VITESSE

SEMICONDUCTOR CORPORATION

Data Sheet

VSC880

High Performance 16x16

Serial Crosspoint Switch

Page 18

G52191-0, Rev 4.2

01/05/01

© VITESSE SEMICONDUCTOR CORPORATION • 741 Calle Plano • Camarillo, CA 93012

Tel: (800) VITESSE • FAX: (805) 987-5896 • Email: prodinfo@vitesse.com

Internet: www.vitesse.com

3.0 Cell Mode

3.1 Overview

In Cell Mode (CMODE = HIGH), a more sophisticated arbitration scheme can be supported by using the

VSC870 and the VSC880 switch in conjunction with a central (user defined) scheduler. In this mode, only fixed

length data packets (cells) can be supported. A cell clock is connected to the switch chip, and the switch chip

distributes the cell clock to all connected transceivers. The transceivers adjust their transmit cell clocks so that all

transceivers send the first word of a cell at such time that it arrives at the switch chip aligned to the switch cell clock

(see serial link operation above). In this mode, messages containing port card queue information are sent to the

central scheduler using an out of band control bus. Arbitration and flow control information are then sent back to the

port cards through the out of band control bus. The scheduler then configures the switch matrix by using the parallel

interface. A picture of a cell based system is shown below. Multiple switch chips can be used in parallel to increase

system bandwidth (see Application Note 32 “Design Guide for a Cell Based Switch with Central Control”).

Figure 8: Cell Based System

3.2 Data Encoding Format

The data word and command word formats are described in section 1.0. Command words use the overhead bits

set to 00. Data words can have overhead bits 01, 10 or 11. The user can use these bits for signaling to the receiving

port card. Information such as start of frame and end of frame can be passed through the switch in this manner.

3.3 Receiver Operation

If CMODE is HIGH, the receiver at each port examines the two overhead bits (B[1:0]) of the received word to

determine the word type. If the word is a command word sent from the port card, the switch will respond based on the

type of command specified in the C[4:0] bit field. If an IDLE word arrives at the end of the cell clock period and it

does not have an embedded cell clock, a cell clock error is flagged. If it is a data word, it is sent to the switch fabric

to be routed to its destination along with the user defined overhead bits.

Port Card

Trans

System

Queuing

Port Card

Trans

Switch

Chip

Switch Card

System

Queuing

Scheduler/

Arbiter

VSC870

VSC870

VSC880

Out-of-band Control Bus