# Synchronous Links/Networks Modems are asynchronous - penalized by start bits and stop bits on each character (and dead time) Synchronous Networks recover.

## Presentation on theme: "Synchronous Links/Networks Modems are asynchronous - penalized by start bits and stop bits on each character (and dead time) Synchronous Networks recover."— Presentation transcript:

Synchronous Links/Networks Modems are asynchronous - penalized by start bits and stop bits on each character (and dead time) Synchronous Networks recover clock timing using sync characters. Network Topologies T2 T6T5 T4 T1 T3 To other Control Stations in Distributed Network Terminals Leased/ Dedicated Lines (Point-to-Point) Shared/Polled Line (Multipoint) Control Station

Message Switching and Delivery Circuit Switching – physical medium is rerouted for duration of message by mechanical switching Message Switching – “store” entire message when received at control station and “forwarded” to the next control station (or destination terminal) when convenient. Packet Switching-Message is broken into standard length packets (~1000 bits). Packets are delivered by “store and forward”, and reassembled at receiving terminal. Packets may take different routes and arrive out of order. T1 T2

What are the odds? Message traffic has been observed to obey Poisson (exponential) distribution (kinda...) For reception of packets at a node, the probability that exactly k packets will arrive at a node in t seconds is: Where r is the average arrival rate (packets/second). The probability that K or fewer packets will be received in time t is:

If a node can transmit an average of  packets per second, then the probability of exactly c packets being transmitted (service completions) is: If a node, on average, is transmitting as many packets as it is receiving, then its internal packet queue will contain, on average, The average packet delay is If r > , the buffer will overflow and the node is “blocked” Node Buffer Requirements

Synchronous Messaging Protocols Message/packet framing Clock Synchronization Master/Slave determination Error detection/Retransmission Flow Control 1.Character/Byte Protocols (e.g. IBM’s BSC/bisync) 2.Byte Count Protocols – Header contains byte count (DDCMP) 3.Bit Frame Oriented Protocols (X.25) contain dedicated fields for Start Stop Destination Address Count Error check Data

Bisync Multipoint: Sender responds with ACK when polled Point-to-Point: Sender requests service (ENQ character) Once Sender is granted use of the line, he sends several SYN (synchronization) characters, Followed (optionally) by a header block preceded by SOH (Start-of-header), Followed by one or more text blocks ending with EOB/ETX and Block Check byte End of Transmission (EOT) terminates the message. Poll Multipoint terminals by address P-P ENQ ACK SYN … SOH Header STX Text message bytesEOB STXText message bytesEOB STXText message bytesETXBCC … EOT … … Link Control Characters: ACK: Acknowledge Error-Free reception of Block NACK: Error Detected WACK: Wait for ACK DLE: Link Escape DLE-STX: Binary data (Transparency) mode

BOP: X.25 Start Flag 01111110 Address Field 8 bits Control Field 8 bits Message bits CRC 16 bits Stop Flag 01111110 Transmitter must insert a zero after any sequence of five consecutive 1s. Receiver must remove any 0 it sees after five consecutive 1s. Address of sending station Next Frame # This Frame # Control Code 1 0 P/F Next Frame #0P/F Control Code 1 1 P/F Supervisory Frame Control Field Data Frame Control Field Un-Numbered Frame Control Field

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