Media Access Methods
In this module, you will learn the concepts related to media access methods and their impact on today’s network. Basically, a Media Access Method is used to process data transfer onto the network cable.
In communication modes of Media Access Methods, there are three types of which the two devices communicate with each other through network. The messaging types are used to send messages to multiple recipients. A protocol that is connectionless can send broadcast or multicast messages and unicast messages. A protocol that is connection-oriented can only use unicast messaging type because the connection exists between a specific pair of hosts.
Process of Media Access Methods
An access method is defined as a set of rules which processes the data transfer onto the network cable. and also receiving the data from the cable. Access methods should be consistent in handling the data. If different computers use different access methods, the network might fail. Access methods prevent simultaneous access of the cable, The two of the commonly used access methods are:
1. Carrier-Sense Multiple Access with Collision Detection (CSMA/CD)
2. Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA)
Token Passing is also a type of accessing method
CSMA/CD is a type of contention protocol that defines the response when a collision is detected, or when two devices attempt to transmit packages simultaneously. Ethernet allows each device to send messages at any time without having to wait for network permission. Thus, there is a high possibility that devices may try to send messages at the same time. In order to manage collisions, Ethernet uses Carrier Sense Multiple Access/Collision Detection (CSMA/CD).
If two devices transmit the data simultaneously on the network, a collision occurs and each computer backs off and waits a random amount of time. When the collision occurs, both the data packets are discarded and the systems must retransmit the data. The collision detection phase of the transmission process is the most important part of the network protocol. If the system fails to detect the error data packets, the corrupted data may reach the destination system and it is treated as valid data.
The Ethernet network cab is designed so that the packets are large enough to fill the entire network cable with signals before the last bit leaves the transmitting computer. Therefore, the maximum length of the cable must be at least 64 bytes. The computer is capable of detecting a collision on the network, as long as a computer is still in the process of transmitting. On a Fiber optic network, if the system detects signals on both its transmit and receive wires simultaneously, the system identifies that the collision has occurred. If the network cable is too long or if the packet is too short, a system might complete transmitting before the collision occurs.
When a system detects a collision, it immediately stops transmitting the data and starts sending the jamming pattern. The jam pattern serves as a signal to every system on the network, if a collision has taken place, the system should discard any partial packets it may have received and should not attempt to transmit any data until the network has cleared. The system meanwhile waits for a specific period of time before attempting to transmit the data again. This is called the backoff period. The systems involved in a collision, compute the length of their own backoff periods using an algorithm called truncated binary exponential backoff. This is done to avoid causing another collision by backing off for the same period of time.
Therefore, the more systems on the network or the more data the systems transmit over the network, the collisions are more to occur. Collisions cause the delay of data because systems have to retransmit the data packets. When the network traffic increases, the number of collisions increases, and the accumulated delays the effect on the network performance. So, it’s advisable not to run an Ethernet network at high traffic levels. The network traffic can be brought down by installing a bridge or a switch. Or another process is by splitting into two LANs and connecting them with a router.
Carrier Sense Multiple Access/Collision Avoidance is the medium access control method that is used in wireless networks. The wireless networks make use of collision avoidance which is used in wired systems such as IEEE 802.3 and Ethernet. It is a protocol for carrier transmission which prevents collisions in advance.
When a node receives a packet that is to be sent, it checks the channel for clearance and that no other node is transmitting at the time. If the channel is clear, then the packet is sent otherwise, the node waits for a randomly chosen period of time, and then checks again to see if the channel is clear. This period of time is called the backoff factor and is counted down by a backoff counter.
If the channel is clear when the backoff counter reaches zero, the node transmits the packet. If the channel is not clear, the backoff factor is set again and the process is repeated.
The mechanism of a Token Ring local area network is called token passing. Token passing is considered the most efficient MAC mechanism. It provides each system on the network with an equal opportunity to transmit its data without generating any collisions and diminished performance at high traffic levels. The mechanism of token passing works by circulating a special packet called a token around the network. The length of the token is 3-bytes long and contains no useful data.
