
PRS28.4G
IBM Packet Routing Switch
Architecture
Page 22 of 131
prs28.03.fm
August 31, 2000
2.2 Expansion Modes
2.2.1 Speed Expansion
The PRS28.4G device is internally composed of two islands operating in speed expansion. It is a 16 x 16
shared-buffer device, with port speed of 1.77Gb/s. Each port is composed of two byte streams, one corre-
sponding to the master island, one to the slave island.
The device supports two-way speed expansion, that is, two devices can be combined to increase the port
speed to 3.2Gb/s. Supported packet lengths range from 64 to 80 bytes, in increments of four. However, when
two devices operate in speed expansion, packets of length 128 to 160, in increments of eight, are also
supported.
Note:
The PRS28.4G implementation allows for only two (2) devices to be combined in external speed
expansion.
Speed expansion allows multiple devices to be connected in parallel to increase the port speed while keeping
the number of ports constant, as shown in the
Speed Expansion Block Diagram
.
In each case, the LU size seen by one island (either the master or the slave island of one device) is equal to
the packet length divided by the speed expansion factor (port speed divided by the island port speed of 0.88
Gb/s)
Figure 3: Speed Expansion Block Diagram
Table 1: Speed Expansion Shared Memory Buffering Capacities
Speed Expansion Mode
Switch
Configuration
Gb/s per
port
Packet Length
(bytes)
LU Size
(bytes)
Buffering
(packets)
Speed
Expansion Factor
Single device, no internal speed
expansion
16 x 16
1.77
64 - 80
32 - 40
256
2
Single device, with internal speed
expansion
8 x 8
3.54
64 - 80
16 - 20
256
4
Two devices speed expanded,
no internal speed expansion
16 x 16
3.54
64 - 80
16 - 20
512
4
Two devices speed expanded,
no internal speed expansion
16 x 16
3.54
128 - 160
32 - 40
256
4
Slave
Buffer
Control
16 Outputs
16 Inputs
Sync + Address
Master
Buffer
Control