The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

17.2. Switch Data Receive Path Functions

Figure 17-3 depicts the path followed by data received from a given input port. The sections that follow examine each of the logical blocks within the receive path.

17.2.1. Port Interfaces (Receive)

The Media Access Control (MAC) and Physical Layer receiver (PHY) comprise the traditional network interface functions. On the receive side, the PHY takes the electrical or optical signals from the medium and decodes them into a bit, nibble, or byte stream as appropriate for the particular technology in use. The PHY will generally be different for different media (for example, twisted pair or optical fiber) and different technologies (for example, data rate and LAN type). The MAC performs the necessary framing and validity checking to reconstruct properly formed frames from data stream presented by the PHY.

Typically, the receiving MAC will compute the Frame Check Sequence (FCS) value in real-time while receiving data from the PHY. In the event of an invalid FCS, the MAC will discard the frame. Other than possibly incrementing an error counter for management purposes, there is no need for a switch to continue to process a frame received in error.^[] The receiving MAC will normally maintain most of the real-time port-based statistics counters discussed in Chapter 14 (for example, RMON Ethernet statistics).

^[] In the special case of a cut-through switch, a frame containing an FCS error may still be processed and forwarded. As discussed in Chapter ...