Port speed This is the speed of the port on the router where the connection
initiates from the central office, and can be as high as a T-1 of
1.536 Mbps.This is sometimes called the burst rate of the connection.
Committed Information Rate (CIR) This is the circuit speed the
provider guarantees you’ll get all the time, regardless of how many subscribers
are on the frame cloud.
Committed Burst Size (Bc) This is the maximum volume of data
the network agrees to move through the frame cloud under normal
working conditions.
Excess Burst Size (Be) Under normal working conditions, this is the
amount of data above and beyond the Bc mentioned in the preceding
bullet.
Discard Eligible (DE) This is the Be data marked as lower priority
than Bc data; if the frame cloud gets congested, Be data marked with its
DE bit set can be discarded to help reduce frame cloud congestion.
For most customers, the CIR is one half of the port speed, so a 256 Kbps circuit
would have a CIR of 128 Kbps.You pay for the CIR, and a marginal
amount higher for the port speed. But, if your traffic flow exceeds the CIR, and
the frame cloud is congested, then the carrier can discard your packets at its own
judgment to reduce the traffic in the cloud.This means your traffic flows must
slow down to account for the congestion.
For the most part, Frame Relay works fairly efficiently. But if your connection
must remain reliable and not experience discarding of packets, then your
only option is to use a leased line circuit (shown in Figure 11.6).
Leased lines can easily exceed three times the cost of a Frame Relay pipe,
because the connection is 100 percent dedicated from the carrier to your connection.
Figure 11.6 shows two sites connected via a leased line, which is directly connected
to the central office. In some leased lines, the router in the central office is a
massive unit that can host hundreds of connections.This figure has been broken
out slightly to show that in a leased line connection, there are patch panels between
devices, but only to create the physical circuit directly between devices.
The benefit is that at whatever speed you subscribe, you get it on a constant
basis regardless of the number of people subscribed to the carrier.Your connection
is truly independent, but you’ll most certainly pay for that privilege. In VoIP
systems, if the sites are within a few miles of one another, leased lines are usually