Repeaters

Repeater's Purpose

Repeaters (also referred to as retiming repeaters) exists for two reasons:

• A long link between ports may experience weakened signal strength and built-up jitter. The repeater compensates for these conditions by reducing the amount of jitter and amplifying and retransmitting the data.

• A repeater can also be used to interface a copper cable and a fibre optic cable.

Repeater's Basic Operation

Refer to Figure 26-36 on page 757. A repeater deserializes the received bit stream into 10-bit symbols and recovers the clock that was used to transmit the data from the bit stream. This clock is the receive clock and is used to clock the 10-bit symbols into the elastic buffer. The symbols are clocked out of the elastic buffer using the repeater's locally generated transmit clock. As previously described in “Receiver Clock Compensation Logic” on page 726, the frequency of the locally generated transmit clock may vary from that of the recovered receive clock by as much as 400ns. The remote port's transmitter periodically inserts a Skip set into the bit stream. In order to properly align the symbols in the elastic buffer with the locally generated transmit clock, when the control logic associated with the buffer detects the presence of a Skip set in the buffer, it takes the following actions:

• If the frequencies of the two clocks are the same, no action is taken.

• If the transmit clock frequency is greater than the receive clock frequency, a Skip symbol is deleted from the buffer.

• If the transmit clock frequency is less than the receive clock frequency, a Skip symbol is added to the buffer.

As the symbol stream is clocked out of the elastic buffer, each symbol is serialized and clocked out to the transmitter using the repeater's locally generated transmit clock.

Notice that the repeater does not convert the 10-bit symbols into 8-bit characters, and it therefore also doesn't check for code violations or disparity errors.

Elastic Buffer Underflow or Overflow

The elastic buffer is not expected to underflow or overflow. If it should, however, the following actions are taken:

• When an underflow condition is detected, the repeater inserts a PAD symbol.

• When an overflow condition is detected, the repeater replaces two arbitrary but consecutive symbols with a pad (PAD) symbol.

Repeaters Are Invisible

Unlike CAs, routers, and switches, a repeater is programmatically invisible. It cannot be addressed as the destination for a packet (including management packets (i.e., SMPs and GMPs) and Flow Control Packets (FCPs).

Two Repeater Limit

Not more than two retiming repeaters are permitted on a link.

Symbol Boundary Alignment

During link training or retraining (see “Link Training” on page 732), repeated TS1 and TS2 training sequences are transmitted between the two end ports. Repeaters use the COM symbols in this stream to detect symbol boundary misalignment. The periodic Skip sets each also incorporates the COM symbol.

Retiming repeaters cannot generate TS1 or TS2 sets on their own. Instead, they detect periodic unaligned comma (COM) symbols within this sequence to determine the loss-of-symbol boundary alignment.

If a repeater detects the COM symbol pattern (0011111010b or 1100000101b) and it doesn't appear to start on a symbol boundary, this is referred to as a misaligned COM symbol. A repeater must reacquire symbol boundary synchronization when three consecutive misaligned COM symbols are detected.

A repeater disables its symbol boundary synchronization circuitry when five consecutive instances of aligned comma symbols (COM) are detected.

Multiple-Lane Repeater

A multiple-lane repeater has the following operational characteristics:

• The repeater must use the same locally generated transmit clock to transmit data on all lanes in a given direction. The lanes in opposed directions, however, may have independent transmit clock sources.

• The repeater is not required to implement lane-to-lane de-skewing (see “Lane-to-Lane De-Skewing” on page 750), but it may optionally implement this capability.

• A multiple-lane repeater can be shared by multiple links that use a subset of the repeater's lanes. For example, a 4x repeater can be used by up to four 1x links. The 12x repeater can be used by multiple combinations of 1x and 4x links.

• Each lane of a multiple-lane repeater operates independently of the other lanes.