CSMA/CD (carrier sense multiple access/collision detection)

In ETHERNET, CSMA is used on a tapped coaxial cable to which all the communicating devices are connected. On the coaxial cable, in addition to sensing carrier, it is possible for the transceivers to detect collisions. 

This variation of CSMA is referred to as carrier sense multiple access with collision detection (CSMA-CD).

CSMA/CD (carrier sense multiple access/collision detection) is a MAC (media access control) protocol. It defines how network devices respond when two devices attempt to use a data channel simultaneously and encounter a data collision. The CSMA/CD rules define how long the device should wait if a collision occurs.

CSMA/CD is a modification of pure carrier-sense multiple access (CSMA). CSMA/CD is used to improve CSMA performance by terminating transmission as soon as a collision is detected, thus shortening the time required before a retry can be attempted.

The collision detection technology detects collisions by sensing transmissions from other stations. On detection of a collision, the station stops transmitting, sends a jam signal, and then waits for a random time interval before retransmission.

The algorithm of CSMA/CD is:

• When a frame is ready, the transmitting station checks whether the channel is idle or busy.
• If the channel is busy, the station waits until the channel becomes idle.
• If the channel is idle, the station starts transmitting and continually monitors the channel to detect collision.
• If a collision is detected, the station starts the collision resolution algorithm.
• The station resets the retransmission counters and completes frame transmission.

The algorithm of Collision Resolution is:

• The station continues transmission of the current frame for a specified time along with a jam signal, to ensure that all the other stations detect collision.
• The station increments the retransmission counters.
• If the maximum number of retransmission attempts is reached, then the station aborts transmission.
• Otherwise, the station waits for a backoff period which is generally a function of the number of collisions and restart main algorithm.

This algorithm detects collisions but it does not reduce the number of collisions. It is not appropriate for large networks; performance degrades exponentially when more stations are added.

Carrier Sense Multiple Access (CSMA)

This method was developed to decrease the chances of collisions when two or more stations start sending their signals over the datalink layer. Carrier Sense multiple access requires that each station first check the state of the medium before sending.

It senses or listens whether the shared channel for transmission is busy or not, and transmits if the channel is not busy. Using CSMA protocols, more than one users or nodes send and receive data through a shared medium that may be a single cable or optical fiber connecting multiple nodes, or a portion of the wireless spectrum.

The versions of CSMA access modes are−

(i) I-persistent CSMA

• In this method, station that wants to transmit data continuously senses the channel to check whether the channel is idle or busy.

• If the channel is busy, the station waits until it becomes idle.

• When the station detects an idle-channel, it immediately transmits the frame with probability 1. Hence it is called I-persistent CSMA.

• This method has the highest chance of collision because two or more stations may find channel to be idle at the same time and transmit their frames.

• When the collision occurs, the stations wait a random amount of time and start allover again.

Drawback of I-persistent
• The propagation delay time greatly affects this protocol.

Let us suppose, just after the station I begins its transmission, station 2 also became ready to send its data and senses the channel. If the station I signal has not yet reached station 2, station 2 will sense the channel to be idle and will begin its transmission. This will result in collision.

Even if propagation delay time is zero, collision will still occur. If two stations became .ready in the middle of third station’s transmission, both stations will wait until the transmission of first station ends and then both will begin their transmission exactly simultaneously. This will also result in collision.

(ii) Non-persistent CSMA

• In this scheme, if a station wants to transmit a frame and it finds that the channel is busy (some other station is transmitting) then it will wait for fixed interval of time. After this time, it again checks the status of the channel and if the channel is free it will transmit.

• A station that has a frame to send senses the channel.

• If the channel is idle, it sends immediately.

• If the channel is busy, it waits a random amount of time and then senses the channel again.

Advantage of non-persistent

• It reduces the chance of collision because the stations wait a random amount of time. It is unlikely that two or more stations will wait for same amount of time and will retransmit at the same time.

Disadvantage of non-persistent

• It reduces the efficiency of network because the channel remains idle when there may be stations with frames to send. This is due to the fact that the stations wait a random amount of time after the collision.

(iii) p-persistent CSMA

• This method is used when channel has time slots such that the time slot duration is equal to or greater than the maximum propagation delay time.

