Media Access Methods



Introduction.  This web page will focus on how networks handle the transmission of signals from multiple computers so that packets reach their destinations.  If there is even a moderate amount of traffic it will be likely that collisions will occur between packets.  What we will examine in particular is who gets to transmit and for how long.

Currently, there are four approaches that are the most popular.  They are contained in the following list.

  • CSMA/CD - Carrier Sense Multiple Access Collision Detection
  • CSMA/CA - Carrier Sense Multiple Access Collision Avoidance
  • Token Passing
  • Demand Priority

Now we will present a short section that focuses on each of these approaches.

CSMA/CD.  This is the most prevalent approach because it is the basis of Ethernet transmission organization.  It is relatively easy to implement and less expensive.  It is not the most efficient.

The following list breaks this into its components.

  • Carrier Sense
    • when a computer wants to transmit it first listens to the cable to see if any other device is currently transmitting
    • it senses the state of the carrier - whether it is busy or not
  • Multiple Sense
    • more than one computer can begin transmitting on the network when no other messages are sensed
  • Collision Detection
    • if at least two devices sense that nothing else is being transmitted they can end up sending at the same instant and causing a collision

When signals collide they are both eliminated from the network.  Then each computer waits a random amount of time and then resends the same signal.  It needs to be a random interval otherwise the devices would likely again transmit simultaneously.

Some authors assess the overall maximum theoretical efficiency of Ethernet LANs to be about 37% of their  bandwidth.  This is not very good at all, but inexpensive relative to other options.  Some authors also state that a maximum of 40 computers should be on an Ethernet LAN segment.  Obviously this depends on the bandwidth available on the network segment.  It also depends on the nature of the traffic and demands on the segment.

CSMA/CA.  This is highly related to CSMA/CD except it is much less trusting.  If a device doesn't sense anything else on the network it doesn't automatically transmit.  Instead it sends a RTS - Request to Send.  This signal announces the devices intention to send.  If another computer attempts the  same thing then these signals will collide so that the actual data will not.  Thus actual data can never collide.

AppleTalk networks make use of this approach.  Unfortunately, it has a negative impact on performance due to the extra overhead of the RTS messages.

Both CSMA/CD and CSMA/CA are known as contention methods because the signals must compete fairly directly for use of the network.  It seems that while CSMA/CA was significantly less efficient for wire-based communications, it has been very important in wireless.

Token Passing.  Token Passing is a non-contention method in that two devices cannot transmit signals at the same time.  Each device needs to wait to get the token before it can transmit.  The token circulates on the network until it reaches a device with data to send.

The most common token passing approach is called Token Ring.  When the token gets to a computer that is waiting to send, the device takes control of the token.  It appends its data to the token signal and puts it back out on the network.  The data has the destination address and the token moves around the network in its established order until it reaches the device with the appropriate address.

When the appropriate receiving device gets the token it takes the data and appends a successful reception message to the token and sends it back around.  The sending computer regains control of the token and then sends more data or puts it back out on the network as free to receive data and direction.

Some authors estimate that Token Rings can make use of about 75% of the network bandwidth.  While considerably more efficient than Ethernet, it is more expensive and slightly more difficult to configure.

Some token passing architectures actually make use of multiple tokens.  One example of this is called FDDI - Fiber Distributed Data Interface.  This will be described in later web pages.

Demand Priority.  This was developed by Hewlett Packard to be used with VG AnyLAN.  This was designed to be a flexible, high speed and efficient replacement to Ethernet.

The demand priority method makes use of multiport hubs that conduct round robin searches of the connected devices looking for requests to transmit.  The underlying topology is a star wired tree.  The hubs can be cascaded off a root hub for centralized control.

The administrator is able to set priorities on certain types of data.  For example, you might choose to give e-media transmissions priority.

It also usually makes use of a cabling with four pairs of wires that enables simultaneous transmission and reception.  Though it can mae use of two pair STP, two or four pair UTP or fiber optic cabling.  The hubs are also configured to limit where the original message is broadcast.  It is restricted to devices connected to the same hub.

Each hub interacts with other hubs and is unaware of specific devices attached to another hub.  Each hub is only aware of devices to which it is attached.

Because not all data goes through all stations it is inherently more secure than Ethernet or Token Ring.

For whatever reasons, there are not a very large selection of vendors who market this LAN product.