Unit 3: Multi-Access Mechanism and Ethernet Standards

Random Access Techniques

Random Access Techniques allow multiple devices to share the same communication medium without centralized control. These methods depend on random transmission attempts and require mechanisms to avoid or handle collisions when two or more devices transmit simultaneously.

CSMA (Carrier Sense Multiple Access)

Carrier Sense Multiple Access (CSMA) is a protocol that helps reduce collisions in random access networks by allowing devices to sense whether the channel is idle before transmitting data.

• Carrier Sense: The device checks if the channel is free before transmitting.
• Multiple Access: Multiple devices are competing for access to the same communication channel.

Example:

In a network using CSMA, if Device A wants to send data, it first checks if the channel is idle. If it is, Device A sends its data. Otherwise, it waits until the channel becomes idle.

CSMA/CD (Carrier Sense Multiple Access with Collision Detection)

CSMA/CD is an enhancement of the CSMA protocol that addresses the issue of data collisions. It is widely used in Ethernet networks. In CSMA/CD, after sending data, a device listens for any collision. If a collision is detected, the devices involved stop transmitting, wait for a random amount of time, and then attempt to retransmit.

• Collision Detection: Devices detect when a collision occurs and take steps to resolve it.

Example:

If Device A and Device B both send data at the same time, a collision occurs. Both devices detect this, stop transmitting, and retry after a random delay.

CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)

CSMA/CA is a protocol designed to avoid collisions by forcing devices to wait for a specific period before attempting to transmit data. Unlike CSMA/CD, which detects collisions after they occur, CSMA/CA aims to prevent collisions from happening in the first place.

• Collision Avoidance: Devices wait for a randomly chosen backoff time before transmitting to reduce the chances of a collision.

Example:

Wi-Fi networks commonly use CSMA/CA to manage collisions. Before transmitting, a device listens to the channel. If it’s idle, the device waits for a random period and then transmits, reducing the chance of a collision.

Controlled Access Techniques

Controlled Access Techniques involve a centralized or distributed control mechanism that manages access to the shared communication medium. These techniques are more orderly than random access methods.

Reservation

In the reservation technique, time slots or channels are reserved for specific devices. Each device is allocated a time to transmit data, ensuring that no collisions occur.

Example:

In satellite communication, each terminal may reserve a time slot for sending data to avoid collisions on the shared uplink.

Polling

Polling involves a central controller (e.g., a server) that queries each device to see if it has data to send. Devices can only transmit data when polled, preventing collisions.

Example:

In a time-division multiplexing (TDM) network, a central hub polls devices one by one to check if they have data to send.

Token Passing

In token passing, a special frame called a token circulates around the network. Only the device that holds the token can transmit data, eliminating the possibility of collisions.

Example:

In a token ring network, the token travels from one device to another. The device holding the token can send data, and once it finishes, it passes the token to the next device.

Channelization Techniques

Channelization refers to the process of dividing the communication medium into separate channels to allow multiple devices to share the same medium simultaneously. Channelization methods are widely used in wireless and mobile networks.

FDMA (Frequency Division Multiple Access)

FDMA divides the available bandwidth into separate frequency bands, with each device transmitting data on its assigned frequency.

• Frequency Division: Different devices use different frequency bands to transmit simultaneously.

Example:

In analog cellular systems, FDMA was used to assign separate frequency bands to different users to avoid interference.

TDMA (Time Division Multiple Access)

TDMA divides the transmission time into time slots, with each device being assigned a specific time slot in which it can transmit.

• Time Division: Devices take turns transmitting in their respective time slots.

Example:

In digital cellular networks like GSM, TDMA is used to allow multiple users to share the same frequency by transmitting in alternating time slots.

CDMA (Code Division Multiple Access)

CDMA allows multiple devices to transmit simultaneously by assigning unique codes to each device. These codes are used to spread the data across the entire available bandwidth, allowing the receiver to distinguish between different transmissions.

• Code Division: Multiple devices share the same frequency spectrum, and unique codes are used to differentiate between transmissions.

Example:

CDMA is commonly used in 3G cellular networks, where multiple users can share the same frequency spectrum by using unique spreading codes.

Ethernet and IEEE Standards

Ethernet is the most widely used LAN (Local Area Network) technology. The IEEE (Institute of Electrical and Electronics Engineers) has defined several standards under the IEEE 802 family that specify how Ethernet networks operate.

IEEE Standards: 802.3, 802.4, 802.5, 802.6

• IEEE 802.3: This standard defines Ethernet and includes CSMA/CD for wired LANs. It supports data rates from 10 Mbps to 100 Gbps over different physical media.
• IEEE 802.4: This standard specifies Token Bus networks, where devices share the communication medium using a token-passing protocol over a bus topology.
• IEEE 802.5: This standard defines Token Ring networks, where a token is passed between devices arranged in a ring topology. Only the device with the token can transmit data.
• IEEE 802.6: This standard specifies Metropolitan Area Networks (MANs) and includes the DQDB (Distributed Queue Dual Bus) protocol for large-scale data communication.

Comparison of Ethernet Standards

Ethernet has evolved over the years to support higher speeds and more advanced features. This section compares the major Ethernet standards with a focus on the MAC layer and the Physical layer.

Standard Ethernet

• Data Rate: 10 Mbps
• MAC Layer: Standard Ethernet uses CSMA/CD for medium access control. Devices sense the carrier before transmitting, and collisions are handled using the CD mechanism.
• Physical Layer: Standard Ethernet typically uses coaxial cable, twisted pair, or fiber optic cables.

Example:

Standard Ethernet is suitable for small networks with moderate data traffic.

Fast Ethernet

• Data Rate: 100 Mbps
• MAC Layer: Fast Ethernet maintains backward compatibility with Standard Ethernet and uses the same CSMA/CD protocol, but with improved efficiency.
• Physical Layer: Fast Ethernet primarily uses twisted-pair cables (100Base-TX) or fiber optic cables (100Base-FX).

Example:

Fast Ethernet is used in networks requiring higher bandwidth than Standard Ethernet, such as small to medium-sized office networks.

Gigabit Ethernet

• Data Rate: 1 Gbps
• MAC Layer: Gigabit Ethernet enhances the CSMA/CD protocol to support the increased data rate while maintaining compatibility with previous Ethernet standards.
• Physical Layer: Gigabit Ethernet can run over twisted-pair cables (1000Base-T) or fiber optic cables (1000Base-LX or 1000Base-SX).

Example:

Gigabit Ethernet is ideal for networks with high data traffic, such as data centers and enterprise-level networks.

Conclusion

Multi-access mechanisms and Ethernet standards are critical for efficient communication in modern networks. Random Access Techniques like CSMA/CD and CSMA/CA manage access to the shared medium, while Controlled Access Techniques like Token Passing ensure orderly communication. Channelization Techniques like FDMA, TDMA, and CDMA provide ways to share the communication medium among multiple devices. Finally, Ethernet standards (IEEE 802.3, 802.4, 802.5, and 802.6) ensure the interoperability and scalability of wired networks, with Gigabit Ethernet being the most common choice for high-performance networks today. Understanding these techniques is essential for building efficient and reliable networks.