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3.4 ATM Backplane / Expansion Unit Connection
As described in Figure 9 on page 18, there are several ATM buses to connect
each component of IBM 8285. The buses between CAP/CADs or CAP/CAD and
the switch chip are called as the ATM backplane. In the IBM 8285, the ATM
backplane is extended to the connection between the base and expansion unit.
When the IBM 8285 is installed with the expansion unit, they are connected by a
special cable, called the
expansion interface cable
, which has 68 pins. On the
other hand, as described in some documents, such as the
IBM 8260 As a
Campus ATM Switch
, all ATM modules have 120 pins for backplane connection.
The difference comes from the number of modules supported by the IBM 8285
and the IBM 8260 whether or not it supports the redundant switch configuration.
Except for the redundant switch function, both of the IBM 8285 and IBM 8260 ATM
backplanes are fully equivalent functionally to an ATM blade.
You can find the pin layout of the IBM 8285 expansion interface in the appendix
of the
IBM 8285 Nways ATM Workgroup Switch: Installation and User
′
s Guide
.
For more information about the IBM 8260 ATM backplane, refer to
IBM 8260 As a
Campus ATM Switch
, SG24-5003.
Table 2 shows the characteristics of typical buses implemented in the IBM 8285.
Table 2. ATM Buses Implemented in the IBM 8285 Nways ATM Workgroup Switch
Bus Location Bus Speed
CAD_Up and CAD_Down (ATM backplane in the base unit) 256 Mbps (32 MHz x 8 bit
parallel)
CP SFE and 25 Mbps HS.SFE_Up/Down 256 Mbps / FDX 1
25 Mbps HS.SFE_Up/Down or 155 Mbps SFE_Up/Down and
CAD_Up/Down
512 Mbps (16 MHz x 32 bit
parallel) / FDX 1
CAD_Up/Down and the switch chip (ATM backplane across the
expansion unit)
256 Mbps / FDX 1
Note
1 FDX means each up (inbound) and down (outbound) works
simultaneously and the maximum capacity should be double (256
Mbps/FDX : 512 Mbps).
As described above, the internal bandwidth for each blade should be 256 Mbps.
However, we have to consider that the IBM 8285 uses a 64-byte internal cell
instead of a 53-byte ATM standardized cell. As a result, the available ATM
bandwidth for the ATM cell transfer should be decreased from the internal
bandwidth, 256 Mbps per blade, to 212 Mbps (256 Mbps x 53/64) per blade by the
overhead of the internal cell header/trailer.
3.5 LAN Emulation Server Functions
The IBM 8285 integrates a Forum-compliant LAN Emulation Server (LES) and
Broadcast and Unknown Server (BUS) functions in the control point. The
LES/BUS functions are performed with or without external a LAN Emulation
Configuration Server (LECS).
The IBM 8285 can support up to two sets of the LES/BUS functions with any
combinations of the types of LAN emulation, token-ring (IEEE 802.5) and Ethernet
Chapter 3. Functional Overview of the IBM 8285 33