IBM SG24-4817-00 User Manual

Page 40

This soft copy for use by IBM employees only.

1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

┌────────────────┬───────────────┬───────┬───────┬────────────────┐
│ TB │

TBE

│LCBAul │

│

LCBAuh

│ ─┐

├────────┬───────┴───────────────┼───────┴───────┼────────────────┤

│ RH (8 byte)

│

│ NBA │

│

│ ─┘

├────────┼───────────────┬───────┴───────────────┴───────┬─────┬──┤
│

GFC

│ VPI │

VCI

│ PT

│CL│ ── ATM Header

├────────┴───────────────┴───────────────────────────────┴─────┴──┤ (4 byte: without HEC)
│

│

ATM payload (48 byte)

│

├─────────────────────────────────────────────────────────────────┤
│

Future Use

│

└─────────────────────────────────────────────────────────────────┘

TB:

Target Blade

TBE:

Target Blade Extension

LCBAul: Leaf Control Block Address up(inbound) lower port
LCBAuh: Leaf Control Block Address up(inbound) higher port
NBA:

Next Buffer Address

F1:

Format Field 1st

F2:

Format Field 2nd

GFC:

Generic Flow Control

VPI:

Virtual Path Identifier

VCI:

Virtual Channel Identifier

PT:

Payload Type

CL:

Cell Loss Priority

HEC:

Header Error Control

Figure 11. Internal Cell Format of the I B M 8285 Nways A T M Workgroup Switch

3.2.1.2 Switching without the Switch Chip

When no expansion chassis is connected, the IBM 8285 Base Unit implements a
direct connection between CAD_up and CAD_down. This means that inbound
cells would undergo the following process:

1. The SFE_up strips the HEC from valid cells and forwards the cell to the

CAD_up.

2. The CAD_up prepares the internal cell and forwards it directly to the

CAD_down.

3. The CAD_down uses the RH information to determine which ports to forward

the cell to, strips the internal header, and forwards the 52-byte cell to the
SFE_down.

ATM Workgroup Solutions: Implementing the 8285 ATM Switch