Switching techniques in data communication

Switching is the method of connecting a sender to a receiver through intermediate nodes of a network, so that every pair of users does not need a dedicated link. Data communication uses three main switching techniques: circuit switching, message switching and packet switching (datagram and virtual circuit).

               Switching
     +-------------+--------------+
  Circuit       Message         Packet
                           +------+------+
                       Datagram    Virtual circuit

1. Circuit switching

A dedicated physical path is set up between the two users before data transfer and kept for the whole call. The telephone network (PSTN) uses it.

Three phases

1. Circuit establishment: signalling sets up a path through the switches.
2. Data transfer: data flows continuously at a fixed rate over the reserved path.
3. Circuit disconnect: the path is released.

 A --[S1]======[S2]======[S3]-- B
   setup    dedicated path   release

• Space-division or time-division (TDM) switches are used.
• Advantages: fixed, small delay; no congestion once connected; data arrive in order; good for real-time voice.
• Disadvantages: setup delay; channel capacity is wasted during silence; both ends must use the same rate; a busy network blocks new calls.

2. Message switching

There is no dedicated path. The whole message is sent with a destination address to the next node, which stores it, checks it and forwards it when the next link is free (store-and-forward). Used in early telegraph and e-mail relay systems.

 A --msg--> [N1 store] --msg--> [N2 store] --msg--> B

• Advantages: good line efficiency (links shared); no blocking (messages queued); priorities and code/speed conversion possible; broadcast possible.
• Disadvantages: long and variable delay; nodes need large storage (disk); not suitable for interactive or real-time traffic.

3. Packet switching

The message is divided into small packets (e.g. up to 1500 bytes) each with a header. Packets are stored and forwarded through the network; links are shared statistically. Packets are small, so they can be held in memory and pipelined over successive links, giving much less delay than message switching.

(a) Datagram approach

• Each packet is routed independently; no setup.
• Packets may take different routes, arrive out of order or be lost; the destination reorders them.
• Example: IP / the internet.
• Robust to node failure, no setup delay; but variable delay and no guaranteed order.

(b) Virtual circuit approach

• A logical route (virtual circuit) is set up first with a call-request/call-accept exchange; each packet carries a short VC identifier.
• All packets follow the same path in order; the circuit is cleared at the end.
• Examples: X.25, Frame Relay, ATM, MPLS (label switched paths).
• Allows QoS and in-order delivery; but setup delay and failure of a node breaks the VC.

 Datagram:      1 -> R1-R2-R4    2 -> R1-R3-R4
 Virtual circ.: 1,2,3 -> R1-R2-R4 (all same path)

Delay comparison (sending a long message over 3 hops)

 Circuit:  |setup|=========data========|
 Message:  |==msg==|==msg==|==msg==|
 Packet:   |p1|p2|p3|
              |p1|p2|p3|
                 |p1|p2|p3|   (pipelined)

Packet switching delivers sooner than message switching because packets overlap on successive links.

Comparison of techniques

Choice of technique

• Real-time, constant-rate traffic (voice) suits circuit switching.
• Bursty computer data suits packet switching, the basis of today's networks.
• Modern networks combine both ideas: e.g. VoIP carries voice in packets but MPLS virtual circuits give it a guaranteed path.