You all might have a basic understanding regarding the Delay in Network, here i will debrief some types of the Delay which are well-known in all Networks, but it will mostly focused in Cisco Environment:


We Know that all the packets in a network experience some delay between when the packets is first sent and when it arrives to destination, and most of the QoS in network are related in some way to delay. In some points of the network the delay is so small that it can just be ignored for practical purposes, but in other cases delay is significant, and there is nothing that we can do about it, here are the type of delay:

  • Serialization Delay (fixed)
  • Propagation Delay (fixed)
  • Queuing Delay (variable)
  • Forwarding/Processing Delay (variable)
  • Shaping Delay (variable)
  • Network Delay (variable)

Let’s start discussing each of the above delay types in detail:

Serialization Delay:

It defines the times it takes to encode the bits of packet onto the physical interface, if the link is fast, the bits can be encoded onto the link more quickly; if the link is slow, it takes longer to encode the bits on the link, likewise if the packet is short, it does not take as long to put the bits on the link as compared with a long packet.

We calculate the Serialization Delay with the following Formula:

#bit sent / Link Speed

Let’s assume that the Router-A sends a 125-byte packet to Server located on other end of the network, it sends the packet over the FastEthernet Link to the Switch, the 125 byte is equal to 1000 bits, ( 125 * 8 = 1000 ) so at the FastEthernet speeds, it takes 1000 bits/100,000,000 bits per second (bps), or 0.01 ms, to serialize the packet onto the Fast Ethernet, another 0.01 ms Serialization Delay occur when Switch sends the 125 byte of the packet toward the next router to reach destination.

The Serialization Components over Fast Ethernet is insignificant, whereas Serialization becomes a more significant number on lower-speed serial links, also Serialization Delay occurs inside the Frame-Relay Cloud, below table shows the Serialization Delay Values:

Clock Rate of Link Serialization Delay (125-Byte) Serialization Delay (1500-Byte)
100 Mbps .01 .12
1.544 Mbps .65 8
512 Kbps 2 24
128 Kbps 7.8 93
56 Kbps 17.85 214

Note: The above values are in Milliseconds.

Propagation Delay:

It defines the time it takes a single bit to get from one end of the link to the other, when an electrical or optical signal is placed onto the cable, the energy does not propagate to the other end of the cable instantaneously, some delay occurs, and the law of physics says that the Speed of energy on electrical and optical interfaces approaches the speed of light and obviously we can’t override this law, the only variable that affects the propagation delay is the length of the link, use the following formula to calculate propagation delay:

Length of Link ( Meters ) / 3.0 X 10 (8) Meter/Second


Length of Link ( Meters ) / 2.1 X 10 (8) Meter/Second

Where 3.0 * 10 (8) is the speed of light in a vacuum, many people use 2.1*10(8) for the speed of light over copper and optical media when a more exact measurement is needed.

Propagation Delay occurs as the bits traverse the physical link. Suppose, for instance, that the Point-to-Point link between Router-A and Router-B is 1000 Kilo Meters ( 1000000 Meters ) long the calculation would be as below:

1,000,000 / 2.1 * 10(8) = 0004.8 ms

The length of the Link affects propagation delay, whereas the size of the packet and link speed affect serialization delay, the serialization delay is larger for larger packets, but the propagation delay is equal for different-sized packets, on the same link.

Below is the Example of Serialization and Propagation Delays:

Step Number From Figure Length of Link Clock Rate of Link Propagation Delay MS Serialization Delay 125 Byte Serialization Delay 1500
1 50 M 100 Mbps .002 .01 .12
2 10 M 100 Mbps .0004 .01 .12
3 1000 KM 56 Kbps 4.8 17.85 214
4 5 KM 128 Kbps .024 7.8 94
5 1000 KM 44.232 Mbps 4.8 .02 .24
6 10 KM 1.544 Mbps .048 .65 7.8

Queuing Delay:

Packets experience queuing delay when they must wait for other packets to be sent, most people think of queuing delay when they think of QoS and most people think of queuing strategies and tools when they think of QoS Tools, but queuing tools are just one category of QoS tool. Queuing Delay Consists of the time spent in the queues inside the device – typically just in output queue in a router, because input queuing is typically negligible in a router, however, the queuing time can be relatively large – hundreds of Milliseconds, or maybe more then that.

