Case Study: The Abacus Switch

CS 4594

Goals and Considerations

• Handles cell relay (fixed-size packets)

• Can be modified to handle variable-sized packets.

• Implements multicasting.

• Scalable from few tens to few thousands of input ports.

• Built from integrated circuits (chips).

• Uses channel grouping to reduce hardware complexity.

• Analyzed for throughput, average cell delay, and cell loss ratio.

History

• Development lead by H J Chao

• Described by a series of papers, starting in 1991.

• See copy of 1997 paper.

• Called Abacus because of appearance of the integrated circuit (chip) that is used the construct it.

Components

• Input port controller (IPC) for each input• Multicast Grouping Network (MGN) is first

stage of switch fabric.• Multicast Translation Tables (MMT) • Small Switch Modules (SSM) at second

stage of switch fabric.• Output port controller (OPC) for each

output.

Overall Architecture (see fig 1)

IPC

MulticastGroup

Network

IPC

IPC

.

.

.

.

.

.

N K groups of LxM

SSM OPCOPC

MTT

SSM OPCOPC

SSM OPCOPC

N

MTTMTTMTT

MTTMTTMTTMTT

MTTMTTMTTMTT

Feedback loop

Overall Architecture

• Input port controller (IPC) for each input• Output port controller (OPC) for each output. • Output ports are divided into K groups of M.• Multicast Grouping Network (MGN) reads cells

from IPCs and send cells to the correct output groups.

• Separate small switch modules (SSM) for each group of outputs to send cells to correct OPC.

Input Port Controller

• The IPC– Reads cells from SONET/ATM line into its input

buffer.

– Takes cells from the head of its input buffer and sends these cells to the switching fabric, tagging, grouping, and replicating the cells.

Location of IPCs

IPC

MulticastGroup

Network

IPC

IPC

.

.

.

.

.

.

N K groups of LxM

SSM OPCOPC

MTT

SSM OPCOPC

SSM OPCOPC

N

MTTMTTMTT

MTTMTTMTTMTT

MTTMTTMTTMTT

Input Controller

CellExtraction Input Buffer(FIFO)

From SONET

Feedback from fabric

to MGN

BusOneCell

buffer

MulticastContentionResolution

Unit

P/S

RoutingTable

K lines

Input Port Controller Process

1. If a new cell is to be sent, it examines the cell at the head of the line (HOL) of input buffer and matches VPI/VCI against routing table to determine the set of output port groups (via MP) and the multicast call (via BCN). The cell is tagged with this information and removed from the input buffer.

2. The cell is temporarily stored in a one-cell holding buffer. It is held here in case it needs to be transmitted again.

3. The cell is sent to the distribution network (MGN).

4. The IPC then reads feedback information from MGN to see if cell needs to be sent again. If so, it loops back to step 3. If not, it goes back to step 1 for a new cell.

Multicasting Group Network

• The MGN distributes the cells from the IPCs to the output groups.

• It consists of K routing modules (RM), one for each of the K output groups.

Location in Overall Architecture

IPC

MulticastGroup

Network

IPC

IPC

.

.

.

.

.

.

N K groups of LxM

SSM OPCOPC

MTT

SSM OPCOPC

SSM OPCOPC

N

MTTMTTMTT

MTTMTTMTTMTT

MTTMTTMTTMTT

Feedback loop

Multicasting Group Network

RoutingModule

RoutingModule

RoutingModule

. . .

LxM LxM LxM

N

Routing Module

• Each routing module (RM) listens to all inputs and sends cells to one group of outputs.

• Each RM has inputs from all IPCs.

• Each RM has LxM outputs to a particular group of M outputs. L is called the group expansion ratio.

Multicasting Group Network

RoutingModule

RoutingModule

RoutingModule

. . .

LxM LxM LxM

N

Routing Module

SWE

SWE

SWE

SWE

SWE

SWE

SWE

SWE

SWE

LxM lines to output group

NLinesfrom N inputs

AB AB AB

Routing Module

• A routing module (RM) consists of a N by LxM crossbar 2D array of switch elements (SWE)

• At the top, a set of address broadcasters (AB) generates empty cells with the correct destination address, but low priority.

• The flow through the RM starts at the top at the set of AB and goes down or to the right. At the bottom cells go to the output groups.

• Each switch element has two states “across” (straight through) or “toggle” (turn). It normally sends old cells down toward their destination and new cells down the line.

Switch Elements

• “Across” is the default state, it lets a downward cell continue toward its destination and sends an entering cell over to the right. It happens when the destination of the cell to the north (old cell) does not match the destination of the cell from the west (left) or the priority of the cell on the west (new cell) is less than the priority of the cell from the north.

• “Toggled” happens when the destinations match and the cell on the left has higher priority. It bumps the downward (old) cell over to the right and lets the western (new) cell start heading down to its destination.

Switch Element

toggle stateacross state

Switch Elements

• Switch elements are small integrated circuits

• Implemented in CMOS as the ARC chip– 32 x 32 per IC– 81,000 transistors– 240 MHz

Tables

• Tables contain routing information (see figure 6.18)

• They are located in the– IPC to map the VPI/VCI to the destination and

to an ID (BCN) that can be used to uniquely identify the virtual circuit in the switch

– MMT to map the BCN to the new VPI/VCI and set up last cell replications

Performance

• Throughput

• Average Delay

• Cell loss

Performance depends on

• The type of buffering (input vs. output)• The number of inputs contending for

outputs• The load• The burstiness of the traffic• The amount of grouping (M)• The amount of parallelism (group

expansion ratio L)

Maximum Throughput

• Maximum throughput = utilization at the output port

• For fixed expansion ratio, as the group size increases, throughput starts at .583 and gradually increases to one and burstiness gradually becomes less important (figure 7.7)

• For fixed group size, as the expansion ratio increases, throughput starts at .583 and gradually increases to one and burstiness gradually becomes less important (figures 7.8, 7.9)

Average Delay

• There are two types of delay

– Input-buffered delay

– Output-buffered delay

• Input-buffered delay is small under a light load and can go to infinity as a switch saturates (figure 7.10).

• Output-buffered delay is larger than input-buffered delay for small loads (figure 7.10).

• Unicast traffic does not do as well as multicast traffic (figure 7.11).

Cell Loss

• Cell loss occurs at the input buffers and output buffers, but not in the fabric (MGN)

• See figure 7.12 for input buffer cell loss probability as a function of burstiness and input buffer size (load .9, N =256, M =16)

• See figure 7.13 for output buffer cell loss probability as a function of burstiness and output buffer size (load .9, N =256, M =16)

• For 10-6 cell loss probability, for burst length of 15 input buffer should be close to 50 cells and the output buffer size should be over 300.

• For 10-10 cell loss probability, for burst length of 15, input buffer should be 100 cells (extrapolating).

Extensions to Abacus

• Variable packet

• Scaling up by multistaging the MGN

• Resequencing cells at the output