CSMA with Collision Detection (CSMA/CD)

CSMA with Collision Detection (CSMA/CD)

Carrier Sense Multiple Access with Collision Detection (CSMA/CD) is a network protocol for carrier transmission that operates in the Medium Access Control (MAC) layer. It senses or listens whether the shared channel for transmission is busy or not, and defers transmissions until the channel is free. 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.

Algorithms

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 counter.

·        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.

The following flowchart summarizes the algorithms:

•             Though this algorithm detects collisions, it does not reduce the number of collisions.

·     It is not appropriate for large networks performance degrades exponentially when more stations are added.

 

Ethernet

 

Ethernet is the traditional technology for connecting devices in a wired local area network (LAN) or wide area network (WAN), enabling them to communicate with each other via a protocol -- a set of rules or common network language. Ethernet describes how network devices can format and transmit data so other devices on the same local or campus area network segment can recognize, receive and process the information. An Ethernet cable is the physical, encased wiring over which the data travels.

Connected devices accessing a geographically localized network with a cable -- that is, with a wired rather than wireless connection -- likely use Ethernet. From businesses to gamers, diverse end users depend on the benefits of Ethernet connectivity, which include reliability and security.

Compared to wireless LAN (WLAN) technology, Ethernet is typically less vulnerable to disruptions. It can also offer a greater degree of network security and control than wireless technology since devices must connect using physical cabling. This makes it difficult for outsiders to access network data or hijack bandwidth for unsanctioned devices.

Why is Ethernet used?

Ethernet is used to connect devices in a network and is still a popular form of network connection. For local networks used by specific organizations -- such as company offices, school campuses and hospitals -- Ethernet is used for its high speed, security and reliability.

Ethernet initially grew popular due to its inexpensive price tag when compared to the competing technology of the time, such as IBM's Token Ring. As network technology advanced, Ethernet's ability to evolve and deliver higher levels of performance, while also maintaining backward compatibility, ensured its sustained popularity. Ethernet's original 10 megabits per second throughput increased tenfold to 100 Mbps in the mid-1990s, and the Institute of Electrical and Electronics Engineers Inc. (IEEE) continues to deliver increased performance with successive updates. Current versions of Ethernet can support operations up to 400 gigabits per second (Gbps).

Advantages and disadvantages

Ethernet has many benefits for users, which is why it grew so popular. However, there are a few disadvantages as well.

Advantages

·        relatively low cost;

·        backward compatibility;

·        generally resistant to noise;

·        good data transfer quality;

·        speed;

·        reliability; and

·        data security -- common firewalls can be used.

Disadvantages

·        It is intended for smaller, shorter distance networks.

·        Mobility is limited.

·        Use of longer cables can create crosstalk.

·        It does not work well with real-time or interactive applications.

·        Increased traffic makes the Ethernet speed go down.

·        Receivers do not acknowledge the reception of data packets.

·        When troubleshooting, it is hard to trace which specific cable or node is causing the issue.

Ethernet vs. Wi-Fi

Wi-Fi is the most popular type of network connection. Unlike wired connection types, such as Ethernet, it does not require a physical cable to be connected; data is transmitted through wireless signals.

Differences between Ethernet and Wi-Fi connections

Ethernet connection

·        transmits data over a cable;

·        limited mobility -- a physical cable is required;

·        more speed, reliability and security than Wi-Fi;

·        consistent speed;

·        data encryption is not required;

·        lower latency; and

·        more complex installation process.

Wi-Fi connection

·        transmits data through wireless signals rather than over a cable;

·        better mobility, as no cables are required;

·        not as fast, reliable or secure as Ethernet;

·        more convenient -- users can connect to the internet from anywhere;

·        inconsistent speed -- Wi-Fi is prone to signal interference;

·        requires data encryption;

·        higher latency than Ethernet; and

·        simpler installation process.

How Ethernet works

IEEE specifies in the family of standards called IEEE 802.3 that the Ethernet protocol touches both Layer 1 (physical layer) and Layer 2 (data link layer) on the Open Systems Interconnection (OSI) network protocol model.

Ethernet defines two units of transmission: packet and frame. The frame includes not just the payload of data being transmitted, but also the following:

·        the physical media access control (MAC) addresses of both the sender and receiver;

·        virtual LAN (VLAN) tagging and quality of service (QoS) information; and

·        error correction information to detect transmission problems.

Each frame is wrapped in a packet that contains several bytes of information to establish the connection and mark where the frame starts.

Engineers at Xerox first developed Ethernet in the 1970s; Ethernet initially ran over coaxial cables. Today, a typical Ethernet LAN uses special grades of twisted-pair cables or fiber optic cabling. Early Ethernet connected multiple devices into network segments through hubs -- Layer 1 devices responsible for transporting network data -- using either a daisy chain or star topology.

However, if two devices that share a hub try to transmit data at the same time, the packets can collide and create connectivity problems. To alleviate these digital traffic jams, IEEE developed the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol, which enables devices to check whether a given line is in use before initiating new transmissions.

Later, Ethernet hubs largely gave way to network switches. Because a hub cannot discriminate between points on a network segment, it can't send data directly from point A to point B. Instead, whenever a network device sends a transmission via an input port, the hub copies the data and distributes it to all the available output ports.

In contrast, a switch intelligently sends any given port only the traffic intended for its devices rather than copies of any and all the transmissions on the network segment, thus improving security and efficiency.

Like with other network types, involved computers must include a network interface card (NIC) to connect to Ethernet.

Types of Ethernet cables

The IEEE 802.3 working group approved the first Ethernet standard in 1983. Since then, the technology has continued to evolve and embrace new media, higher transmission speeds and changes in frame content:

·        802.3ac was introduced to accommodate VLAN and priority tagging.

·        802.3af defines Power over Ethernet (PoE), which is crucial to most Wi-Fi and Internet Protocol (IP) telephony deployments.

·        802.11a, b, g, n, ac and ax define the equivalent of Ethernet for WLANs.

·        802.3u ushered in 100BASE-T -- also known as Fast Ethernet -- with data transmission speeds of up to 100 Mbps. The term BASE-T indicates the use of twisted-pair cabling.

Gigabit Ethernet boasts speeds of 1,000 Mbps -- 1 gigabit or 1 billion bits per second (bps) -- 10 GbE, up to 10 Gbps, and so on. Network engineers use 100BASE-T largely to connect end-user computers, printers and other devices; to manage servers and storage; and to achieve higher speeds for network backbone segments. Over time, the typical speed of each connection tends to increase.

Ethernet cables connect network devices to the appropriate routers or modems, with different cables working with different standards and speeds. For example, the Category 5 (Cat5) cable supports traditional and 100BASE-T Ethernet, the Category 5e (Cat5e) cable can handle GbE and Category 6 (Cat6) works with 10 GbE.

Ethernet crossover cables, which connect two devices of the same type, also exist, enabling two computers to be connected without a switch or router between them.

 

 

Gigabit Ethernet

, a transmission technology based on the Ethernet frame format and protocol used in local area networks (LANs), provides a data rate of 1 billion bits per second (one gigabit). Gigabit Ethernet is defined in the IEEE 802.3 standard and is currently being used as the backbone in many enterprise networks.