MAC Protocols & Multiple Access Methods : MCQs & PYQs with Explanation

Option D

IEEE 802.11ac is a wireless networking standard used in Wi-Fi technology that operates primarily in the 5 GHz frequency band. It provides higher data rates, better bandwidth, and less interference compared to older standards like 802.11n.
It is commonly known as Wi-Fi 5 and supports advanced features like MIMO (Multiple Input Multiple Output) and wider channel bandwidth.

Option C

A Contention System is a communication system in which multiple users or devices share a common communication channel and compete for access to that channel.
Because more than one user may try to use the channel at the same time, conflicts or collisions can occur. A contention-based system therefore needs a method to control or resolve such conflicts.
For example, in a shared network, if two computers try to transmit data over the same channel simultaneously, their signals may interfere with each other. This situation is handled using contention-based access methods such as CSMA/CD or CSMA/CA.

• Carrier refers to a signal or frequency used to carry information.
• Communication Network, a general term for interconnected devices that communicate with one another.
• Bus, a bus is a shared communication medium/topology.

Option B

Bit Stuffing is a technique used in data communication to prevent the data from being confused with a special delimiter or flag pattern. 
Here, the delimiter is 01111110. The rule is that whenever five consecutive 1s occur in the original data, the sender automatically inserts a 0 after those five 1s.
The given output bit-string is 01111100101. We can see five consecutive 1s followed by a 0. The 0 immediately after the five consecutive 1s is the stuffed bit, so it must be removed to recover the original input.
Therefore, the original input bit-string is 0111110101.

Option A

In an Ethernet frame, the Start Frame Delimiter (SFD) is an 8-bit field that indicates the actual beginning of the Ethernet frame. It comes immediately after the 7-byte Preamble. 
The preamble consists of repeated 10101010 patterns, while the SFD is 10101011. The change in the final two bits from the preamble pattern helps the receiver identify the exact point where the Ethernet frame begins.

Option D

The statement (I) is true, CDMA spreads the signal over the entire available spectrum using spread-spectrum technology.
The statement (II) is true, every user has a unique pseudo-random code, which allows multiple users to share the same frequency simultaneously.
The statement (III) is false, CDMA technology offers more security compared to GSM because it uses spread spectrum technology and encryption, making signal interception and detection harder. 
The statement (IV) is false, A SIM card is not always required for CDMA devices. CDMA phones traditionally do not rely on SIM cards, unlike GSM phones.

Option C

In computer networks, multiple devices share a common communication medium. To avoid conflicts when two devices try to transmit simultaneously, access control mechanisms are used. These mechanisms can be Collision-Prone or Collision-Free:

• Collision-prone protocols allow the possibility that two devices transmit at the same time, causing a collision, and then require retransmission. ALOHA,  CSMA/CD, CSMA/CA are example of collision-prone protocols.
• Collision-free protocols use a structured approach to ensure only one device transmits at a time, preventing collisions entirely. Token Passing (Token Ring or Token Bus) is collision-free mechanism, only the station holding the token may transmit, so concurrent transmissions cannot occur.

Option B

CSMA/CD (Carrier Sense Multiple Access with Collision Detection) is a method used in traditional Ethernet networks to detect and handle collisions when multiple devices try to transmit data at the same time. 
When a device detects a collision, it sends a special Jamming Signal to inform all other devices on the network that a collision has occurred. This ensures that all communicating devices become aware of the collision and stop their current transmission. 
After sending the jamming signal, the devices wait for a randomly determined backoff time before attempting to transmit again.

Option C

In CSMA/CD, the minimum frame size must be large enough for the sender to detect a collision. Therefore, the frame transmission time must be at least equal to the round-trip propagation delay. 
The one-way propagation delay is 1 / (2*10⁵) = 5*10⁻⁶ seconds. Hence, the round-trip delay is 2*5*10⁻⁶ = 10⁻⁵ seconds.
Minimum Frame Size = Data Rate * Round-Trip Delay
= 1*10⁹*10⁻⁵
= 10,000 bits
Therefore, the minimum frame size is 10,000 bits.