Important Short Questions & Answers: Media Access & Internetworking | Computer Networks - Computer Science Engineering (CSE) PDF Download

1. What are the functions of MAC?
The MAC sublayer arbitrates access to a shared medium and resolves contention among nodes. It provides frame delimiting and synchronization, implements flow control and error detection, and carries the physical (MAC) address information used to identify the next station for delivery or forwarding of a frame.

2. What are the functions of LLC?
The IEEE 802.2 Logical Link Control (LLC) layer handles the end-user portion of the link layer functions separated from the MAC. It provides logical addressing, control information for managing link connections, and a framing interface for upper-layer protocols to send and receive data over different IEEE 802 networks.

3. What is Ethernet?
Ethernet is a family of multiple-access LAN technologies in which a set of nodes send and receive frames over a shared physical medium. It defines frame formats, addressing and media access rules so multiple hosts can communicate on the same link.

4. Define the term carrier sense in CSMA/CD?
Carrier sense means that each node can detect whether the medium is idle or busy before transmitting. In CSMA/CD a node listens to the medium and delays transmission if it senses activity; collision detection additionally lets a transmitting node detect interference from another transmission and stop to reduce wasted transmission time.

5. Define Repeater?
A repeater is a simple physical-layer device that regenerates and forwards digital signals to extend the reach of a network. Because repeaters do not perform any frame filtering, Ethernet standards limit the number of repeaters between hosts (typically up to four) to keep timing and collision detection effective.

6. Define collision detection?
Collision detection is the ability of a node to sense that its transmission has interfered with another node's transmission on the same medium. In shared Ethernet, collision detection lets a station stop transmitting when a collision is seen and then follow a retry procedure to retransmit later.

7. Why Ethernet is said to be a 1-persistent protocol?
Ethernet (original CSMA/CD) is called 1-persistent because when a station finds the line idle it attempts to transmit immediately with probability 1. If the medium is busy it defers until the medium becomes idle, which may lead to collisions that are then handled by collision detection and backoff.

8. What is exponential back off?
After a collision, a station waits a random delay before trying again. Each time retransmission fails, the range of the random delay is doubled (up to a limit), so the expected waiting time grows exponentially with the number of consecutive collisions; this reduces the chance of repeated collisions under heavy load.

9. What is token holding time (THT)?
Token Holding Time defines how long a node may transmit while it holds the token in token-passing networks. It limits the amount of data (or time) a node can use the medium per token possession, ensuring fair access and preventing a single node from monopolising the token.

• Synchronous
• Asynchronous

 11. What are the four prominent wireless technologies

• Bluetooth
• Wi-Fi (formally known as IEEE 802.11)
• WiMAX (IEEE 802.16)
• Third generation (3G) cellular wireless

12.Define Bluetooth?
Bluetooth provides very short-range wireless links for personal devices such as mobile phones, headsets, PDAs and printers. It is intended for low-power, low-cost device-to-device connections and supports simple services like audio streaming and file transfer.

13.What are the four steps involved in scanning?
1.     The node sends a Probe frame to discover nearby access points.
2.     All access points within radio range reply with a Probe Response frame.
3.     The node selects an access point and sends it an Association Request frame.
4.     The access point replies with an Association Response frame confirming the association.

14.Explain the term handoff?
Handoff (or handover) is the process of transferring an ongoing call or data session from one base station to another as a mobile node moves. If a call is active, handoff preserves continuity by switching the connection to the new base station with minimal interruption.

15. Define satphones?
Satphones (satellite phones) use communication satellites as their network infrastructure rather than terrestrial base stations. They operate on internationally allocated satellite frequency bands and provide coverage where terrestrial cellular networks are unavailable.

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16. How to mediate access to a shared link?
Common media access methods include contention-based schemes such as Ethernet (CSMA/CD) and centralised or token-based schemes such as token ring. Wireless networks add complexity because some nodes may be hidden from others (hidden terminal problem), so specialised MAC protocols and mechanisms (e.g., RTS/CTS in 802.11) are used to reduce collisions.

17. Define Aggregation points?
An aggregation point collects and processes data from neighbouring nodes before forwarding it upstream. By performing in-network processing or summarisation, aggregation reduces overall network traffic and improves scalability, especially in sensor or distributed networks.

18. Define Beacons?
Beacons are reference nodes that know their absolute position (for example via GPS or manual configuration). Other nodes estimate their own absolute location by measuring their position relative to these beacons and combining that with the beacons' known locations.

19. What is the use of Switch?
A LAN switch forwards frames between multiple shared-media LAN segments at the data-link layer. Switches learn the MAC addresses of attached devices and forward frames only to the appropriate port, reducing unnecessary traffic compared with simple hubs or repeaters.

20. Explain Bridge?
A bridge connects two or more LAN segments to form an extended LAN and selectively forwards frames between them. Simple bridges forward received frames out on all other ports, while learning bridges build a forwarding table of source addresses to avoid flooding and improve efficiency.

21. What is Spanning tree?
The Spanning Tree Protocol (STP) is used by bridges to select a subset of links that guarantees a loop-free topology. Bridges exchange configuration messages and agree which ports should forward frames so the extended LAN forms a single spanning tree with no cycles.

22. What are the three pieces of information in the configuration messages?
1.     The ID of the bridge sending the message.
2.     The ID that the sending bridge believes to be the root bridge.
3.     The distance, measured in hops or path cost, from the sending bridge to the root bridge.

23.What is broadcast?
Broadcast means delivering a frame to all nodes on the LAN. Each bridge forwards a frame with a broadcast destination address out on every selected (active) port except the one on which the frame was received, ensuring the frame reaches all segments.

24. What is multicast?
Multicast delivers a frame to a specific group of interested hosts rather than to all hosts. Each host decides whether to accept messages for a given multicast address; multicast delivery can be optimised by network switches or routers to forward frames only to segments containing group members.

25. How does a given bridge learn whether it should forward a multicast frame over a given port?
A bridge learns by observing source addresses on each port, just as it does for unicast frames. By recording which hosts (or group-membership reports) are reachable via a port, the bridge decides whether forwarding a multicast frame out that port is necessary.

26.What are the limitations of bridges?

• Scale - Bridges and extended LANs have limited scalability; performance and manageability degrade as the network grows.
• Heterogeneity - Integrating diverse network technologies or protocols can be difficult, since bridges operate at the data-link layer and cannot resolve all interoperability issues between different network types.