scope:

This document defines a transport layer of the Serial Storage Architecture (SSA) that runs SSA-S2P and SSA-S3P while running on SSA-PH2.

The goals of SSA-TL2 are to:

a) provide an Extended Distance Option;

b) provide support for higher data rates in the physical layer 2 (SSA-PH2);

c) enhance packet formats and addressing methods;

d) define a transport layer acceptable to vendors looking for an evolution from parallel SCSI and systems designers looking for opportunities to more fully exploit the capabilities inherent to a serial bus; and

e) cover other capabilities that fit within the scope of SSA-TL2 that may be proposed during the development phase by the participants in the project.

This document defines the transport layer of the Serial Storage Architecture (SSA). SSA defines a serial interface hierarchy to be used for purposes within its distance and performance characteristics, including but not limited to storage subsystems. This standard is intended to be used with an upper layer protocol [e.g., SCSI-2 Protocol (ANSI X3.294-1996) or SCSI-3 Protocol (ANSI NCITS 309-1997)] and a physical layer [e.g., SSA Physical Layer 2 (ANSI NCITS 307-1997)].

SSA-TL2 characteristics

The essential characteristics of SSA-TL2 are as follows:

a) SSA-TL2 is optimized to minimize gate count while maintaining function and performance;

b) SSA-TL2 provides a flexible addressing scheme to allow the configuration of a Web into dedicated connections, strings, loops, and complex configurations including a switch;

c) the Web supports peer-to-peer communication with frame multiplexing between any pair of ports;

d) the Web is self-configuring without the use of address switches;

e) the link supports point-to-point physical connections only. Strings and loops are created by using dual port nodes with cut-through routing;

f) an Extended Distance Option exists that allows 10 km between nodes through the use of additional buffering and an increased ACK time-out;

g) allows operation at a distance of 2.5 km at a line data rate of 10 MB/s, and at a distance of 340 m at a line data rate of 40 MB/s. The Extended Distance Option allows operation at a distance of 10 km at a line data rate of 40 MB/s. The actual data rates and distances depend on the cable assembly and the drivers and receivers used;

h) the link makes optimum use of the physical medium by using full-duplex communication to avoid arbitration overhead and turn-around delay;

i) full duplex communication achieves an aggregate 80 MB/sec bandwidth (40 MB/sec in each direction) between two ports;

j) the frames are constructed with a minimum framing overhead (e.g., only 8 characters of overhead for a 128-byte data field);

k) an architected error recovery procedure provides transparent frame recovery after a transmission error;

l) a Wrap mode is provided to allow fault isolation at a node;

m) an optional line fault detector detects failures in the link;

n) cyclical paths allow a Web to be configured providing redundant access to a node;

o) the transport layer is capable of simultaneously supporting multiple upper-level protocols (e.g., SCSI-2, SCSI-3, IPI-3, TCP/IP, etc.);

p) the 128-byte frame size minimizes the gate count needed for buffers.

This standard defines the following functions:

a) the protocol (e.g., framing, addressing, and flow control);

b) link management (e.g., buffering, port states, resets, configuration, and error recovery).