Bursting is one of the features that has made Frame Relay so popular. Bursting enables a user
to transmit data faster than the CIR for a short period of time. Figure 29.5 shows the difference
between the CIR and the access rate and how the burst traffic rate can increase beyond the CIR.
The network controls this bursting capability, and it usually does not result in any additional fees
on the user. There is a catch, though. Some burst traffic has the Discard Eligibility (DE) bit turned
on, indicating excess traffic above CIR. If a Frame Relay switch becomes congested, traffic with
the DE bit set (excess burst traffic) is the first to be dropped.
You will see the following symbols: Bc, Be, and Tc. Committed burst size (Bc)
and excess burst size (Be) are the two types of burst sizes. Each of these sizes is measured over
the committed rate measurement interval (Tc). Bc is the maximum amount of data that the network
can guarantee will be delivered during the time Tc. Be is the amount of traffic by which the
user can exceed the committed burst size.
For example, take a user who buys a Frame Relay circuit with the following characteristics:
1,544Kbps access rate
256Kbps committed information rate
Four-second committed time interval
The user is guaranteed a CIR of 256Kbps over a four-second period. The user could transmit
256Kbps for four seconds, and the network would ensure delivery. The user could alternately
send 1,024Kbps for one second, representing the committed burst. However, for the remaining
three seconds, there would be no guarantee of delivery for the excess burst traffic.
IT Certification CCIE,CCNP,CCIP,CCNA,CCSP,Cisco Network Optimization and Security Tips
Committed Information Rate (CIR) network
The committed information rate (CIR) is the rate at which the provider guarantees to deliver
network traffic. The CIR is always less than or equal to the access rate. The CIR is advertised
in Kbps and is actually averaged over a specified time period, referred to as committed rate
measurement interval (Tc). This is what the cost of the Frame Relay connection is normally
based upon.
network traffic. The CIR is always less than or equal to the access rate. The CIR is advertised
in Kbps and is actually averaged over a specified time period, referred to as committed rate
measurement interval (Tc). This is what the cost of the Frame Relay connection is normally
based upon.
Access Rate
Access rate is the maximum speed at which data can be transferred to the Frame Relay network.
This number denotes the actual line speed of the connection to the provider. In a dedicated circuit,
you would consider this the actual data rate. However, in a Frame Relay network, this is
considered the maximum data rate.
This number denotes the actual line speed of the connection to the provider. In a dedicated circuit,
you would consider this the actual data rate. However, in a Frame Relay network, this is
considered the maximum data rate.
Factors Affecting Performance
Network performance at the router level is affected by three primary factors:
Access rate
Committed information rate (CIR)
Bursting
Each of these has an effect on Frame Relay.
Access rate
Committed information rate (CIR)
Bursting
Each of these has an effect on Frame Relay.
Frame Relay Congestion Control
Frame Relay is optimized for speed and contains little error control. Frame Relay will discard
errored frames and will not attempt to recover from the error either through retransmission or
repair. Ideally, users can send as much data as they want across the network without interference.
However, because user requests for bandwidth often outstrip the network’s capability to
provide bandwidth, a mechanism is needed to handle congestion in the frame switch.
In this section, you will learn about the factors that affect network performance, as well as
methods for handling Frame Relay congestion. The two primary methods of congestion handling
use Frame Relay switches and routers.
errored frames and will not attempt to recover from the error either through retransmission or
repair. Ideally, users can send as much data as they want across the network without interference.
However, because user requests for bandwidth often outstrip the network’s capability to
provide bandwidth, a mechanism is needed to handle congestion in the frame switch.
In this section, you will learn about the factors that affect network performance, as well as
methods for handling Frame Relay congestion. The two primary methods of congestion handling
use Frame Relay switches and routers.
Subscribe to:
Posts (Atom)