Sybex - CCNA 2.0 Study Guide (640-507).pdf - Cifo

500 Chapter 10 Wide Area Networking Protocols
Frame Relay Congestion Control
In this section we will define how the Frame Relay switch handles congestion
problems.
DE (Discard Eligibility) When a Frame Relay router detects congestion
on the Frame Relay network, it will turn the DE bit on in a Frame Relay
packet header. If the switch is congested, the Frame Relay switch will discard
the packets with the DE bit set first. If your bandwidth is configured
with a CIR of zero, the DE will always be on.
FECN (Forward-Explicit Congestion Notification) When the Frame
Relay network recognizes congestion in the cloud, the switch will set the
FECN bit to 1 in a Frame Relay packet header. This will indicate to the
destination DCE that the path just traversed is congested.
BECN (Backward-Explicit Congestion Notification) When the switch
detects congestion in the Frame Relay network, it will set the BECN bit in
a Frame Relay packet and send it to the source router, telling it to slow
down the rate at which it is transmitting packets.
Committed Information Rate (CIR)
Frame Relay provides a packet-switched network to many different customers
at the same time. This is a great idea because it spreads the cost of the
switches among many customers. However, Frame Relay is based on the
assumption that not all customers need to transmit constant data all at the
same time. Frame Relay works best with bursty traffic.
Think of Frame Relay as a party line. Remember party lines? That is when
many people on your block had to share the same phone number. Okay, I am
showing my age here, but understand that party lines were created on the
assumption that few people needed to use the phone each day. If you needed
to talk excessively, you had to pay for the more expensive dedicated circuit.
Frame Relay works somewhat on the same principle, except many devices
can transmit at the same time. However, if you need a constant data-stream
connection, then Frame Relay is not for you. Buy a dedicated, point-to-point
T-1 instead.
Frame Relay works by providing a dedicated bandwidth to each user,
who is committed to that bandwidth at any given time. Frame Relay providers
allow customers to buy a lower amount of bandwidth than what they