J.GODWIN PONSAM & S.CHRISTOBEL DIANA ASST.PROFESSOR SRM University, Kattankulathur

Transcription

1 8/22/2011 School of Computing, Department of IT IT 0305 COMPUTER NETWORKS FIFTH SEMESTER UNIT III J.GODWIN PONSAM & S.CHRISTOBEL DIANA ASST.PROFESSOR SRM University, Kattankulathur 1

2 Unit iii School of Computing, Department of IT 8/22/2011 STATIC CHANNEL ALLOCATION DYNAMIC CHANNEL ALLOCATION CSMA IEEE IEEE IEEE IEEE BRIDGES 2

3 Static Channel Allocation Problem FDM Allocates a single channel among multiple competing users If there are N users then the bandwidth is divided into N equal sized portions No interference Problem Bursty traffic TDM Each user is statistically allocated every Nth time slot.

4 Dynamic Channel Allocation Model consists of n independent stations. 2. A single channel is available for communications. 3. Collision Assumption :: If two frames are transmitted simultaneously, they overlap in time and the resulting signal is garbled. This event is a collision. 4a. Continuous Time Assumption :: frame transmissions can begin at any time instant. 4b. Slotted Time Assumption :: time is divided into discrete intervals (slots). Frame transmissions always begin at the start of a time slot.

5 5a. Carrier Sense Assumption :: Stations can tell if the channel is busy (in use) before trying to use it. If the channel is busy, no station will attempt to use the channel until it is idle. 5b. No Carrier Sense Assumption :: Stations are unable to sense channel before attempting to send a frame. They just go ahead and transmit a frame.

6 ALOHA PURE ALOHA Users transmits whenever they have data to be sent Whenever 2 frames try to occupy the channel at the same time there will be collision A sender can always find out the whether or not its frame was destroyed by listening to the channel

7 ALOHA SLOTTED ALOHA uses discrete time intervals as slots A computer is not permitted to send whenever it has data to send instead it is required to wait for the beginning of the next slot

8 CARRIER SENSE MULTIPLE ACCESS 1 persistent CSMAis selfish Sense the channel. IF the channel is idle, THEN transmit. IF the channel is busy, THEN continue to listen until channel is idle. Now transmit immediately.

9 CARRIER SENSE MULTIPLE ACCESS nonpersistent CSMA is less greedy Sense the channel. IF the channel is idle, THEN transmit. If the channel is busy, THEN wait a random amount of time and start over.

10 CARRIER SENSE MULTIPLE ACCESS p persistent CSMA is a slotted approximation Sense the channel. IF the channel is idle, THEN with probability p transmit and with probability (1 p) delay for one time slot and start over. IF the channel is busy, THENdelayone timeslot and start over.

11 CSMA/CD the time slot is usually set to the maximum propagation delay. as p decreases, stations wait longer to transmit but the number of collisions decreases Considerations for the choice of p: (n x p) must be <1for stability, where n is maximum number of stations In all three cases a collision is possible.

12 CSMA/CD If a collision is detected during transmission, THEN immediately cease transmitting the frame. The first station to detect a collision sends a jam signal to all stations to indicate that there has been a collision. After receiving a jam signal, a station that was attempting to transmit waits a random amount of time before attempting to retransmit. The maximum time needed to detect a collision = 2 x propagation delay.

13 Collision Free Protocols Bit Map Protocol Binary Countdown Protocol

14 Collision Free Protocols Contention and data transmission periods alternate The contention period is divided into slots, with 1 bit wide slots for each host in the network. If a host wants to transmit a packet, it sets its contention slot equal to 1. Otherwise, it sets it to 0. The slots pass all hosts in sequence, so every host is aware of who will transmit

15 Collision Free Protocols But what if there are a large number of hosts in the network? The contention period will have to grow to include them all With a large number of hosts, the contention period may be very long, leading to inefficiency

16 Collision Free Protocols Binary Countdown Protocol To overcome the problem of Bitmap protocol During 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 together Arbitration rule: If a host sent a zero bit but the boolean OR results in a one bit, the host gives up and stops sending address bits Whichever host remains after the entire address has been broadcast gets access to the medium

17 Collision Free Protocols Host Addresses Bit Time

18 IEEE ETHERNET CABLING ENCODING MAC SUBLAYER PROTOCOL OPERATION

19 IEEE ETHERNET The most common kinds of Ethernet cabling.

20 IEEE ETHERNET 10Base5 or Thick Ethernet. Connections onto the cable are done using some vampire taps. 10 represents the speed Base represents the transmission method (base or broadband) 5 represents the maximum length of a single, non amplified sector (500 meters or 200 meters). Transeiver,Transeiver cable, Interface board

