Packet Switching: Store-and-Forward Transmission and Forwarding
Store-and-Forward Transmission
End systems exchange messages containing data. A long message is divided into smaller packets, which travel through communication links and packet switches. Sending a packet of bits over a link operating at bits per second takes seconds.
Most packet switches use store-and-forward transmission: before starting to transmit the packet's first bit onto an outgoing link, the switch must receive the entire packet.
Assemble a packet before forwarding it
This model packet contains three bits. Move them into the router's buffer, then decide when the router can forward the packet. Trying too early reveals what is still missing.
3/3
0/3
0/3
The router is still receiving the packet.
Ignore propagation delay and let the source begin at time zero. The router has the complete packet at time and can then start transmitting it. The destination receives the entire packet at time .
For one packet crossing links of equal rate , with routers between source and destination, the end-to-end delay under these assumptions is
KNOWLEDGE QUIZ
Forwarding Tables
A router receives a packet on an input link and sends it onto an output link. How does it choose the output?
The sender puts the destination IP address into the packet header. At each router, the forwarding table is consulted to choose an appropriate outgoing link for that destination.
An analogy is asking for directions along a journey: each person you ask helps choose the next part of the route, much as a router chooses a packet's next outgoing link.
Be the router: forward by destination address
Each router knows only its own forwarding table: destination address → next hop. Send a packet, then act as each router it reaches. Afterwards, cut link R1–R3 and repair R1's table so packets still arrive. Addresses are documentation examples.
| Destination | R1 next hop | R2 next hop | R3 next hop |
|---|---|---|---|
| 198.51.100.7 | R2 | Host A | R2 |
| 203.0.113.5 | R3 | R3 | Host B |
Choose a destination address. At each router, find that address in the router's own forwarding table and click the highlighted link it selects.
KNOWLEDGE QUIZ
Time one packet across three links
A 12,000-bit packet crosses three 6 Mbps links and two store-and-forward routers. Ignore propagation, processing, and queuing delay. Calculate transmission time per link and final arrival time. Can the first router begin forwarding at 1 ms? Explain.