Image Processing Reference
In-Depth Information
Ethernet is an RA network, also often referred to as CSMA. Each node listens to the network and
can start transmitting at any time that the network is free. Typically, once the network is clear, a
node must wait for a specified amount of time (the interframe time) before sending a message.
To reduce collisions on the network, nodes wait an additional random amount of time called the
backoff time before they start transmitting. Some types of messages (e.g., MAC layer ACKs) may be
sent after a shorter interframe time. Priorities can be implemented by allowing for shorter interframe
times for higher priority traic. However, if two nodes start sending messages at the exact same time
(or if the second node starts transmitting before the first message arrives at the second node), there
will be a collision on the network. Collisions in Ethernet are destructive; the data are corrupted and
the messages must be resent.
There are three common flavors of Ethernet: () hub-based Ethernet, which is common in office
environments and is the most widely implemented form of Ethernet, () switched Ethernet, which is
more common in manufacturing and control environments, and () wireless Ethernet. Each of these
is discussed in detail below.
23.3.4.1 Hub-Based Ethernet (CSMA/CD)
Hub-based Ethernet uses hub(s) to interconnect the devices on a network; this type of Ethernet is
common in the office environment. When a packet comes into one hub interface, the hub simply
broadcasts the packet to all other hub interfaces. Hence, all of the devices on the same network receive
thesamepacketsimultaneously,andmessagecollisionsarepossible.Collisionsaredealtwithutilizing
the CSMA/CD protocol as specified in the IEEE . network standard [,,].
This protocol operates as follows: when a node wants to transmit, it listens to the network. If the
networkisbusy,thenodewaitsuntilthenetworkisidle;otherwise,itcantransmitimmediately
(assuming an interframe delay has elapsed since the last message on the network). If two or more
nodes listen to the idle network and decide to transmit simultaneously, the messages of these trans-
mitting nodes collide and the messages are corrupted. While transmitting, a node must also listen
to detect a message collision. On detecting a collision between two or more messages, a transmit-
ting node transmits jam bits and waits a random length of time to retry its transmission. This
random time is determined by the standard binary exponential backoff (BEB) algorithm: the retrans-
mission time is randomly chosen between and
i
slot times, where
i
denotes the
i
ith collision
event detected by the node and one slot time is the minimum time needed for a round-trip trans-
mission. However, after collisions have been reached, the interval is fixed at a maximum of
slots. After collisions, the node stops attempting to transmit and reports failure back to the node
microprocessor. Further recovery may be attempted in higher layers [].
The Ethernet frame format is shown in Figure . []. The total overhead is
(
)
)
bytes. The data packet frame size is between and bytes. There is a nonzero minimum data
size requirement because the standard states that valid frames must be at least bytes long, from
(=
+
Bytes
7
1
6
6
2
0-1500
0-46
4
Destination
address
Source
address
Start of
delimiter
Data
length
Preamble
Data
Pad
Checksum
46-1500 bytes
Overhead = 22 bytes
OH = 4 bytes
FIGURE .
Ethernet (CSMA/CD) frame format; Byte interframe space not shown.
