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Data and Computer

Communications

Chapter 10 – Circuit Switching and

Packet Switching

Switched Network

Circuit Switching

 uses a dedicated path between two stations

 has three phases

 establish  transfer  disconnect

 inefficient

 channel capacity dedicated for duration of connection  if no data, capacity wasted

 set up (connection) takes time

 once connected, transfer is transparent

Public Circuit Switched

Network

Circuit

Switch

Elements

Blocking or Non-blocking

 blocking network

 may be unable to connect stations because all paths are in use  used on voice systems

 non-blocking network

 permits all stations to connect at once  used for some data connections

3 Stage Space Division Switch

Time Division Switching

 modern digital systems use intelligent

control of space & time division elements

 use digital time division techniques to set

up and maintain virtual circuits

 partition low speed bit stream into pieces

that share higher speed stream

 individual pieces manipulated by control

logic to flow from input to output

Traditional Circuit Switching

Packet Switching

 circuit switching was designed for voice

 packet switching was designed for data

 transmitted in small packets

 packets contains user data and control info  user data may be part of a larger message  control info includes routing (addressing) info

 packets are received, stored briefly (buffered) and past on to the next node

Advantages

 line efficiency  single link shared by many packets over time  packets queued and transmitted as fast as possible

 data rate conversion

 stations connects to local node at own speed  nodes buffer data if required to equalize rates

 packets accepted even when network is busy

 priorities can be used

Switching Techniques

 station breaks long message into packets

 packets sent one at a time to the network

 packets can be handled in two ways

 datagram  virtual circuit

Virtual

Circuit

Diagram

Virtual Circuits v Datagram

 virtual circuits

 network can provide sequencing and error control  packets are forwarded more quickly  less reliable

 datagram

 no call setup phase  more flexible  more reliable