Congestion Control Overview; Current Restrictions - Cabletron Systems CSX1200 User Manual

USER'S GUIDE
-- the rate at which data frames may be sent into the network without incurring congestion. This is
generally accepted as the end-to-end available bandwidth at which frame relay service devices may
enjoy sustained frame transmission. By definition this must be less than the throughput that the
actual physical access link can support. However, for short periods of time, service devices may
exceed this rate by defined values. This excess is known as the excess information rate and is
defined as the bandwidth available above and beyond the committed rate. The reason this is
possible is because statistically, not each PVC within the access will make use of its complete
bandwidth allocation. Busy PVCs may essentially borrow bandwidth from underutilized PVCs.
The Frame Relay software has the capability to transmit data above the committed information rate
up to the excess information rate. Note that the sum of the committed and the excess information
rates must not exceed the rate defined by the physical link. These rates are user-configurable
options.
C C O
ONGESTION ONTROL
Congestion occurs when traffic arriving at a resource, whether network or user equipment, exceeds
that node's capacity. Congestion notification in the device plane is used to inform the equipment
(at the ingress point to the network) of the congestion, and allows the user equipment to initiate
congestion avoidance procedures. The intent is to reduce the negative effects on both network and
user equipment: the user equipment should take corrective action to reduce the congestion, or to
notify the source that throughput has been exceeded. Congestion control is very important in
providing reliable frame relay services. Congestion can be detected in two ways, implicitly and
explicitly. Implicit indications are provided by lost frames whereas explicit congestion indications
are provided for within the frame relay protocol.
C R
URRENT ESTRICTIONS
Currently, only PVC (Permanent Virtual Circuit) frame relay connections are implemented.
Through configuration, PVC connections establish a permanent association between two DTEs.
The only types of facilities to be supported for frame relay access are channeled interfaces such as
T1 and BRI. However, currently only 1 port per channeled interface is supported until SVC
standards are available.
Frame relay supports only a single Permanent Virtual Circuit connecting any two given systems.
To illustrate this point, the following diagram shows a frame relay network configuration that
would be allowed:
CSX1200
LAN
POWER SERVICE TX RX 10BASE - T B1 B3 B5 B7
B2 B4 B6 B8
"SITE1"
ALLOWED
216 CyberSWITCH
VERVIEW
DLCI 1 -> NE2
B-CHANNELS E1 ONLY
B9 B11 B13 B15 B17 B19 B21 B23 B25 B27 B29 B31
E1 D D T1
B10 B12 B14 B16 B18 B20 B22 B24 B26 B28 B30 L1
DLCI 2 ->NE 3
Frame Relay
CSX1200
LAN B-CHANNELS E1 ONLY
POWER B1 B3 B5 B7 B9 B11 B13 B15 B17 B19 B21 B23 B25 B27 B29 B31
SERVICE TX RX 10BASE - T
E1 T1
D D
B2 B4 B6 B8 B10 B12 B14 B16 B18 B20 B22 B24 B26 B28 B30 L1
"SITE2"
CSX1200
LAN B-CHANNELS E1 ONLY
POWER B1 B3 B5 B7 B9 B11 B13 B15 B17 B19 B21 B23 B25 B27 B29 B31
SERVICE TX RX 10BASE - T
E1 T1
D D
B2 B4 B6 B8 B10 B12 B14 B16 B18 B20 B22 B24 B26 B28 B30 L1
"SITE3"