• Whenever a station becomes ready to send, it senses the channel.

• If channel is busy, station waits until next slot.

• If channel is idle, it transmits with a probability p.

• With the probability q=l-p, the station then waits for the beginning of the next time slot.

• If the next slot is also idle, it either transmits or waits again with probabilities p and q.

• This process is repeated till either frame has been transmitted or another station has begun transmitting.

• In case of the transmission by another station, the station acts as though a collision has occurred and it waits a random amount of time and starts again.

Advantage of p-persistent

• It reduces the chance of collision and improves the efficiency of the network.

Ethernet

The original Ethernet was created in 1976 at Xerox's Palo Alto Research Center (PARC).

Ethernet is a set of technologies and protocols that are used primarily in LANs. Ethernet can also be used in MANs and even WANs. It was first standardized in the 1980s as IEEE 802.3 standard.

Classic Ethernet is the original form of Ethernet that provides data rates between 3 to 10 Mbps. The varieties are commonly referred as 10BASE-X. Here, 10 is the maximum throughput, i.e. 10 Mbps, BASE denoted use of baseband transmission, and X is the type of medium used.

There are a number of versions of IEEE 802.3 protocol. The most popular ones are -

IEEE 802.3: This was the original standard given for 10BASE-5. It used a thick single coaxial cable into which a connection can be tapped by drilling into the cable to the core. Here, 10 is the maximum throughput, i.e. 10 Mbps, BASE denoted use of baseband transmission, and 5 refers to the maximum segment length of 500m.

IEEE 802.3a: This gave the standard for thin coax (10BASE-2), which is a thinner variety where the segments of coaxial cables are connected by BNC connectors. The 2 refers to the maximum segment length of about 200m (185m to be precise).

IEEE 802.3i: This gave the standard for twisted pair (10BASE-T) that uses unshielded twisted pair (UTP) copper wires as physical layer medium. The further variations were given by IEEE 802.3u for 100BASE-TX, 100BASE-T4 and 100BASE-FX.

IEEE 802.3i: This gave the standard for Ethernet over Fiber (10BASE-F) that uses fiber optic cables as medium of transmission.

Frame Format of Classic Ethernet

 

The main fields of a frame of classic Ethernet are -

• Preamble: It is the starting field that provides alert and timing pulse for transmission. In case of classic Ethernet it is an 8 byte field and in case of IEEE 802.3 it is of 7 bytes.
• Start of Frame Delimiter: It is a 1 byte field in a IEEE 802.3 frame that contains an alternating pattern of ones and zeros ending with two ones.
• Destination Address: It is a 6 byte field containing physical address of destination stations.
• Source Address: It is a 6 byte field containing the physical address of the sending station.
• Length: It a 7 bytes field that stores the number of bytes in the data field.
• Data: This is a variable sized field carries the data from the upper layers. The maximum size of data field is 1500 bytes.
• Padding: This is added to the data to bring its length to the minimum requirement of 46 bytes.
• CRC: CRC stands for cyclic redundancy check. It contains the error detection information.

Frame Format of IEEE 802.3 Ethernet

Preamble:  The first field of the 802.3 frame contains 7 bytes (56 bits) of alternating Os and Is that alerts the receiving system to the coming frame and enables it to synchronize its input timing. The pattern provides only an alert and a timing pulse. The 56-bit pattern allows the stations to miss some bits at the beginning of the frame. The preamble is actually added at the physical layer and is not (formally) part of the frame.

Start frame delimiter (SFD): The second field (l byte: 10101011) signals the beginning of the frame. The SFD warns the station or stations that this is the last chance for synchronization. The last 2 bits is 11 and alerts the receiver that the next field is the destination address.

Destination address (DA): The DA field is 6 bytes and contains the physical address of the destination station or stations to receive the packet.

Source address (SA): The SA field is also 6 bytes and contains the physical address of the sender of the packet.

Length or type: This field is defined as a type field or length field. The original Ethernet used this field as the type field to define the upper-layer protocol using the MAC frame. The IEEE standard used it as the length field to define the number of bytes in the data field. Both uses are common today.

Data: This field carries data encapsulated from the upper-layer protocols. It is a minimum of 46 and a maximum of 1500 bytes.

