Network Delays and Losses

Network delays are usually small. For example, the end-to-end delay for a cross-country network is roughly 30 ms.

However, packets can get lost in a network. Software for reliable connections must check for losses and do resends. If a resend is needed, the overall delay is at least doubled: the another round-trip time is added for a resend request and response. For higher speed reliable data transfer protocols the impact can be even greater.

This web page examines first the delay equation which breaks down network delays into easier to analyse components. It then examines some of the causes of packet losses.

The Delay Equation

The delay equation below describes the packet delay at a single node along its route from source to destination.

Nodal Delay

To simplify the analysis of network delay times, the packet delay is broken up into a sequence of nodal delays. Each nodal delay is the time between the arrival of a packet at a node and its arrival at the next node. The above equation decomposes the nodal delay into components that are simpler to analyze.

Processing Delay

The nodal processing delay d_proc is the time that a node spends processing a packet. This includes time for error checking, time for reading the packet header, and time for looking up the link to the next node, based on the destination address. Although the processing may sound complicated, the nodal processing delay is usually negligible compared to other terms in the delay equation.

Queuing Delay

The queuing delay d_queue is the time that a packet spends in a queue at a node while waiting for other packets to be transmitted. If the node is a high-speed router then there is one queue for each outgoing link, so a packet waits only for other packets that are going across the same link.

The queuing delay is related to the transmission delay d_trans by the following approximate equation.

 = 

Here, l_queue is the average length of the queue. The average queue length depends on the load factor, which is the ratio of the attempted link transmission rate to the link maximum transmission rate. The average queue length is typically less than 1 for a load factor less than 1/2. When the load factor exceeds 1, the queue length grows without bound.

Transmission Delay

The transmission delay d_trans is the time required to put an entire packet into the communication media. It can be computed by the following equation.

 = 

Here, L is the length of a packet in bits and R is the transmission rate in bits per time unit. The time unit in d_trans and R should be the same.

Propagation Delay

The propagation delay d_prop is the time that it takes a signal change to propagate through the communication media from a node to the next node. It can be computed using the following equation.

 = 

Here, D is the distance from the node to the next node and s is propagation speed of the media.

For links using radio broadcast, a signal change propagates at close to the speed of light, which is about 186,000 miles per second. For copper and fiber links, a signal change propagates at 60% to 80% of the speed of light. For back-of-the-envelope calculations in copper or fiber links, 100,000 miles per second is a convenient approximation to the propagation speed.

Packet Losses

Lost packets are the result of two main causes:

• Queue overflow
• Noise

Overflow

Queue overflow occurs when an output link from a node has a load factor that exceeds 1.0. That is, data is arriving at the queue for the link faster than it can be transmitted. As a result, queue length grows until there is no more space in the queue. At that time, a node has no choice but to discard a packet.

Noise

A small amount of noise is present in any electronic circuit. Even more noise may be generated by nearby appliances or lightning activity. This noise can cause bits to be changed from 0 to 1 or vice versa. Normally, this causes error checks to fail. Most nodes will then just discard the packet that contains the error.

Different media have different susceptibility to noise. Fiber media generally have the lowest noise levels. Coaxial cable has low noise, but not as low as fiber. Twisted pair cabling is somewhat more susceptible to noise than coaxial cable. Unguided media (radio and satellite broadcast) are the most susceptible to noise for two reasons:

• The media is unselective about the signals that it carries, and
• The signal strength drops rapidly with distance.

Back-of-the-Envelope Calculations

The following delay calculations assume a packet length of 1Kbits and a media propagation speed of 100,000 miles per second. Then the reciprocal of the propagation speed is 10 microseconds per mile. Dividing by the speed is the same as multiplying by the reciprocal of the speed.

LAN Delay

The following values are typical for a LAN:

• Transmission rate: R = 100 Mbits/second
• Bit transmission time: 1/R = 10 nsec/bit
• Distance: D = 0.1 miles

Then we have

• d_trans = 1Kbits * 10 ns/bit = 10 µsec
• d_prop = 0.1 mi * 10 µsec/mi = 1 µsec

The queue delay d_queue depends on the load factor. If the load factor is less than 0.1 then the queue delay is almost always smaller than the transmission delay. If the load factor is less than 0.5 then the queue delay is no more than 2 or 3 times the transmission delay.

For modern network equipment, the nodal processing delay is insignificant. The above calculations show that the transmission delay is the most significant component of the nodal delay for a lightly loaded link, but the queue delay dominates for a heavily loaded link.

WAN Delay

The following values are typical for a WAN:

• Transmission rate: R = 10 Gbits/second
• Bit transmission time: 1/R = 0.1 nsec/bit
• Distance: D = 100 miles

Then we have

• d_trans = 1Kbits * 0.1 ns/bit = 100 nsec
• d_prop = 100 mi * 10 µsec/mi = 1 msec

The queue delay d_queue depends on the load factor, but is small compared to the propagation delay unless the load factor is close to 1.0.

For modern network equipment, the nodal processing delay is insignificant. The above calculations show that the propagation delay is the most significant component of the nodal delay.

Summary

The delay equation breaks down network delays into easier to analyse components. Network delays are usually small, as indicated by example calculations. However, packets can get lost in a network, which increases the impact of delays.

Nodal Delay

• The packet delay is broken up into a sequence of nodal delays.

• Each nodal delay, d_nodal, can be calculated with the following equation.

  dnodal

  =

  dproc

  +

  dqueue

  +

  dtrans

  +

  dprop

Components of Nodal Delay

• The nodal processing delay, d_proc, is the time that a node spends processing a packet. It is usually negligible compared to other terms.

• The queuing delay, d_queue, is the time that a packet spends in a queue at a node while waiting for other packets to be transmitted. It is related to the transmission delay d_trans by the following approximate equation.

  dqueue

  =

  dtrans

  *

  lqueue

• The transmission delay d_trans is the time required to put an entire packet into the communication media. It can be computed by the following equation.

  dtrans

  =

  L/R

• The propagation delay d_prop is the time that it takes a signal change to propagate through the communication media from a node to the next node. It can be computed using the following equation.

  dprop

  =

  D/s

Comparisons

• In a LAN, the transmission delay dominates for a lightly loaded link, but the queuing delay dominates for a heavily loaded link.

• In a WAN, the propagation delay is the dominant component.

Packet Losses

• Packet loss can have a significant impact on data transfer rates.

• Packets are lost for two reasons:

  • queue overflow
  • noise

Queue overflow only occurs for heavily loaded links. Packet loss due to noise is primarily dependent on the link media. It is more significant for unguided media.