Voltage transition from previous bit

63 Manchester Encoding vs. Differential Manchester Encoding

64 Ethernet Synchronization 56-bit frame preamble used to synchronize reception 7 bytes of Manchester encoded, the preamble appears appears like a sine wave Followed by a start of frame delimiter

65 Ethernet MAC Layer Data Encapsulation Frame Format Addressing Error Detection Link management CSMA/CD Backoff Algorithm

66 802.3 Ethernet Frame Format Must be >= 64 bytes Preamble 7 bytes containing Start of frame delimiter Destination Address 6 bytes types of destination addresses Physical address: unique for each user Multicast address: group of users First bit of address determines which type of address 0 = physical address 1 = multicast address Broadcast address = all 1s

67 802.3 Ethernet Frame Format Source Address 6 bytes Length of data field 2 bytes Data field bytes Frame Check Sequence Field 4 bytes CRC - 32

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

69 Ethernet CSMA/CD Carrier sense reduces the number of collisions Collision detection reduces the impact of collisions

70 Ethernet CSMA/CD Ethernet Short end-to-end propagation delay broadcast channel Ethernet access protocol 1-Persistent CSMA/CD with Binary Exponential Backoff Algorithm

71 Ethernet Backoff Algorithm Binary Exponential Backoff If collision: Divide wait time into slots of length = worst case propagation delay After 1 collision wait 0-1 slots and try again After n collisions where n < 10 wait 0-2^n -1 slots and try again After n collisions where n > 9 and N < 16 wiat slots and try again After 16 collisions give up

72 Ethernet Features and Inexpensive Broadcast Advantages All users can listen Distributed Control Each users makes own decision Simple Reliable Relatively easy to configure and maintain

73 Ethernet Disadvantages Lack of priority levels Security issues

74 Ethernet Switching Recent Development Connect many Ethernet segments through a switch

75 Why Ethernet Switching? LANs may grow very large Switch has a very fast backplane Can forward frames very quickly from one segment to another Cheaper than upgrading all host interfaces to use a faster network Cuts collision domains down Virtual LANs (V-LAN)

76 Token Ring IEEE Layers specified by Token Ring Physical Layer Token Ring MAC Layer Sublayer

77 Token Ring Token Ring, unlike Ethernet, requires an active interface Each bit arrives at the Ring Interface and is copied out onto the ring again Induces a 1-bit delay Appears as a collection of pointto-point links that form a circle

78 Token Ring Physical Ring Interface Listen Mode Layer Transmit Mode Circulating token is seized Channel Logic Differential Manchester Encoding 0 is indicated by a transition at the start of the bit interval. 1 is indicated by the absence of a transition at the start of the bit interval.

79 Token Ring MAC Sublayer Token Passing protocol Frame Format Token Format

80 Token Passing Protocol A 3 byte token circulates around the ring Tokens have Reservation and Priority fields Stations can set priority and reserve tokens for transmitting their frames.

81 Token Passing Protocol 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

82 Token Ring Delimiters SD AC ED Token SD AC FC Destination Address Source Address DATA Checksum ED FS Frame SD = Starting Delimiter ED= Ending Delimiter

83 Token Ring Access Control Field SD AC ED P = Priority Bits Provides up to 8 levels of priority when accessing the ring T = Token Bit PPPTMRRR T=0: Token T=1: Frame

84 Token Ring Access Control Fields SD AC ED PPPTMRRR M = Monitor Bit Prevents tokens and frames from circulating indefinitely All frames and tokens are issued with M=0 On passing the monitor station M is set to 1 All other stations repeat the bit as set A token or frame that reaches the monitor station with M=1 is considered invalid and purged

85 Token Ring Access Control Fields SD AC ED PPPTMRRR R = Reservation Bits Allows stations with high priority to request (in frames and tokens as they are repeated) that the next token be issued at the requested priority.

86 Token Ring Frame Control Field SD AC FC Destination Address Source Address DATA Checksum ED FS FC = Frame Control 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

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

88 Token Ring Checksum and Frame Status SD AC FC Destination Address Source Address DATA Checksum ED FS Checksum: 32-bit CRC FS = Frame Status 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

89 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

90 Using Priority in Token Ring If a host wants to send data of priority N, it may only grab a token with a priority value of N or lower. A host may reserve a token of priority N by marking 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

91 High Speed Networks FDDI DQDB Fast Ethernet

92 FDDI FDDI = Fiber Distributed Data Interface 100 Mbs data rate Distances of up to 200 km Up to 1000 hosts attached Based on fiber optic cabling

93 FDDI Dual Ring Topology Two fiber rings: Fiber channels are unidirectional so 2 are needed One transmits clockwise, the other counterclockwise FDDI Fault Tolerance Two rings can be merged into one after a failure

94 FDDI Physical Layer Coding 4B5B (4 out of 5) coding is used Each group of 4, 1-bit data messages requires 5 bits to send Some of the unused 5-bit codes are used for controlling the fiber rings or for synchronization

95 FDDI Token Passing Protocol In Token Ring, a new token is not generated until the frame is received again at the transmitted host In FDDI, multiple tokens may be on the ring simultaneously After a FDDI host transmits a frame, it may put another token on the ring Timers are used to implement token holding time, token rotation (loss), valid transmission (timeout)

96 DQDB DQDB = Distributed Queue Dual Bus DQDB is a MAN DQBD is covered by IEEE 802.6

97 DQDB MAC Sublayer Head-ends generate cells in both directions To transmit, a host must know whether the destination is to its left or right If left, the host must send on one bus If right, the host sends on the other bus A Distributed Queue is used to make sure that cells are transmitted on a FIFO basis. Stations queue up in the order they became ready to send and transmit in that order

98 Fast Ethernet IEEE 802.3u 100 Mbps Ethernet MAC sublayer for Fast Ethernet is the same as normal Ethernet Physical layer is slightly different: no more Manchester encoding Cat-3 uses 8B6T (half-duplex) Cat-5 uses 4B5B (half or full duplex)

99 Gigabit Ethernet IEEE 802.3z Utilizes fiber optic and twistedpair cables Support full-duplex and halfduplex operation Cat-5 8B/10B encoding

100 Gigabit Ethernet

101 1000Base-T Objectives: Provide line transmission which supports full and half duplex Support operation over 100 meters of Cat-5 UTP Support Auto-Negotiation

102 10 Gigabit 802.3ae IEEE 802.3ae Ratified March 2002 Supports a speed of 10 Gbps at the service interface between the MAC/ and the physical layer signaling (PLS) sub-layer A PHY operating at a data rate of 10 Gbps A PHY operating at a data rate compatible with the payload rate of OC-192c/SDH VC-4-64c Provide physical layer specifications which support line distances of: At least 100 meters over installed multimode fiber At least 2 km over single mode fiber

103 Comparison 1 Gigabit IEEE 802.3z CSMA/CD + Full Duplex Carrier Extension Optical/Copper Media Leverage Fibre Channel PMD's Reuse 8B/10B Coding Support LAN to 5 km 10 Gigabit IEEE 802.3ae Full Duplex Only Throttle MAC Speed Optical Media Only Create New Optical PMD's From Scratch New Coding Schemes Support LAN to 40 km; Provide Direct Attach to SONET/SDH Gear

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