Physical Layer Overview

Detailed Physical Layer Description

This section provides an overview of the Physical Layer. A detailed description of the Physical Layer can be found in the chapter entitled “Detailed Physical Layer Description” on page 681.

General

The Physical Layer establishes the physical communications (via link training) with the entity (i.e., the CA, switch, or router) on the opposite end of the link. This includes:

• Maximum path width determination (1, 4, or 12 lanes; a lane is a serial data transmit/receive signal pair).

• Maximum speed determination (currently, only 2.5Gb/s is supported).

• Multi-lane deskew (to align the data on all lanes).

In addition, the Physical Layer:

• Performs training of the physical link to sync up the transmitter in the local port with the receiver in the other port.

• Notifies the Link Layer of the physical link's status.

• On transmission, receives the packet byte stream from the Link Layer, performs the 8-bit to 10-bit character encode of data, serializes the 10-bit character stream, and transmits the serial stream over the wire (or fiber).

• On receive, converts the serial stream to a 10-bit character stream, decodes the 10-bit character stream into an 8-bit byte stream, and passes the packet byte stream to the port's Link Layer.

• Permits transfer rates of 250MB/s in each direction (and, optionally, up to 1000MB/s or 3000MB/s in each direction if four or 12 lanes are implemented).

• Checks incoming packets for code (i.e., character formation) errors and timing (skew) errors.

Three Types of Ports Are Supported

The three types of port implementations are:

• Backplane port.

• Copper cable port.

• Fiber optic port.

Full-Duplex Connection

In a copper (rather than a fiber) implementation, information is transmitted and received across the physical link on two twisted pairs of wires (one for transmit and one for receive; see Figure 6-7 on page 117). Note that a fiber implementation, however, only uses a single conductor for transmit and another for receive (see Figure 6-8 on page 117). Two unassociated data streams may be transferred simultaneously in opposite directions.

Link/Physical Layer Interface

Refer to Figure 6-9 on this page:

• The receive bit stream(s) enters the Physical Layer on 1, 4, or 12 lanes:

  - Each of the bit streams is deserialized into a 10-bit character stream.

  - The 10-bit encoded characters are decoded into 8-bit data or control characters.

  - Each byte is then presented to the Link Layer (on Rcv Stream) along with a Rcv Control signal indicating whether it is a data or a control character.

• The byte-wide transmit stream (Xmit Stream) is presented a byte at a time from the Link Layer to the Physical Layer along with a control signal (Xmit Control) indicating whether the current byte is a control or a data character. The 8-bit byte plus the control/data signal is encoded into a 10-bit character, is serialized, and is transmitted over 1, 4, or 12 lanes.