Causes of packet delay

A packet passing through a network suffers four kinds of delay at each node. Total nodal delay is the sum of these.

1. Processing delay: time to examine the packet header, check for bit errors and decide the output link. It is usually microseconds.
2. Queuing delay: time the packet waits in the output buffer before transmission. It depends on traffic load and varies; heavy congestion makes it large.
3. Transmission delay: time to push all bits of the packet onto the link = L/R, where L is packet length in bits and R is link rate in bps.
4. Propagation delay: time for a bit to travel along the link = d/s, where d is link length and s is propagation speed (about 2 × 10⁸ m/s in cable).

d_nodal = d_proc + d_queue + d_trans + d_prop

For example, a 1000-byte packet on a 1 Mbps link has a transmission delay of 8000/10⁶ = 8 ms.