Switching
Overview
A network layer function
Routing is a related but distinct network layer function
Two important types of switching:
Circuit switching
Packet switching
Two important types of network layer delivery service
Connection-oriented (telephone metaphor)
Connectionless (postal metaphor)
Note: connection-oriented transport layer service can be built on connectionless network layer service (e.g. TCP/IP)
Circuit Switching
Physical circuit is "direct connection"
Switch device has fixed # input lines and output lines (may differ)
Path between sender and receiver may include many switches
Space-division switches for dedicated line
Crossbar switch
Adv: simple switch devices
Dis: many such switches required (n * m) -- most idle
Multistage switch
Adv: number of switches grows slowly as # supported lines increases (e.g. n log n rather than n * n)
Dis: individual switches more complex, wiring more complex
Time-division switches for TDM
Time Slot Interchange (TSI) required between sender and receiver to reorder frame slots. Mux sends slots in sender order, TSI reorders to receiver order for Demux.
Consider advantages and disadvantages of circuit switching for transmitting various types of information (e.g. voice, video, data)
Packet Switching
Transmission partitioned into packets
Two approaches: datagram, virtual circuit
Datagram
Each packet independent
Each can take different route
May arrive out of order
Virtual Circuit
Route determined at beginning
Each packet takes same route
Route fixed, but not necessarily dedicated! Switches can be changed between packets to serve different circuits
Network Services
Connection-oriented
Telephone metaphor
May be built on circuit- or packet-switched (virtual circuit)
Basic transmission protocol
- Establish connection
- Transmit over connection
- Terminate connection
- If packet-switched, steps 1 and 3 require packet exchanges (for request and acknowledgement)
- Adv: simplified flow control and sequencing, little packet overhead bytes
- Dis: fixed route not adaptable to dynamic network loads, complex protocol to establish and maintain connection
Connectionless
- Postal metaphor
- Basic transmission protocol: transmit
- Each packet independent
- No delivery promises (may be damaged, lost, out of order)
- Adv: xmit protocols simple, packets can be rerouted depending on network load
- Dis: much packet overhead since it needs to carry full addressing and flow info
Related Home Pages:[ notes | CSC 465 | Peter Sanderson | Computer Science | SMSU ]
Last reviewed: 25 March 1998
Peter Sanderson ( pete@csc.smsu.edu )