Packet Delay Within Our Networks

The first step to understanding QoS is understanding the concepts of Delay, Jitter, Packet Loss and bandwidth. Only once you understand these key topics can you begin to understand what service levels different types of traffic will require. Below is a short list of the most common forms of delay you will find on your networks. It is far from detailed as I just wanted to provide you with a short list to refresh your memory….

Serialization Delay (Fixed Delay)
The Time taken to clock a frame onto the physical interface.
How long your router takes to encode an entire frame onto the circuit.

Propagation Delay (Fixed Delay)
Time taken for a single bit to traverse the physical connection.
How long does a single bit take to get from Router1 to Router2 once it has been placed on the physical media (Copper and Optical cabling, etc).

Queuing Delay (Variable Delay)
Time Spent in the routers software queue.
Assuming congestion exists & queuing has been enabled, queuing is in effect, how long will packets wait within software queues before being sent to the hardware queue (TX-Ring).

Processing/Forwarding Delay (Variable Delay)
Time taken from when a packet has been fully received by the router, until it is placed into the output queue.
Delay experienced as the router receives a packet, determines if it has a route the the destination, what interface it should send this packet out of, etc.

Shaping Delay (Variable Delay)
Time spent in a shaping queue.
When Shaping has been enabled to limit the maximum average output rate, packets may be delayed so that this maximum average output rate is achieved.

Network Delay (Variable Delay)
Delay incurred by packets as they traverse the Provider network.
Serialization Delay, Propagation delay and more are all experienced within the Service Provider network as well. I like to think of this as “cloud delay”.

Codec Delay (Fixed Delay)
Time taken to analyze and encode voice with the desired codec.
Encoding audio received by the router results in added delay as the audio processing algorithms work through their encoding and other related functions.

Compression Delay (Variable Delay)
Time taken to compress and decompress a packet.
A compression algorithm analyzing and compressing a packet results in added delay.


~ by networkingza on May 11, 2008.

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