Telecommunications & Networking
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Transcript of Telecommunications & Networking
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Telecommunications & Networking
Lesson 3
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ISO Open System Interconnect (OSI)
Application
Presentation
Session
Transport
Network
Data Link
Physical
Application
Presentation
Session
Transport
Network
Data Link
Physical
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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
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Flow Control
• Rate of transfer• Sender must not overwhelm the receiver• Half-duplex vs Full-duplex• Buffer size
– 0– n– Infinity
• Set of procedures
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Flow Control
• Handshaking
• ENQ/ACK/NAK
• Poll/Select
• Stop-and-Wait
• Sliding Window
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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
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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
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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
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Sliding Window
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Error Control
• For ‘lost frame’ and ‘damaged frame’
• What to do:– Error detection– Positive Acknowledgement – Retransmission After Timeout– Negative Acknowledgement & Retransmission
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Error Control
• ARQ (Automatic Repeat Request)– Stop-and-Wait ARQ– Sliding Window ARQ
• Go-Back-n ARQ
• Selective-Reject ARQ
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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
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Stop-and-Wait ARQ, lost ACK frame
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Stop-and-Wait ARQ
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Stop-and-wait ARQ
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Go-back-n, damaged data frame
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Go-back-n, lost data frame
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Go-back-n, lost ACK
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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
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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
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Error Detection• Longitudinal Redundancy Check (LRC)
01001101101010010011100111011101
Assuming even parity
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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
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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
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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