Switching Lab

Data Communication, Lecture 10. Time a message through circuit and packet switching, follow datagrams and virtual circuits through switches, and size a switch fabric. Works offline.

Circuit switching: setup, then the whole message flows straight through
Packet switching: each switch stores a packet, then forwards it (packets pipeline)

Which finishes first?

Circuit totalPacket totalWinner

Circuit: setup + L/R + links × propagation. Packet: (N + links − 1) × packet time + links × propagation + waiting. Headers are ignored here.

Four datagrams from A to X; each switch chooses a route on its own

Send four datagrams

Arrival order at XDelivered in order?

Datagrams are independent: they can take different paths, see different waiting times and arrive out of order. The transport layer at X puts them back in order.

A frame crosses three switches; each swaps the VCI using its table

Follow the VCI

A VCI is only meaningful on one link. Each switch looks up (incoming port, incoming VCI) and sends the frame out on a new port with a new VCI. The values match Forouzan's Figures 8.12–8.15.

Three-stage switch: N/n crossbars (n × k), k crossbars (N/n × N/n), N/n crossbars (k × n)

How many crosspoints?

Single crossbar N²Three-stage 2kN + k(N/n)²Share of crossbarBlocking?

Example 8.3: N = 200, n = 20, k = 4 → 2,000. Example 8.4 (Clos): n = √(N/2) = 10, k = 2n − 1 = 19 → 15,200, and no blocking.