Information Technology Reference
In-Depth Information
Each node on the network must be able to detect collisions and be capable of transmitting
and receiving simultaneously. These nodes either connect onto a common Ethernet connec-
tion or can connect to an Ethernet hub. Nodes thus contend for the network and are not guar-
anteed access to it. Collisions generally slow the network.
CSMA/CD
Ethernet
uses carrier sense, multiple access with
collision detection (CSMA/CD).
Nodes
monitor the bus (or Ether) to determine if
it is busy. A node wishing to send data waits for an
idle condition then transmits its message.
Collisions
can occur when two nodes transmit at
the same time, thus nodes must monitor the cable
when they transmit.
When
a collision occurs, both nodes stop
transmitting frames and transmit a jamming signal.
This
informs all nodes on the network that a
collision has occurred.
Each
of the nodes involved in the collision then
waits a random period of time before attempting a
re-transmission.
As each
node has a random delay time then there
can be a prioritisation of the nodes on the network.
CSMA/CD
Ethernet
uses carrier sense, multiple access with
collision detection (CSMA/CD).
Nodes
monitor the bus (or Ether) to determine if
it is busy. A node wishing to send data waits for an
idle condition then transmits its message.
Collisions
can occur when two nodes transmit at
the same time, thus nodes must monitor the cable
when they transmit.
When
a collision occurs, both nodes stop
transmitting frames and transmit a jamming signal.
This
informs all nodes on the network that a
collision has occurred.
Each
of the nodes involved in the collision then
waits a random period of time before attempting a
re-transmission.
As each
node has a random delay time then there
can be a prioritisation of the nodes on the network.
Two nodes transmit
at the same time
Two nodes transmit
at the same time
Node detect there
has been a collision
Node detect there
has been a collision
Nodes transmit a
jamming signal
Nodes transmit a
jamming signal
All computers have access to
a common bus at the same time
All computers have access to
a common bus at the same time
All computers have access to
a common bus at the same time
Nodes wait a random
period before retransmitted
Nodes wait a random
period before retransmitted
Common bus
Common bus
Common bus
Figure 26.1
Ethernet transmission
26.2 IEEE standards
The IEEE are the main standards organization for LANs and they refer to the standard for
Ethernet as CSMA/CD. Figure 26.
2
shows how the IEEE standards for CSMA/CD fit into
the OSI model. The two layers of the IEEE standards correspond to the physical and data
link layers of the OSI model. On Ethernet networks, most hardware complies with IEEE
802.3 standard. The MAC layer allows many nodes to share a single communication chan-
nel. It also adds the start and end frame delimiters, error detection bits, access control infor-
mation, and source and destination addresses. Each frame also has an error detection scheme
known as cyclic redundancy check (CRC).
LLC (IEEE 802.2)
Data link
Media access
control (IEEE 802.3)
Physical
Figure 26.2
Standards for IEEE 802 LANs
