Lecture 17 cn_aizaz_final_TEA

Lecture 17 cn_aizaz_final_TEA - •Data Communication and...

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Unformatted text preview: •Data Communication and Networks 1 EXAMPLES OF DATA LINK PROTOCOLS HDLC ­ a classical bit­oriented protocol HDLC ­ a classical bit­oriented protocol HDLC—High­Level Data Link Control HDLC—High­Level Data Link Control PPP ­ To connect home computers to the Internet PPP ­ To connect home computers to the Internet PPP – Point­to­Point Protocol PPP – Point­to­Point Protocol 2 HDLC – HIGH­LEVEL DATA LINK CONTROL Initially IBM developed a data link protocol for use in the IBM Initially IBM developed a data link protocol for use in the IBM mainframe world: SDLC (Synchronous Data Link Control) protocol. After developing SDLC, IBM submitted it to ANSI and ISO for After developing SDLC, IBM submitted it to ANSI and ISO for acceptance as U.S. and international standards, respectively. ANSI modified it to become ADCCP (Advanced Data ANSI modified it to become ADCCP (Advanced Data Communication Control Procedure) ISO modified it to become HDLC (High­level Data Link ISO modified it to become HDLC (High­level Data Link Control). CCITT then adopted and modified HDLC for its LAP (Link CCITT then adopted and modified HDLC for its LAP (Link Access Procedure) as part of the X.25 network interface standard Later CCITT modified it again to LAPB (LAP Balanced) Later CCITT modified it again to LAPB (LAP Balanced) 3 • Frame format for bit­oriented protocols. Flag sequence: 01111110 Address: To identify one of the terminals on lines with multiple terminals To distinguish commands from responses for point­to­point lines 4 Control: sequence numbers, acknowledgements Data: any information, may be arbitrarily long Checksum: cyclic redundancy code The minimum frame contains three fields and totals The minimum frame contains three fields and totals 32 bits, excluding the flags on either end. 5 There are three kinds of frames Information Information Supervisory Supervisory Unnumbered Unnumbered 6 Control fields of (a) An information frame. (b) A supervisory frame. (c) An unnumbered frame. 7 • FIELDS of INFORMATION FRAME: The Seq field is the frame sequence number. The P/F bit stands for Poll/Final. The Next field is a piggybacked acknowledgement. All the frames sent by the terminal, except the final one, have the P/F bit set to P. The final one is set to F. The P/F bit is also used to force the other machine to send a Supervisory frame immediately rather than waiting for reverse traffic onto which to piggyback the window information. 8 • TYPE FIELD IN A SUPERVISORY FRAME: Type 0: Acknowledgement Frame Officially called RECEIVE READY. Used to indicate the next frame expected This frame is used when there is no reverse traffic to use for piggybacking. Type 1: Negative Acknowledgement Frame Officially called REJECT It is used to indicate that a transmission error has been detected. The sender is required to retransmit all outstanding frames starting at Next. 9 Type 2: RECEIVE NOT READY It acknowledges all frames up to but not including Next, just as RECEIVE READY does, but it tells the sender to stop sending. It is intended to signal certain temporary problems with the receiver, such as a shortage of buffers, etc. Type 3: SELECTIVE REJECT Retransmit only the frame specified It is like our protocol 6 Used by HDLC and ADCCP , but not by SDLC and LAPB • UNNUMBERED FRAME ­ used for control purposes but can also carry data in unreliable connectionless service. 10 • COMMANDS: DISC (DISCconnet) Allows a machine to announce that it is going down. SNRM (Set Normal Response Mode) Allows a machine that has just come back on­line to announce its presence. FRMR (FRaMe Reject) Indicates a frame with a correct checksum but impossible semantics. EXAMPLE: a frame shorter than 32 bits 11 Control frames can also be lost or damaged, just like data frames, so they must be acknowledged too. UA (Unnumbered Acknowledgement) UA (Unnumbered Acknowledgement) It is a special control frame which is used for acknowledgement of control frames. 12 There are control frames which deal with initialization, polling, and status reporting. UI (Unnumbered Information) UI (Unnumbered Information) These data are not passed to the network layer but are for the receiving data link layer itself. 13 NUMERICALS CHAPTER # 3 CHAPTER # 3 END PROBLEMS related to Data Link Protocols END PROBLEMS related to Data Link Protocols 17, 18, 26, 29, 30, 31, 32 17, 18, 26, 29, 30, 31, 32 14 The Medium Access Control Sublayer • Network Classification 1. Use of point­to­point connections – mostly in • This chapter deals with broadcast WANs, except satellite. 2. Use of broadcast channels – mostly in LANs. networks and their protocol. • The objective is to allocate the channel to: maximize channel utilization, and minimize channel access delay. 15 Channel Allocation • Static Channel Allocation in LANs and MANs • Dynamic Channel Allocation in LANs and MANs 16 Static Channel Allocation • In static channel allocation, a subchannel is statically assigned to each station (computer or terminal). • For example, in FDM, a frequency band is assigned to each station. • This is inherently inefficient (w.r.t. channel utilization) for bursty traffic. • Note, however, the channel access delay is minimal. 17 • Use the queueing system to analyze above scheme. • Single station case: Static Channel Allocation Let C = channel capacity (in bps) From QUEUEING theory, we obtain: T = 1 / (μ C ­ λ) For example, if 1/ μ = 10,000 bits/frame, T = mean time delay to send one frame (in sec.) λ = arrival rate (in frames/sec.) 1/μ = mean frame size (in bits/frame) C = 100 Mbps, and λ = 5000 frames/sec, then T = 1 / ((1/104)(108) ­ 5000) = 1 / 5000 = 200 μsec per 18 frame. Static Channel Allocation N station case: Divide the channel up into N subchannels, each with capacity C/N. Let λ /N = arrival rate at each station (divide the load). Then, TFDM = 1 / (μ(C/N) ­ λ/N) = N / (μC ­ λ) = N T. So, the N station case is N times worse than the 1 station case. For example, as above, N = 10 TFDM = 2 19 • 1. Station Model – N independent stations Assumptions: Dynamic Channel Allocation generate frames. 1. Single Channel – A single channel is available for all communication. 1. Collision – Frames that overlap in time destroy each other; this is called a COLLISION. All stations can detect collisions. A collided frame must be transmitted again later. 20 1. Continuous Time means that transmission of frames can begin at any time. Slotted Time means that time is divided into discrete intervals, and frame transmission always begins at the start of a slot. 1. Carrier Sense means that stations can tell if the channel is in use by listening to the channel. No Carrier Sense means that stations cannot tell if the channel is in use by listening to the channel. 21 Quiz # 3 CRC M(x)=x9 + x8 + x6 + x4 + x3 + x + 1 C(x)=x4 + x + 1 Find T(x)=? Hamming Code Hamming Code Show the bit pattern transmitted for the message 10100101. (Assume odd parity used in Hamming code) 22 ...
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