21 IEEE ETHERNET CABLING Three kinds of Ethernet cabling. (a) 10Base5, (b) 10Base2, (c) 10Base-T.

22 IEEE ETHERNET CABLING In 10Base5 the transceiver and the interface board are separated. The interface board goes inside the computer, while the transceiver goes on the cable (close to the vampire tap). The transceiver contains the electronics that handle the collision detection. In 10Base2, the connection to the cable is just a passive BNC T junction. The transceiver electronics are situated onto the controller board (inside the PC). In 10BaseT there is no shared cable at all, just the hub. Each station is connected to the hub, using a dedicated, non shared cable.

23 IEEE ETHERNET CABLING 10: 10Mbps; 2: under 200 meters max cable length between two nodes without repeater repeaters used to connect up to multiple segments Time domain Reflectometry

24 IEEE ETHERNET CABLING T stands for Twisted Pair 10/100 Mbps rate; Hub to which nodes are connected by twisted pair, thus star topology

25 IEEE ETHERNET CABLING Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.

26 IEEE ETHERNET ENCODING (a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding.

27 MAC SUBLAYER PROTOCOL Preamble SOFD (7 bytes) (1 byte) Dest. Address (2/6 bytes) Source Address (2/6 bytes Length (2 bytes) Data ( bytes) Pad (0-46) Frame Checksum (4 bytes) IEEE Frame Format

28 MAC SUBLAYER PROTOCOL Preamble :Eachframestartswithapreambleof 7 bytes, each byte containing the bit pattern Manchester encoding is employed here and this enables the receiver's clock to synchronize with the sender's and initialise itself. Start of Frame Delimiter :This field containing a byte sequence denotes the start of the frame itself.

29 MAC SUBLAYER PROTOCOL Source and Dest. Address :The standard allows 2 byte and 6 byte addresses. Note that the 2 byte addresses are always local addresses while the 6 byte ones can be Local or global. 2 Byte Address Manually assigned address Individual(0)/Group(1) (1 bit) Address of the machine (15 bits) 6 Byte Address address Every Ethernet card with globally unique Individual(0)/Group(1) (1 bit) Universal(0)/Local(1) Address of the machine (1 bit) (46 bits)

30 MAC SUBLAYER PROTOCOL Multicast : Sending to group of stations. This is ensured by setting the first bit in either 2 byte/6 byte addresses to 1. Broadcast : Sending to all stations. This can be done by setting all bits in the address field to 1.All Ethernet cards(nodes) are a member of this group. Length : The Length field tells how many bytes are present in the data field, from a minimum of 0 to a maximum of Data :Actually this field can be split up into two parts Data( bytes) Padding(0 46 bytes).

31 MAC SUBLAYER PROTOCOL Frame Checksum : It is a 32 bit hash code of the data. If some bits are erroneously received by the destination (due to noise on the cable), the checksum computed by the destination wouldn't match with the checksum sent and therefore the error will be detected.

32 Binary Exponential Backoff Algorithm After a collision the time is divided in discrete slots (equal to worst round trip propagation, which is 512 bits time or 51.2 us) After the first collision, each station waits 0 or 1 slot time before tries again If two station collide and they pick same number, they will collide again After a second collision, each station waits 0, 1, 2 or 3 at random and waits that number of slot times. After a third collision will happen, the next number to pick is between 0 and and that number of slots is skipped. After 10 collisions have been reached, the number interval is frozen at After 16 collisions, the station gives up to send the frame and reports the failure. Further recovery it is up to the higher layers.

33 MAC SUBLAYER PROTOCOL

34 net Transmission Flowchart transmit packet assemble packet yes deferring on? no start transmission send jam signal transmission done? no collision detect? yes increment attempts yes yes too many attempts? no done transmit ok done excessive collision errors compute and wait backoff time

35 IEEE Token Bus Token Bus is a linear or tree shaped cable in which all the stations are attached Logically, the stations are organized into a ring. A Token is passed from a station to its logical neighbour irrespective of physical location of latter Only token holder is permitted to send frame. When station acquires token it can transmit frames for a certain amount of time (several short frames)

36 IEEE Token Bus Station has 4 possible priorities, 0, 2, 4, 6; station maintains 4 queues for requests. Within each station, Token comes first to priority 6 queue. Sends occur until nothing to send OR timer expires. Token goes next to priority 4 queue. Sends occur until nothing to send OR timer expires. And so on.... Proper setting of the various timers ensures that high priority requests happen first.

