Vergita --
An interesting question, as it covers the three major techniques for
managing data collision among network participants. You may find this
quick reference to IEEE standards for each of the networking protocols
helpful:
Rx4Tech
"Technical Glossary" (Dec. 13, 2002)
http://www.rx4tech.com/Glossary/zero.htm
A. CSMA
Carrier Sense Multiple Access (CSMA) is the technique used by Ethernet
networks. As Byte Magazine wrote in 1991, it uses the same etiquette
as a polite conversation: "listen before talking."
All network nodes monitor the carrier signal. When there is no
signal, the channel is assumed to be free. Ethernet actually uses a
CSMA/CD or "collision detection" scheme. Collisions can still occur
when two net nodes start sending (after detecting silence on the
channel). In the event of a collision, each node backs off and waits
a different period of time before resending. The amount of time for
each node is pre-set by an algorithm.
The wireless 802.11 standard uses CSMA/CA or "collision avoidance."
The method is used because the wireless stations have no way to detect
collisions WHILE sending. The sender transmits an intention to send
(some term it a "jamming signal") then transmits afterwards. An ACK
or acknowledgement message is sent by the receiving station to ensure
that no collision occurs. If no ACK is received, the message is
re-transmitted.
You’ll find a definition of both forms of CSMA in the Austin Wireless
glossary:
Austin Wireless
"Glossary"
http://www.austinwireless.net/cgi-bin/index.cgi/Glossary
B. TOKEN PASSING
In a token-passing scheme, the right to the network is controlled by a
"token" (special bit patterns) that move from node to node. The token
is created by one device on the network and if it is turned off or
fails, another node takes on the job of token creation.
Tokens are held during transmission, then passed during silence or
when a transmission is complete. One device on the networks is
usually designated the token manager and certain machines can be given
more access to tokens.
Token ring networks have received large commercial support from IBM.
These networks are popular in some types of installations, such as
machine controls, because they are deterministic: a node can hold the
token during a critical period.
C. ALOHA
This protocol has an interesting history: according to Newton’s
Telecom Dictionary,(Telecom Library Inc., New York, 1991) this
“transmit at will” protocol got its name when the University of Hawaii
was given a dying satellite with lots of bandwidth but relatively
little data to send. It was used to send data among researchers in
the Pacific islands.
Aloha allows the network device to transmit whenever it wants to (as
opposed to collision sensing or token passing networks). If an
acknowledgement is received, it continues to transmit. If not, it
starts again. This type of NAP is perfect in a broadcast or radio net
where channels may be difficult to assign to a node.
There are actually two versions of Aloha: "pure ALOHA" and "slotted
Aloha".
With "pure Aloha":
1. node sends out a packet at any time
2. node waits for acknowledgement from destination
3. if no acknowledgement received, message resent after a random
period
With "slotted Aloha":
1. time is divided into “slots” and nodes send out packets at the
start of slots.
2. if re-transmission is necessary, the node waits for a random number
of time slots before re-sending.
The "slotted Aloha" technique saves wasted time in the cases where two
nodes transmit simultaneously, as can happen with pure Aloha.
An excellent diagram of these two techniques (and the general CSMA
case) is shown in Dr. David Al-Dabass lecture notes:
Nottingham Trent University
"MSc in Industrial Computing Systems – Computer Communications Lecture
6"
ducati.doc.ntu.ac.uk/uksim/dad/Lecture%20Notes/
Comp%20Comms/Comp%20Comms%20-6.ppt
D. EMPTY SLOT
This is a special type of ring network, in which the ring has “slots”
into which data can be transmitted. At the outset all slots are
empty, then they are filled by nodes which are transmitting. When
data is received, the slot is marked "empty" again and passed along
the network.
This page includes some interesting Macromedia animations showing the
operation of a token ring and a token bus network to demonstrate it:
Informatics Institute
"Medium Access Control Layers"
http://www.ii.metu.edu.tr/~ion504/demo/lan/html/medium.html
Google search strategy:
"network access protocols" + Aloha
CSMA + "collision detection" + "collision avoidance"
"empty slot" + "ring network"
If any portion of this answer needs clarification, please don’t
hesitate to ask before rating the answer.
Best regards,
Omnivorous-GA |
