COMPUTER NETWORKS

Lecture # 7

Multiple Access Techniques

• Multiple Access: Refers to the methods that allow multiple devices to access a shared communication channel or medium.
• Importance: With the proliferation of networked devices, especially in wireless communications, effective multiple access techniques are essential for enabling smooth and efficient communication.

Types of Multiple Access Techniques

There are several key techniques used in multiple access systems, each with its advantages and appropriate use cases:

1. Time Division Multiple Access (TDMA)
2. Frequency Division Multiple Access (FDMA)
3. Code Division Multiple Access (CDMA)
4. Carrier Sense Multiple Access (CSMA)
5. Space Division Multiple Access (SDMA)

1. Time Division Multiple Access (TDMA)

• Definition: In TDMA, the available time on the channel is divided into distinct time slots. Each user is allocated a specific time slot to transmit data.
• How It Works:
• Users send their data in their designated time slot.
• For example, in a system with four users, each user may be assigned a time slot of 1/4 of the total transmission time.

• Advantages:
• Eliminates interference between users because they transmit at different times.
• Efficient time utilization when the number of users is stable.
• Disadvantages:
• If a user has no data to send during their time slot, that time is wasted.
• Adding or removing users can be complicated.
• Example Use Cases: Used in satellite communications and some cellular networks.

2. Frequency Division Multiple Access (FDMA)

• Definition: In FDMA, the available bandwidth is divided into several frequency bands, and each user is allocated a unique frequency for their transmissions.
• How It Works:
• Each user transmits simultaneously on different frequencies.
• For example, in a radio communication system, each station might operate on a different frequency.

• Advantages:
• Continuous transmission is possible since users are on different frequencies.
• Minimal delay and more flexibility in handling users concurrently.
• Disadvantages:
• Requires a larger bandwidth, which may be inefficient if usage is intermittent.
• Limited scalability due to the fixed allocation of frequencies.
• Example Use Cases: Used in traditional analog telephony and radio broadcasting.

. Code Division Multiple Access (CDMA)

• Definition: CDMA allows multiple users to share the same frequency band by assigning unique spreading codes to each user’s signal.
• How It Works:
• Each user’s data is encoded with a unique code (a sequence of bits) before transmission.
• At the receiver end, the intended user’s code is used to decode the signal, filtering out other users’ signals.

• Advantages:
• High capacity, as many users can share the same frequency simultaneously.
• Robust against interference and eavesdropping.
• Disadvantages:
• Complexity in encoding and decoding.
• Requires sophisticated technology to manage codes.
• Example Use Cases: Used in modern cellular networks (e.g., CDMA2000 and WCDMA), GPS, and certain wireless communication systems.

. Carrier Sense Multiple Access (CSMA)

• Definition: CSMA is a protocol that listens for carrier signals (indicating whether the channel is free) before transmitting data.
• How It Works:
• CSMA/CD (Collision Detection): Used in wired networks (e.g., Ethernet). If two devices transmit simultaneously, they detect the collision, stop transmitting, and retry after a random time.
• CSMA/CA (Collision Avoidance): Used in wireless networks (e.g., Wi-Fi). Before transmitting, a device sends a request to send (RTS), and the receiver responds with a clear to send (CTS).

• Advantages:
• Reduces the probability of collisions.
• Efficient when the load is lower, allowing for flexible data transmissions.
• Disadvantages:
• Collision handling can introduce delays.
• Performance degrades as the number of users increases, leading to contention.
• Example Use Cases: Widely used in Ethernet and Wi-Fi networks.

Space Division Multiple Access (SDMA)

• Definition: SDMA separates users based on geographical area or distance, allowing multiple users to use the same frequency and time.
• How It Works:
• Spatial separation is achieved through directional antennas or beamforming techniques.
• Users in different locations can transmit simultaneously without interference.

• Advantages:
• Efficient use of resources by allowing simultaneous transmissions.
• It's beneficial for systems with non-overlapping coverage areas.
• Disadvantages:
• Requires advanced technology such as phased array antennas.
• Complexity in managing spatial resources.
• Example Use Cases: Used in mobile communication and satellite systems.

Comparison of Multiple Access Techniques

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