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Telecommunications & Networking

Telecommunications & Networking. Lesson 3. ISO Open System Interconnect (OSI). Application. Application. Presentation. Presentation. Session. Session. Transport. Transport. Network. Network. Data Link. Data Link. Physical. Physical. Objectives & Requirements.

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Telecommunications & Networking

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  1. Telecommunications & Networking Lesson 3

  2. ISO Open System Interconnect (OSI) Application Application Presentation Presentation Session Session Transport Transport Network Network Data Link Data Link Physical Physical

  3. Objectives & Requirements • Frame synchronization : knows when to send and when to receive • Flow control : not send more than can be received • Error control : detect and correct • Addressing : specific line in a multiport • Control and data : be able to distinguish • Link management

  4. Flow Control • Rate of transfer • Sender must not overwhelm the receiver • Half-duplex vs Full-duplex • Buffer size • 0 • n • Infinity • Set of procedures

  5. Flow Control • Handshaking • ENQ/ACK/NAK • Poll/Select • Stop-and-Wait • Sliding Window

  6. Stop-and-Wait Flow Control • Sender sends again only an acknowledgement of the previous frame sent is received by the receiver • Receiver sends back an acknowledgement to indicate its readiness to receive another frame • Receiver withhold an acknowledgement if it is not ready to receive the next frame • flip between 0 and 1 for labelling of frame

  7. Stop-and-Wait : Inadequacy • Only one frame in transit at any one time • Bit length of the network is now usually >> the frame length • Example: transmission time = 1 and propagation delay = a; if a>1, then propagation delay causes the frame to arrive IMMEDIATELY AFTER or AFTER the transmission is complete. If a < 1, then propagation delay is less than the transmission time, meaning that the frame could arrive before the transmission is complete • Point: Inefficient with high transfer rate line

  8. Sliding-Window Flow Control • Size of the ‘sliding window’ is defined by the buffer size of both ends • Sending window shrinks from the left (once frames are sent) and expands to the right (once ACK is received) • Receiver window shrinks from the left (once frames are received) and expands to the right (once ACK is sent) • modulo-n for n-1 buffer size

  9. Sliding Window

  10. Error Control • For ‘lost frame’ and ‘damaged frame’ • What to do: • Error detection • Positive Acknowledgement • Retransmission After Timeout • Negative Acknowledgement & Retransmission

  11. Error Control • ARQ (Automatic Repeat Request) • Stop-and-Wait ARQ • Sliding Window ARQ • Go-Back-n ARQ • Selective-Reject ARQ

  12. Stop-and-Wait ARQ • Damaged Frame • Receiver detects error in frame; discard • Sender timeout for expected acknowledgement • Sender re-send frame (thus requiring a buffer size of 1 frame); frame is labelled alternatively with 1 and 0 • Damaged ACK/NAK • ACK0 for reception of frame 1 and waiting for frame 0 next; similar for ACK1

  13. Stop-and-Wait ARQ, lost ACK frame

  14. Stop-and-Wait ARQ

  15. Stop-and-wait ARQ

  16. Go-back-n, damaged data frame

  17. Go-back-n, lost data frame

  18. Go-back-n, lost ACK

  19. Other DLC Protocols • HDLC (ISO 33009, ISO 4335) - High-Level Data Link Control : many important DLC protocols based on this • LAPB : Link Access Procedure, Balanced - ITU-T/X.25; subset of HDLC; same frame format • LAPD : Link Access Procedure, D-Channel - ITU-T/ISDN • Frame Relay • ATM - cell based and not based on HDLC

  20. Error Detection • VRC (Vertical Redundancy Check) • Even Parity (even number of one’s) • 1 0 1 0 1 1 0 0 • Odd Parity (odd number of one’s) • 0 0 1 0 1 1 0 0 • Detect all one-bit errors; all odd number of bits errors

  21. Error Detection • Longitudinal Redundancy Check (LRC) 01001101101010010011100111011101 Assuming even parity

  22. Error Detection • Cyclic Redundancy Check (CRC) • The redundancy bits are the remainder (at least n-1 bits) given a predetermined divisor of n bits for a k bit datum, where k > n • The receiver accepts if the division leads to no remainder

  23. CRC (cont’d) 1 0 1 1 1 0 0 1 1 0 0 1 0 1 0 0 0 • Note: • left-most is a one, then subtract • left-most is NOT a one, then subtract zero • positional subtraction ONLY 1 0 1 1 1 0 1 0 1 0 1 1 1 0 1 0 1 0 1 1 1 1 0 0 1 0 1 1 1 1 1 0 1 0 1 1 1 0 1

  24. CRC (cont’d) 1 0 1 1 1 0 0 1 1 0 0 1 0 1 1 0 1 1 0 1 1 1 0 1 0 1 0 1 1 1 0 1 0 1 0 1 1 1 1 10 1 0 1 1 1 0 1 1 1 0 1 1

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