CA2200008A1

CA2200008A1 - Packet delay measurements

Info

Publication number: CA2200008A1
Application number: CA002200008A
Authority: CA
Inventors: Rex Benning; Mark Wolff
Original assignee: Newbridge Networks Corp
Current assignee: Nokia Canada Inc
Priority date: 1997-03-14
Filing date: 1997-03-14
Publication date: 1998-09-14

Abstract

A method of measuring packet delay in a packet switched network involves measuring the queuing time for each packet at each node on a connection through the network.
Statistical methods are used to determine a representative value at each node, and these values are summed across the connection. A value representing transmission delay and switching delay is then added to the summed representative values.

Description

~ 02200 008 Packet Delay Measurements This invention relates to a method of measuring the delay of encountered by packets passing through a packet switched network.
A packet network traditionally comprises a series of intermediate nodes that fol ~v~d individual data packets to the next node along the route until the packets reach their destination. Measuring the delay encountered by data frames as they are traverse through such packet networks, such as frame relay networks, traditionally has been solved by several methods. One such method involves occasionally sending a test packet, or ping, through the network. The packet is retnrnefl~ back through the network, to the sender when it reaches the receiving node, and the sending node uses the time elapsed between sending and receiving the packet to give an indication of the net~,vork delay.
Another method is to add timing information, which can be used by the receiving node to determine network delay, into the. packet itself.
While the two methods cited above can give indications of delay, they have shortcomings. For example, the ping method suffers from numerous limitations including the fact that the size of the ping packet likely does not match a given data frame's size and that it is sent at a di~e~ instant in time than the data frame. It may therefore have a di~elelll delay associated with it, and more seriously, the ping packet may not follow the same route as a user data frame, again resulting in a dirr~ L network delay value.
The second method of adding timing information into the packet results in a proprietary implemen~ation that would not work in standards based networks, such as frarne relay networks.
An object of the invention is to alleviate these problems.
According to the present invention there is provided a method of measuring packet delay in a packet switched network comprising the steps of measuring the queuing time for each packet at each node on a connection through the network; applying statistical methods to determine a representative value at each node; summing said representative values across the connection; and adding to said summed representative values a value representing tr~n~mi~sion delay and switching delay.

~~ 02200008 The delay of a packet through a packet network is the sllmm~tion of the time it takes for the packet to traverse each physical link of the network (T_tr~nqmi.cqion for each link), which is a function of the line speed of the link and the packet size, plus the switching delay incurred when the packet is moved from one physical link to another through the network (T_switching for each sw;tch) normally a constant value incurred at each switching point, plus queing delay at each switching point (T_queing), which is a function of how many other packets are attempting to use the same physical link at the same time. T_tr~nsmisqion and T_switching are essentially fixed values for a given packet route.
The invention is based on the realization that in effect the only variable is in fact T_queing. Thus by measuring and storing over set time intervals the exact T_queing at each switch for every packet of a connection where timing information is desired, applying statistical mathematics to the resulting values such as mean, variance and standard deviation, and ~.".. i--~ up these values across the involved switches, then adding in T_tr~nqmiqsion and T switching values, an accurate statistical view of the delay encountered by packets of a connection can be realized.
This statistical view is equivalent to the information arrived at by using test packets, for although a test packet returns an actual delay value, several test packets must be sent to arrive at a true value since a single test packet cannot be trusted to be typical of delays encountered, similar statistical analysis of the test packets results must be done.
The invention will now be described in more detail, by way of example only, withreference to the accompanying drawings, in which the single ~igure shows a packet switched network.
In the Figure, users I, 2, are communicating over a virtual connection 2 established over a frame relay network 3 consisting of switch nodes A, B, C. The virtual connection is carried over physical links 4, 5.
As each packet traverses the virtual connection it encounters a tr~nqmi.q~ion delay T-tr~n~mi.qqion, which is the time taken to traverse the physical links 4, 5, and a switching delay T-switching, which is a substantially constant delay incurred at each switching point. These values can be treated as constants that can be determined for each connection.

-T_queing, for exarnple at node B, is calculated by using the line speed of the link 4 and the number of bytes of data from other frames that are using the physical link but are queued ahead of the frame in question. The switch is capable of knowing thisinformation when it processes the frame. For example, a frarne arrives at a switch after taking T_tr~n~mi~ion to arrive from its previous location. This frarne is switched inside the switch B taking T_switching time, and is queued to the physical link 5. Due to its arrival time (and other factors), this frarne is behind 3 frarnes of identical length. ~t follows then that r_queuing would equal 3 * T_tr~n~mi~ion, as the other frames must leave the switch before the target frame can use the physical link.
The total delay for this switch (or "hop") would be T_tr~n~mi~sion + T_switching+ T_queing where for this exarnple T_queuing = 3 * T_tr~n~mi~ion This value is stored in the switch for a given connection, and for each frame switched on the connection the results are stat;stically added. By adding tlle statistical results from all switches involved in a connection path, ~he statistical delay encountered by frames on a connection is accurately measured.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of measuring packet delay in a packet switched network comprising the steps of:
a) measuring the queuing time for each packet at each node on a connection through the network;
b) applying statistical methods to determine a representative value at each node, c) summing said representative values across the connection; and d) adding to said summed representative values a value representing transmissiondelay and switching delay.

2. A method as claimed in claim 1, wherein the queuing delay of a connection is determined from the line speed of the link and the number of bytes of data from other packets that are using the physical link carrying said connection and are queued ahead of the packet in question.

3. A method as claimed in claim 2, wherein the queuing delay is stored in the associated node, and the delays incurred by each packet switched on the connection the are statistically added.