HP auroraTempo, User Manual

Page: 231 / 300

Issue 1a - 10/99 8-17
Introduction to Frame Relay
There is also a mechanism in Frame Relay for an end user
device to prioritize its frames. It does this by setting the
DE (Discard Eligibility) bit in the frame header. The DE
bit tells the network to ‘discard these frames first’. By
marking some traffic as DE, Frame Relay users can
prioritize their non-DE traffic.
The network also has the ability to set a frame’s DE bit.
For example, this may be used to prevent a high
bandwidth device, such as a graphics workstation, from
taking all the bandwidth from a low bandwidth device.
Committed Information Rate & Burst Size
In order to be able to guarantee a certain level of service and
control access to the Frame Relay network service providers
offer a number of service parameters. These are :
• Committed Information Rate (CIR). The CIR is the
minimum rate of service which can be expected
from a virtual circuit, under normal conditions.
Transmissions in excess of the CIR may be
discarded if there is congestion in the network.
However, it is also possible to get a better rate than the
CIR when there is available bandwidth.
• Committed Burst Size (Bc). This is the maximum
number of bits that the network attempts to deliver,
under normal conditions, within a given time period
(Tc). The CIR is equal to the committed burst size
divided by time interval Tc (CIR = Bc/Tc)
• Excess Burst Size (Be). This is the maximum number of
bits by which the user may exceed the Committed
Burst Size, within a given time period (Tc). The
network will attempt to send these but they are likely
to be discarded during periods of congestion.