Texas Instruments TMS320C645x DSP - Manual (Page 25)

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2.4.2 Multiple Access Protocol
EMAC Functional Architecture
Nodes in an ethernet local area network are interconnected by a broadcast channel. As a result, when an
EMAC port transmits a frame, all of the adapters on the local network receive the frame. Carrier sense
multiple access with collision detection (CSMA/CD) algorithms are used when the EMAC operates in
half-duplex mode. When operating in full-duplex mode, there is no contention for use of a shared medium,
because there are exactly two ports on the local network.
Each port runs the CSMA/CD protocol without explicit coordination with the other ports on the ethernet
network.
Within a specific port, the CSMA/CD protocol is as follows:
1. The port obtains data from upper layer protocols at its node, prepares an ethernet frame, and puts the
frame in a buffer.
2. If the port senses that the medium is idle, it starts to transmit the frame. If the port senses that the
transmission medium is busy, it waits until it senses no signal energy (plus an inter-packet gap time)
and then starts to transmit the frame.
3. While transmitting, the port monitors for the presence of signal energy coming from other ports. If the
port transmits the entire frame without detecting signal energy from other ethernet devices, the port is
finished with the frame.
4. If the port detects signal energy from other ports while transmitting, it stops transmitting its frame and
instead transmits a 48-bit jam signal.
5. After transmitting the jam signal, the port enters an exponential back off phase. Specifically, when
transmitting a given frame, after experiencing a number of collisions in a row for the frame, the port
chooses a random value that is dependent on the number of collisions. The port then waits an amount
of time which is multiple of this random value, and returns to Step 1.
SPRU975B – August 2006 Ethernet Media Access Controller (EMAC)/Management Data Input/Output (MDIO) 25
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