Image Processing Reference
In-Depth Information
destination address to checksum ( bytes including preamble and start of delimiter). If the data
portionofaframeislessthanbytes,thepadieldisusedtoillouttheframetotheminimumsize.
There are two reasons for this minimum size limitation. First, it makes it easier to distinguish valid
frames from “garbage.” Because of frame truncation, stray bits and pieces of frames frequently appear
on the cable. Second, it prevents a node from completing the transmission of a short frame before the
first bit has reached the far end of the cable, where it may collide with another frame. For a -Mbps
Ethernet with a maximum length of  m and four repeaters, the minimum allowed frame time or
slot time is . µs, which is the time required to transmit  bytes at  Mbps [].
Because of low medium access overhead, Ethernet uses a simple algorithm for operation of the
network and has almost no delay at low network loads []. No communication bandwidth is used
to gain access to the network compared with TP protocols. However, Ethernet is a nondeterministic
protocol and does not support any message prioritization. At high network loads, message colli-
sions are a major problem because they greatly affect data throughput and time delays may become
unbounded []. The Ethernet “capture” effect existing in the standard BEB algorithm, in which a
node transmits packets exclusively for a prolonged time despite other nodes waiting for medium
access, causes unfairness and substantial performance degradation []. Based on the BEB algorithm,
a message may be discarded after a series of collisions; therefore, end-to-end communication is not
guaranteed. Because of the required minimum valid frame size, Ethernet uses a large message size to
transmit a small amount of data.
Several solutions have been proposed for using this form of Ethernet in control applications [].
For example, every message could be time-stamped before it is sent. This requires clock synchro-
nization, however, which has not traditionally been easy to accomplish [], although the IEEE 
standard has recently emerged to enable clock synchronization over networks []. Various schemes
based on deterministic retransmission delays for the collided packets of a CSMA/CD protocol result
in an upper-bounded delay for all the transmitted packets. However, this is achieved at the expense of
inferior performance to CSMA/CD at low-to-moderate channel utilization in terms of delay through-
put []. Other solutions also try to prioritize CSMA/CD (e.g., LonWorks) to improve the response
time of critical packets []. To a large extent these solutions have been rendered moot with the
proliferation of switched Ethernet as described below. On the other hand, many of the same issues
reappear with the migration to wireless Ethernet for control.
23.3.4.2 Switched Ethernet (CSMA/CA)
Switched Ethernet utilizes switches to subdivide the network architecture, thereby avoiding colli-
sions, increasing network efficiency, and improving determinism. It is widely used in manufacturing
applications. The main difference between switch- and hub-based Ethernet networks is the intelli-
gence of forwarding packets. Hubs simply pass on incoming traffic from any port to all other ports,
whereas switches learn the topology of the network and forward packets to the destination port only.
In a starlike network layout, every node is connected with a single cable to the switch as a full-duplex
point-to-point link. Thus, collisions can no longer occur on any network cable. Switched Ethernet
relies on this star cluster layout to achieve this collision-free property.
Switches employ the cut-through or store-and-forward technique to forward packets from one
port to another, using per-port buffers for packets waiting to be sent on that port. Switches with cut-
through first read the MAC address and then forward the packet to the destination port according
to the MAC address of the destination and the forwarding table on the switch. On the other hand,
switches with store-and-forward examine the complete packet first. Using the CRC code, the switch
will first verify that the frame has been correctly transmitted before forwarding the packet to the
destination port. If there is an error, the frame will be discarded. Store-and-forward switches are
slower, but will not forward any corrupted packets.
 
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