37 Token Bus Operation Transmit A C B D Token sequence: C,A,D,B

38 Token Bus Operation Transmit Token A C B D Token sequence: C,A,D,B

39 Token Bus Operation Receive Token A C B D Token sequence: C,A,D,B

40 Token Bus Operation Transmit A C B D Token sequence: C,A,D,B

41 Token Bus Operation Transmit Token A C B D Token sequence: C,A,D,B

42 Token Bus Operation A C B D Receive Token Token sequence: C,A,D,B

43 Token Bus Operation A C B D Transmit Token sequence: C,A,D,B

44 Token Bus Mac Sub Layer Protocol Preamble (1 byte) Start Delmiter( 1byte) Frame Control (1byte) Dest. Address (2/6 bytes) Source Address (2/6 bytes Data ( bytes) Frame Checksum (4 bytes) End Delimiter (1 byte) IEEE Token Bus Frame Format

45 IEEE Token Bus Preamble used to synchronize receiver clock. Start/End Delimiter Used to mark frame boundaries (Analog Encoding) Frame control shows control or data. Destination Address (same as 802.3) usually 6 bytes. Source Address (same as 802.3) usually 6 bytes. Data BIG 8182 or 8174 bytes Checksum (Same as 802.3)

46 IEEE Logical Ring Maintanance

47 SOLICIT_SUCCESSOR Computer Networks IEEE Logical Ring Maintanance Gives sender s address and the current successor's address. Stations not in thering,withaddressbetweenthesetwoareinvitedtobidtobe inserted. No response within given time ==> go on as before. One response ==> newcomer is inserted; becomes new successor. Two or more responses ==> answers collide so garbled. So Resolve_Contention frame is used

48 IEEE Logical Ring Maintanance RESOLVE_CONTENTION Causes responding stations to NOT immediately try to be successors, but use binary countdown by 0, 1, 2, or 3 slots. Mechanism also ensures that traffic isn't slowed down by solicitation. SET_SUCCESSOR Used by a leaving station. Sent to the predecessor to say the leaver's successor is now the predecessor's successor. WHO_FOLLOWS The token sender listens to make sure the successor got and then passed on the token. If doesn't happen, it sends a WHO_FOLLOWS and failed station's successor sends a SET_SUCCESSOR to the failed one's predecessor.

49 IEEE Logical Ring Maintanance SOLICIT_SUCCESSOR_2 The token sender can't find the successor and there's no response from WHO_FOLLOWS; This causes ALL stations to once again bid for a place in the ring this is like starting from scratch. CLAIM_TOKEN If the token holder crashes, then nothing appears on the ring. All station's timers go off and the contention algorithm determines who gets to generate the token.

50 FDDI Use optical fibre cabling as a shared communication medium Optical fibre cables are made of glass Because they are so thin, they are fairly flexible Capable of 100 Mbps Light is used to transmit data Light is not susceptible to electrical interference Optical cabling can span longer distances Optical cabling does not need to be shielded near devices which generate electromagnetic interference

51 FDDI 100Mbps, distance up to 200km, 1000 hosts mainly used as a backbone

52 FDDI Is a token ring category network A token is passed from station to station When a station receives the token, it may transmit data If a station has no data, it allows the token to pass to the next station FDDI uses 2 rings of cabling, moving in opposite directions The second ring is used to allow twice the flow of data The purpose of the second ring is to allow data to reach its destination, even when one station has failed (and cannot forward messages)

53 FDDI Ring Technology

54 FDDI With Node Failure

55 FDDI Token Passing T S:12 D: S:12 2 S:12 3 S:12 D:07 D:07 D:07 4 S:12 S:12 D:07 D:07 S:12 D:07 S:12 D:07 S:12 D:07 5 S:12 D:07 11 S:12 D: S:12 D:07 6 S:12 D:07 S:12 D:07

56 FDDI Token Passing T T

57 FDDI 2 Classes of stations A and B Class A stations connects to both rings Class B stations connects to one of the ring 4 out of 5 encoding is used 16 out of 32 for data 3 are for delimiters 2 are for control 3 are for hardware signaling 8 reserved Data Used to Synchronize Stations Indicates Start of Frame Identifies the Type of Frame Address of the Destination Node Address of the Source Node Status of Frame Holds J. ack Godwin bits Ponsam

58 FDDI Preamble ( 8byte) Start Delmiter( 1byte) Frame Control (1byte) Dest. Address (2/6 bytes) Source Address (2/6 bytes Data ( bytes) Frame End ChecksumDelimiter (4 bytes) (1 byte) Frame Status (1 byte) FDDI Frame Format

