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In today's economy, corporations need to make the most of opportunities made possible by the Internet, while managing performance issues (access, reliability, and availability) and seeking suppliers who can deliver quality services. AT&T understands the criticality of these issues; we know that there are vital business applications running on our IP backbone. Therefore, we have provided this site for you to check on and compare the performance measurements of the AT&T's US IP backbone as well as the performance measurements across Trans Atlantic and Trans Pacific paths.
Descriptions of Performance Measurements
In order for you to understand the performance measurements given on this site, following is an explanation of the AT&T Global IP network performance measurements of Delay (or latency), Loss (or its converse deliverability) and IP Backbone Availability.
Delay (or latency)
is the round-trip transmission time for a data packet to travel between
two end points on the Internet. Delay is measured in milliseconds
and is most noticeable when an interactive application is being used,
such as a real time order entry or inventory checking system.
Delay can be the result
of several different factors. These factors include the speed
of the computers involved, the size of the facility connecting the end
computer to the Internet, overloaded Web servers, poor network routing
(too many hops or miles to travel) or congestion on the network. Only
some of these factors are within the Internet Service Provider’s control
(routing and congestion). Others, such as overloaded Web servers and
the size of the facility connecting the end computer, have nothing to
do with the performance of an IP backbone.
To judge and compare
IP network performance, one needs a measurement that is not influenced
by delays that are outside of the provider's control. This measure is
called network roundtrip delay. Network roundtrip delay measures
the round-tip transmission between two points on the Internet Service
Provider’s network.
Network delay can be affected by transmission delay (the speed of the network circuits) or insertion delay (the rate at which routers can forward the packets). It can also be affected by queuing delay (the rate at which packets arrive at the router) or by propagation delay (the time it takes for light to traverse the physical distance between source and destination).
Loss is defined as
the percentage of packets lost in a transmission. The opposite of loss,
deliverability, is defined as 100 minus the percentage packet loss and
is the percentage of successfully delivered packets. Thus, a network
with a one percent packet loss would be said to have a 99 percent deliverability
rating. To understand the intricacies of packet loss measurement, an
explanation of how backbone routers operate is helpful.
Backbone routers process
thousands of simultaneous packet transmissions. As part of this
processing the routers use temporary spaces called “buffers” to
store the packets until they can be forwarded to their destinations.
If the buffers fill up because the routers cannot transmit the packets
fast enough or because the outgoing circuits are too slow, packets can
be dropped for lack of buffer space.
The nature of the TCP/IP
protocol, on which the Internet runs, allows for the possibility of
packets being lost or dropped and instructs that those dropped packets
be retransmitted. However, a high level of retransmission can
increase the level of traffic on the network, causing network congestion
and resulting in a slowing down of the network. If the original
loss is due to congestion on the network, the resulting flood of retransmitted
packets can heighten the congestion on the network. Therefore,
packet loss significantly affects the network’s ability to transport
data. Thus, a network which offered low delay and high availability,
but which had a high packet loss will provide a low level of performance.
However, it should be noted that as in the case of delay, several factors other than packet loss and retransmission could contribute to an apparent network slow down. Once again to measure the Internet Service Provider, it is important to measure network loss – loss that occurs within the IP network that is not due to other non-network causes such as overloaded Web servers.
Backbone availability
is the time that the backbone is available to route customer packets
to every edge of the backbone. Availability is mostly governed by physical
factors in the network but also may be affected by routing failures.
AT&T’s IP Global Network Performance Measurements
Delay, Loss and Availability
are the three significant measures of an Internet Service Provider’s
network performance. These measurements can be used to compare the AT&T
IP network with other providers on IP network performance related issues.
In general, our measurement methodology uses active probes to measure
most of our metrics. Below we describe the measurements and how they
are presented on the AT&T Global IP Network Performance site.
To start with, a general
understanding of or familiarity with the site’s performance measurement
pages is helpful. The AT&T Global IP Network Performance Site provides
five pages with measurement information: the Home page, the Current
Performance page, the Network Delay page, Network Loss page and the
Previous three Months’ Averages page.
The average (city-pair)
round-trip delay is used to summarize the round-trip delay time between
different source and destination city-pairs. The average round-trip
delay is measured in milliseconds. The measurements are made from
one edge of the backbone to another for U.S domestic, from U.S to cities
in Europe for Trans Atlantic and from U.S to cities in Asia Pacific
region. They are taken continuously but are aggregated and reported
every 15 minutes across U.S all city-pair links, Trans Atlantic city
pairs and Trans Pacific city pairs. When delay data is unavailable,
no value is shown.
Delay Thresholds
The AT&T Global IP network has threshold standards for the network delay metrics. These threshold values are utilized to signal to our network technicians and engineers that a potential problem may be arising before it affects the applications running on the network.
Thresholds are represented on the Current Performance page and the Network Delay and Loss pages by various colors. For example, if a box is colored in yellow, this represents that the performance for that city pair is rated as fair. Similarly if a box is colored in red, this represents that the performance for that city pair has reached the alert threshold.
The city pair delay thresholds on the AT&T Global IP network will differ slightly depending on the city pair. These differences exist because as described earlier, delay is highly dependent on the distance between the cities. This is due to the fact that propagation delay, which is defined as the time it takes for light to traverse the physical distance between the source and destination, is one of the major components of delay.
To help minimize the AT&T Global IP network delay, our thresholds are set very competitively – The highest boundary setting of a yellow threshold for the round trip delay between a U.S domestic city-pair is 95 milliseconds. The highest boundary setting of a red threshold for a U.S domestic city-pair is set at 120 milliseconds. It should be noted that in order to provide a high standard level of performance many city-pairs have significantly lower thresholds.
The percentage loss
is used to summarize deliverability. For example, if the percentage
loss is shown as 1 percent, it implies that the deliverability is 99
percent.
The loss probes are
sent continuously, although the city-pair loss figures are updated every
15 minutes. Once again the measurement is a round trip measurement between
one city to another. Thus the loss between each city-pair is measured.
When loss data is unavailable, no value is shown.
Note that it is possible
to have values for delay but not for loss and vice versa.
Loss Thresholds
The AT&T Global IP network also has threshold standards for the IP backbone loss metrics. These threshold measurements are utilized to signal to our network technicians and engineers that a potential problem may be arising before it affects the applications running on the network.
Thresholds are represented on the Current Performance page and the Network Delay and Loss pages using various colors. For example, if a box is colored in yellow, this represents that the performance for that city pair is rated as fair. Similarly if a box is colored in red, this represents that the performance for that city pair is at the alert threshold.
The yellow threshold is set at loss greater than 5% for a given city-pair, while the red threshold is triggered when loss is greater than 10% between source-destination city-pairs.
Monthly Averages
Measurements for the previous 3 calendar months’ averages are also presented on the site for the following metrics:
U.S domestic: Round trip average delay, loss, and availability.
The monthly average round trip delay is the round trip delay averaged over all the city-pairs in continental U.S network over all the measurements made in each of the previous 3 calendar months.
The monthly average loss is the round trip loss averaged over all the city-pairs in continental U.S network averaged over all the measurements made in each of the previous 3 calendar months.
Average Availability
The Backbone Availability
is the percentage of time that the continental U.S. AT&T IP backbone
was available to accept customer traffic. It is computed network-wide
and averaged over all the measurements made in each of the previous
3 calendar months.
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