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)

• 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

Error Detection• Longitudinal Redundancy Check (LRC)

01001101101010010011100111011101

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)

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

CRC (cont’d)

1 0 1 1 1 0 0 1 1 0 0 1 0 1 1 0 11 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