59 FDDI Timers Token Holding Timer How long a station can transmit Token Rotation Timer Restarted every time the token is seen Valid Transmission Timer Used to time out and recover from certain transient ring errors

60 IEEE LLC (a) Position of LLC. (b) Protocol formats.

61 IEEE LLC Logical Link Control protocol runs in the top of all IEEE 802 protocols. It hides all the aspects of the 802 protocols, and it provides to the network layer a single format and interface. Usage of LLC: 1. The network layer of sending machine passes a network layer packet onto the LLC layer, using the LLC layer access primitives 2. The LLC sublayer adds the LLC header, containing sequences and acknowledgement numbers 3. The resulting structure is then inserted into the payload filed of an 802 frame and transmitted. 4. At receiver, the reverse process takes place. LLC provides three service options: 1. Unreliable datagram service 2. Acknowledged datagram service 3. Reliable connection oriented service

62 IEEE 802.3U Fast Ethernet

63 IEEE 802.3U Fast Ethernet Reduces the bit time from 10nsec to 100 nsec 100Base T4 Category 3 UTP scheme called 100 Base T4 uses a signaling speed of 25 MHz Needs four twisted pairs, 100Base TX Category 5 Wiring the design 100Base TX is simpler because the wires can handle clock rates upto 125 MHZ and beyond Needs only two twisted pairs 4B5B Encoding scheme is used

64 100Base FX Computer Networks IEEE 802.3U Fast Ethernet Uses two strands of multimode fiber,one for each direction Full duplex with 100 Mbps in each direction Distance between the hub and the station can be 2 km Shared Hub Switched Hub

65 Spanning Tree Bridges Two parallel transparent bridges.

66 Spanning Tree Bridges (2) (a) Interconnected LANs. (b) A spanning tree covering the LANs. The dotted lines are not part of the spanning tree.

67 Remote Bridges Remote bridges can be used to interconnect distant LANs.

68 Repeaters, Hubs, Bridges, Switches, Routers and Gateways (a) Which device is in which layer. (b) Frames, packets, and headers.

69 Repeaters, Hubs, Bridges, Switches, Routers and Gateways (2) (a) A hub. (b) A bridge. (c) a switch.

70 Figure 3.8 BSSs bsss BSS without an access point is called an adhoc network BSS with an access point is called an infrastructure network BSS made of wireless stations and optional central station AP

71 Figure 3.9 ESS Made of two or more BSS with APs ESS uses two types of stations mobile and stationary 72

72 Csma/CA FLOW CHART

73 dcf DCF Uses CSMA/CA NAV How much time must pass before the station are allowed to check the channel for idleness Each time a station sends RTS other stations starts their NAV Collision During HandShaking

74 pcf PCF Contention Free polling access method AP performs polling for stations that are capable of being polled Same time a station wants to use DCF and an AP wants to use PCF, AP has priority Repetition Interval Starts with beacon frame At the end of Contention free period PC sends a CF end Frame

75 Repetition interval

76 Csma/ca and nav

77 Frame format

78 Subfields in fc field

79 Frame format D Duration of transmission that is used to set value of NAV Addresses To DS and From DS Sequence Control Sequence nooftheframe Frame body 2312 bytes FCS 4 bytes CRC

80 Frame Types Management Frames Used for initial communication between stations and APs Control Frames Used for accessing the channel

81 Control frames

82 addressing

83 Figure 3.10 Physical layer 84

84 Figure 3.13 MAC layers in IEEE standard 85

85 Table 3.1 Addresses in IEEE TCP/IP Protocol Suite 86

86 Hidden Station Problem

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90 Physical Layer

91 Disclaimer School of Computing, Department of IT 8/22/2011 The contents of the slides are solely for the purpose of teaching students at SRM University. All copyrights and Trademarks of organizations/persons apply even if not specified explicitly. 92

92 School of Computing, Department of IT Review questions 8/22/ List the advantages and disadvantages of Static Channel Allocation 2. Define Pure Aloha 3. Define 1 Persistent CSMA 4. Why Bit Map protocol is called as a reservation protocol? 5.Define Time domain reflectometry 6. List the difference between BSSs and ESSs 7.Why CSMA/CD cannot be implemented in Wireless Lan? 8. Define Hidden terminal problem 9. List the service options provided by LLC 10. Define Exposed terminal problem 93

93 bibliography School of Computing, Department of IT 8/22/ Andrew S. Tanenbaum, Computer Networks, Fourth Edition, Prentice Hall of India, Cisco Network Fundamentals CCNA Exploration Companion Guide, Pearson Education, William Stallings, Data and Computer Communications, Fourth Edition, Prentice Hall of India,