Types of delay in computer networks

When a packet travels from source to destination through a network, it experiences four types of delay at each hop (router/link):

1. Processing delay — time a router takes to examine the packet header, check for bit errors, and determine the outgoing link (routing table lookup).

2. Queuing delay — time a packet waits in the router's output buffer before it can be transmitted, depending on current congestion at that router.

3. Transmission delay — time to push all of the packet's bits onto the link, equal to Packet size / Bandwidth.

4. Propagation delay — time for a bit to physically travel from one end of the link to the other, equal to Distance / Propagation speed.

Total delay per hop = Processing + Queuing + Transmission + Propagation delay.

Numerical problem

Data to send    = 200 Mb = 200 x 10^6 bits
Bandwidth        = 1 Gbps = 1 x 10^9 bits/sec
Routers in between = 3  ->  number of links (hops) = 3 + 1 = 4
                     (A -> R1 -> R2 -> R3 -> B)
Other delays (processing, queuing, propagation) = 0

Since only transmission delay is non-zero, and (assuming store-and-forward switching at each router, the standard assumption) each router must fully receive the data before forwarding it, the data must be transmitted in full once per link:

Transmission delay per link = Data size / Bandwidth
                             = (200 x 10^6) / (1 x 10^9)
                             = 0.2 sec  (200 ms)

Total delay = Transmission delay per link x Number of links
            = 0.2 x 4
            = 0.8 sec  (800 ms)

Total delay involved = 0.8 seconds (800 ms)