Telecommunications & Networking Lesson 3

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

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

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

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

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

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

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

Sliding Window

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

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

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

Stop-and-Wait ARQ, lost ACK frame

Stop-and-Wait ARQ

Stop-and-wait ARQ

Go-back-n, damaged data frame

Go-back-n, lost data frame

Go-back-n, lost ACK

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

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

Error Detection Longitudinal Redundancy Check (LRC) Assuming even parity

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

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

CRC (cont’d)