Parvus PRV-1059, User Manual

Page: 12 / 27

PRV-1059 User Manual
Page 12 of 27 MNL-0490-01 RevD4 REF. ECO-3163 Effective: 01 Feb 10
Chapter 4 Operational Description
This chapter explains the operation of the switch.
Switch Data Flow
The switching portion of the PRV-1059 receives good packets from the MAC’s, processes them, and forwards
them to the appropriate MACs for transmission. Processing the frames is the key activity, and involves the
Ingress Policy, the Queue Controller, the Output Queues, and the Egress Policy blocks shown below. Each port
has it’s own Ingress and Egress Policies
.
Media Access Controllers
Five independent Media Access Controllers (MACs) perform the 802.3 protocol functions, including frame
formatting, frame stripping, CRC checking, CSMA/CD enforcement, collision handling, etc. Before a packet can
be sent out, the transmit block must check if the line is available for transmission. The transmit line is available
all the time when the port is in full-duplex mode, but the line could be busy receiving a packet if the port is in half-
duplex mode. If the line is busy, the transmitter waits by deferring its transmission. When the line is available, the
transmission insures a minimum interpacket gap of at least 96 bits has occurred and then transmits a 56-bit
preamble and an 8-bit Start of Frame Delimiter (SFD) ahead the basic frame.
For half-duplex mode, the switch also monitors the collision signals while it is transmitting. If a collision is
detected (i.e. both transmitter and receiver of a half-duplex MAC are active at the same time), the MAC transmits
a JAM pattern and then delays the retransmission for a random time period determined by the IEE 802.3 backoff
algorithm. In full-duplex mode, the collision signal and backoff algorithm is ignored.
Backoff
In half-duplex mode, the Ethernet Switch’s MACs implement a truncated binary exponential backoff
algorithm, starting with a randomly small backoff time followed by progressively longer and longer
random backoff times. The random times prevent two or more MACs from always attempting re-
transmission at exactly the same time. The progressively longer backoff times give a wider random
range giving congested links a better chance of finding a winning transmitter. The MACs will rest the
progressively longer backoff time circuit after 16 consecutive re-transmit trials.