Suppose The PC Sends 4 Packets with each size of 1500 Byts toward the Router-A, now Router-A Queue all these 4 Packets like below:

1500 * 8 / 56,000 or 214 ms to serialize each 1500-Byte packets, the other packets need to be stored either in memory or discarded, therefore the more well-known queuing form that the router uses is FIFO (First In first Out) logic, which means that after 856 MS all four Packets would have been sent out to the Serial Link, ( 214 * 4 = 856 ). So, some queuing helps prevent packet drops, but large queues can cause too much delay.

Suppose These 4 Packets reach the Router-A but it will take 856 MS to reach Router-B, which is is 214 ms of Serialization and 4.8 MS of Propagation Delay and 642 MS of Queuing Delay.

Forwarding Delay:

It is refers to the processing time between when a frame is fully-received, and when the packet has been placed in an output queue, so forwarding delay does not include the time the packet sits in the output queue waiting to leave the router, it does not include the time required for the router to process the router, or forward the packet.

Forwarding delay is typically a small enough component to ignore in overall delay budget calculations.

Shaping Delay:

Traffic Shaping cause additional delays by serving queues more slowly than if traffic shaping were not used, will the question comes that why we need to shape the traffic? and which of the below are better:

  • Sending packets really fast and having them be dropped
  • Sending packets more slowly, but not having them be dropped

The answer might be different and differ to the each cases, carries can drop the packets if they are exceeded suppose you have a contract of 64 KBPS with the Frame-Relay Cloud and rest of the Packets will be discarded, and you have a T1 Connection to the Router where the next end of your router is only having 56 Kbps of the link speed, in this case the Service Provider Router will be queued and stack which in deed make the router to work slowly in this case the traffic has to be shared before it leaves the Customer-End and reaches ISP Router or even other Site of your network.

Network Delay:

Mostly you all draw your network and when it is connected to the outside you simple put a cloud or a frame-relay switch but what does it mean ? Network Engineer’s should consider delay inside the Cloud which are the Network of iSP and it has to be well-measured, briefly speaking the total number of the Network delay is depend on total number of circuits, and the good calculation of the Network Delay is that i take the Propagation Delay and the Serialization Delay of the Ingress and Serialization of the Egress Routers.

Propagation Delay + Ingress Serialization Delay + Egress Serialization Delay = Network Delay

Queuing delay inside the cloud creates the most variability in network delay, just as it does outside the cloud. These delays are traffic dependent, and hard to predict.

Let’s See the following Table as a summary of all the above definitions:

Delay Components Definition Where it Occurs
Serialization Delay ( Fixed ) Time taken to place all bits of a frame onto the physical medium. Function of frame size and physical link speed. Outbound on every physical interface, typically negligible on T3 and faster links.
Propagation Delay ( Fixed ) Time taken for a single bit to traverse the physical medium from one end to the other. Based on the speed of light over that medium, and the length of the link. Every physical link. Typicallynegligible on LAN links and shorter WAN links.
Queuing delay (variable) Time spent in a queue awaiting the opportunity to be forwarded (output queuing), or awaiting a chance to cross the switch fabric (input queuing). Possible on every output interface. Input queuing unlikely in routers, more likely in LAN switches.
Forwarding or processing delay(variable) Time required from receipt of theincoming frame, until the frame/ packet has been queued for transmission. On every piece of switching equipment, including routers, LAN switches, Frame Relay switches, and ATM switches
Shaping delay (variable) Shaping (if configured) delays transmission of packets to avoid packet loss in the middle of a Frame Relay or ATM network. Anywhere that shaping is configured, which is most likely on a router, when sending packets to a Frame Relay or ATM network.
Network delay (variable) Delays created by the components of the carrier’s network when using a service. For instance, the delay of a Frame Relay frame as it traverses the Frame Relay network. Inside the service provider’s network.



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