The main function of the token is to designate which system on the network is allowed to transmit the data. When the computers are in an inactive state, a token ring network is called repeat mode. The systems in this mode receive packets from the network and forward them immediately back to the MAU for transmission to the next port. When the packets are not received, the ring is effectively broken and the network communication stops. When the designated system generates the data packets, the token circulates around the ring to each system. When a system has to transmit the data, it waits for the token to arrive before it can send its data. Only one system on the network can transmit at a time and therefore there can be no collisions on a token ring network unless something is wrong.
Communication modes of Media Access Methods
the communication modes refer to the manner in which the two devices in a network communicate with each other. There are three different modes of communication. They are
- Simplex Communication
- Half-Duplex Communication
- Duplex Communication
In this mode of communication, the network cable or the communication channel can transfer data in only one direction. Simplex communication is permanent unidirectional communication. This type of communication was used by computers in earlier days. For example, the Mainframe computers communicate with the printer in simplex mode. The printer just sends the data and the print command to the printer. The Computer (sender) is not aware of whether the printer is ready for printing or if there are any errors while printing etc. In this mode of communication, the sender is not aware of the receiver’s status.
Connectionless protocols should be used in simplex mode. This mode does not allow any acknowledgment to be sent back to the sender. The best examples of simplex communications are the radio, television, and other public announcement systems. The radio signal is transmitted across and the receiver receives the signal. It is the responsibility of the receiver to check if the message is received correctly and there are no errors. Televisions also use the simplex mode of communication since they cannot talk back to the satellites that are transmitting. Simplex mode o communication can thus be used for broadcasting.
The half-duplex mode communication can send the data in both directions but not simultaneously. It can either use the communication channel to send the data or the same channel can be used to receive the data at a time. In this mode of operation, the receiver detects the error in the data and asks the transmitter to send the corrupted data again.
Walkie-talkies and CB radios are the best examples of half-duplex communication where you will not be able to hear the other person’s conversation currently. A single network cable between any two devices can be considered as the best example for this mode of communication. The network cable can be used for communication between the two devices but cannot be done simultaneously. The figure below shows the half-duplex operation.
Full duplex communication
The full-duplex communication permits data to be sent and received by the same communication channel simultaneously. The full-duplex communication can be compared to a two-way bridge on a two-lane highway. The telephone is the best example of full-duplex communication where both the sender and the receiver can talk simultaneously. The user can üse the cable connection not only for receiving signals for television but the same cable can be used for surfing the internet and telephone connectivity. The figure below shows the full-duplex communication mode.
In networking technology, the messages are sent using a number of tasks that are undertaken in order to successfully transmit the data from one place to another. One of the methods is simply the addressing of the message that is, putting an address on it so that the system transmits it to the right place. The other is transmitting the message, which is sending it to the intended recipient. There are three different kinds of messages which have to be sent from one device to another through the network. They are:
Unicast: is the communication between a single sender and a single receiver through the network. The unicast messages are sent from a single source to a specified destination on LANs within the Internet. The local area network and the IP networks maintain unicast transfer mode and most users are familiar with the standard unicast applications like HTTP, SMTP, FTP, and telnet employ the TCP transport protocol.
Multicast: is the delivery of information to a group of destination systems simultaneously using the most efficient strategy to deliver the messages over each link of the network. The multicast information is sent from one or more points to a set of other points. In this case, there may be more than one senders and the information is distributed to a set of receivers. Multicasting is the networking technique of delivering the same packet simultaneously to a group of destination systems. Multicast messages are sent to group stations that meet a particular set of criteria. These stations are usually related to each other such as serving a common function or being set up into a particular multicast group.
Broadcast: Broadcast is used to send information from one source to multiple systems. One sender sends the information to all connected receivers. Broadcast transmission is supported on most LANs and can be used to send messages to all computers on the LAN. The Network layer protocols also support a form of the broadcast which allows the same packet to be sent to every system in a logical network. The messages are sent to every device on a network. They are used when the information actually communicates to every system on the network, or it is also used when the sending station needs to send to just one recipient, but the address is not recognized.