December 13, 2000, 3:30 PM — In previous columns, I discussed methods for prioritizing traffic on IP networks to
achieve multiple levels of Quality of Service (QoS).
Following QoS technologies is like reading a series of novels in which there is
always one more book -- after a while, each new book starts to resemble the others.
Similarly, vendors are constantly trying to improve the QoS capabilities of their
products, but the more things change, the more they stay the same.
A case in point is Cisco's introduction of the Committed Access Rate (CAR) feature
in IOS 12.0. CAR is designed to give network operators a tool for guaranteeing
bandwidth to specific traffic flows on an IP network. CAR implements both
classification services and policing through rate limiting. CAR is, in a sense, a kind
of time division multiplexing (TDM), in which a network device transmits various
traffic streams through a single link for specific time intervals. TDM has been around
for decades.
CAR uses a
t4/qcpolts.htm#19709">token bucket to limit traffic rates. With token
buckets, an administrator tells the router how much bandwidth each type of data may use
on a specified link. The system then puts one token into a bucket for every bit per
second that each traffic flow is configured for. When a packet arrives from the
specified traffic flow, a token is removed from the bucket for every bit in the packet,
until the bucket is empty. Once the tokens are used up, packets are discarded unless
burst mode is enabled. This keeps traffic levels proportional to the amount of tokens.
Cisco lets you configure the router to support traffic bursts by borrowing tokens up to
a specified limit.
This type of traffic policing emulates TDM, but is more flexible since it allows for
bursting. But before implementing CAR on your network, consider that only the
designated traffic flow can take tokens out of the bucket. If there is no
CAR-designated traffic, the bandwidth may not be used by another traffic flow.
This may not be a problem if you have broadband pipes and are willing to reserve
some bandwidth for specific applications, or if you want to ensure that
latency-sensitive traffic always has enough bandwidth. However, you should always
retain enough unreserved bandwidth to support your other applications, or you may make
the problem worse.
Cisco is not the only vendor with bandwidth reservation capabilities; Extreme has a
similar method, called IP TDM, and many others support similar capabilities based on
priority queuing, with guaranteed transmission intervals for the highest-priority
queue.
It's a little ironic: vendors have worked hard to build packet networks with
multiple types of Layer 3 QoS features, but when push comes to shove and your traffic
must get through, TDM lives on!