CRC: The last field contains error detection information, in this case a CRC-32.

IEEE 802

In 1985, the Computer Society of the IEEE started a project, called Project 802, to set standards to enable intercommunication among equipment from a variety of manufacturers.

IEEE 802 is a family of Institute of Electrical and Electronics Engineers (IEEE) standards for local area networks (LAN), personal area network (PAN), and metropolitan area networks (MAN). The IEEE 802 LAN/MAN Standards Committee (LMSC) maintains these standards.

The services and protocols specified in IEEE 802 map to the lower two layers (data link and physical) of the seven-layer Open Systems Interconnection (OSI) networking reference model.

IEEE 802 LAN/MAN  Standards:

IEEE 802.1     Standards for LAN/MAN bridging and management and remote media access control (MAC) bridging

IEEE 802.2     Standards for Logical Link Control (LLC) standards for connectivity

IEEE 802.3     Ethernet Standards for Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

IEEE 802.4     Standards for token passing bus access

IEEE 802.5     Standards for token ring access and for communications between LANs and MANs

IEEE 802.6     Standards for information exchange between systems

IEEE 802.7     Standards for broadband LAN cabling

IEEE 802.8     Fiber-optic connection

IEEE 802.9     Standards for integrated services, like voice and data

IEEE 802.10   Standards for LAN/MAN security implementations

IEEE 802.11  Wireless Networking – "WiFi"

IEEE 802.12   Standards for demand priority access method

IEEE 802.14   Standards for cable television broadband communications

IEEE 802.15.2            Bluetooth and Wi-Fi coexistence mechanism

IEEE 802.15.4            Wireless Sensor/Control Networks – "ZigBee"

IEEE 802.15.6            Wireless Body Area Network[16] (BAN) – (e.g. Bluetooth low energy)

IEEE 802.16   Wireless Networking – "WiMAX"

IEEE 802.24   Standards for Logical Link Control (LLC) standards for connectivity

The relationship of the 802 Standard to the traditional OSI model is

The IEEE has subdivided the data link layer into two sublayers:

- logical link control (LLC) and

- media access control (MAC).

Logical link control (LLC) : 

In IEEE Project 802, flow control, error control, and part of the framing duties are collected into one sublayer called the logical link control. Framing is handled in both the LLC sublayer and the MAC sublayer.

A single LLC protocol can provide interconnectivity between different LANs because it makes the MAC sublayer transparent.

LLC defines a protocol data unit (PDU). The header contains a control field used for flow and error control. The two other header fields define the upper-layer protocol at the source and destination that uses LLC. These fields are called the destination service access point (DSAP) and the source service access point (SSAP).

Media access control (MAC): 

Media access control defines the specific access method for each LAN. For example, it defines CSMA/CD as the media access method for Ethernet LANs and the tokenpassing method for Token Ring and Token Bus LANs.

In contrast to the LLC sublayer, the MAC sublayer contains a number of distinct modules; each defines the access method and the framing format specific to the corresponding LAN protocol.

Local Area Network

LAN stands for Local Area Network, is a group of computers located in the same room, on the same floor, or in the same building that are connected to form a single network. Local area networks (LANs) allow users to share storage devices, printers, applications, data, and other network resources.

Features of LAN’s :

• Network size is limited to a small geographical area, presently to a few kilometers.
• Data transfer rate is generally high. They range from 100 Mbps to 1000 Mbps.
• In general, a LAN uses only one type of transmission medium, commonly category 5 coaxial cables.
• A LAN is distinguished from other networks by their topologies. The common topologies are bus, ring, mesh, and star.
• The number of computers connected to a LAN is usually restricted. In other words, LANs are limitedly scalable.
• IEEE 802.3 or Ethernet is the most common LAN. They use a wired medium in conjuncture with a switch or a hub. Originally, coaxial cables were used for communications. But now twisted pair cables and fiber optic cables are also used. Ethernet’s speed has increased from 2.9 Mbps to 400 Gbps.

Wireless LANs (WLAN)

Wireless LANs use high-frequency radio waves instead of cables for communications.They have an Access Point or a wireless router or a base station for transferring packets to and from the wireless computers and the internet.

Most WLANs are based on the standard IEEE 802.11 or WiFi.