Data Communication Access Methods: A Comprehensive Guide
Data Communication
Access Methods
1. Types of Access Methods
Access methods are categorized into three main types:
- Round Robin: Each station gets a chance to transmit data one at a time in a sequential manner.
- Reservation: Time is divided into segments called slots. A station must reserve a slot before transmitting and can only transmit during its reserved slot.
- Contention: There’s no fixed order for transmission. Stations transmit data whenever they have it, potentially leading to collisions.
2. Evaluating Media Access Methods
The most relevant aspects for evaluating media access methods are:
- Capacity: The total transmission capacity the method allows.
- Fairness: Equal access opportunity for stations within the same priority class.
- Stability: How the network’s delay behaves under increasing load (whether it remains stable or not).
Options (a), (b), and (c) are incorrect because they don’t include all three of these crucial aspects.
3. Evaluating Physical Media Access Methods
Similar to the previous question, the relevant aspects for evaluating physical media access methods are capacity, fairness, and stability. Options (b), (c), and (d) are incorrect because they don’t encompass all three.
4. Key Aspects of Physical Environment Access Methods
The evaluation of methods for accessing the physical environment should consider:
- Capacity: The total transmission capacity enabled by the method.
- Fairness: Equal access opportunity for stations within the same priority class.
- Stability: The network’s behavior, specifically whether delay remains stable under increasing load.
5. Capacity in Physical Medium Access
Capacity, a crucial aspect in evaluating physical medium access methods, is influenced by the physical environment’s capabilities, the network’s size, and the number of stations on the network.
6. Fairness in Physical Medium Access
Fairness ensures that all stations within the same priority class have equal opportunities to access the physical environment.
7. Stability in Physical Medium Access
Stability refers to the network’s behavior, particularly whether delay remains consistent as the network load increases.
8. Contention-Based Access
Contention-based access methods allow multiple stations to compete for access to the shared medium. This can lead to collisions if two or more stations try to transmit simultaneously.
9. Contention-Based Access Characteristics
Regarding contention-based access:
- There is no chronological access to the physical environment.
- It can be used with multiplexing but doesn’t necessarily work best with it.
- There is no predefined order of medium access.
Time Segmented (Slotted): Time is divided into intervals.
Continuous Time: Transmission frames can start at any time.
10. Reservation-Based Access Characteristics
Regarding reservation-based access methods:
- Collisions are avoided as stations reserve slots before transmitting.
- They are not limited to ring topologies and can be used in satellite networks where propagation delays are significant.
- Stations monitor the physical environment before transmitting to ensure it’s free.
11. Main Feature of Reservation-Based Access
The primary feature of reservation-based access is that stations must reserve a time slot before transmitting and can only transmit during their allocated slot.
12. Main Characteristic of Ordered Access Without Restraint
Ordered access without restraint methods implement orderly access to the physical environment to prevent collisions.
13. Aloha Contention-Based Access
The Aloha method, developed in 1970, linked the University of Hawaii’s computer center in Honolulu to terminals across the islands. It’s a radio and satellite broadcast network that influenced many contention-based access methods.
14. Collision Detection in Aloha
In Aloha, a transmitting station detects message loss due to collision by sending a message and waiting for an acknowledgment. If no acknowledgment is received within a specific time, the station retransmits the message.
15. Aloha vs. Slotted Aloha
Slotted Aloha improves upon Aloha by synchronizing terminal transmissions to reduce collisions. Terminals start transmitting at the beginning of predefined time slots, minimizing the chances of overlapping transmissions.
16. Collision Detection in Slotted Aloha
The central station in Slotted Aloha detects collisions by checking the Cyclic Redundancy Check (CRC) of the received message. A mismatch in the CRC value indicates a collision.
17. Main Feature of CSMA Family
The CSMA (Carrier Sense Multiple Access) family’s main feature is that stations can monitor the physical environment to detect ongoing transmissions and avoid collisions.
18. Collision Detection in np-CSMA
In np-CSMA (non-persistent CSMA), stations detect collisions by waiting for an acknowledgment after transmission. If the acknowledgment doesn’t arrive within a timeout period, the station assumes a collision occurred and reattempts transmission after monitoring the environment.
19. np-CSMA vs. p-CSMA
The key difference between np-CSMA and p-CSMA (persistent CSMA) lies in their behavior when the medium is busy. In np-CSMA, the station waits for a random interval before reattempting transmission. In p-CSMA, the station continuously monitors the medium to determine when the current transmission ends.
20. CSMA/CD Contention-Based Access
CSMA/CD (Carrier Sense Multiple Access with Collision Detection) allows stations to monitor the physical environment even while transmitting. This enables them to detect collisions by comparing their transmitted data with the received data and immediately stop transmission upon collision detection.
21. Frame Size Limitation in CSMA/CD
Frames in CSMA/CD have a minimum size limitation to ensure that a collision can be detected before the entire frame is transmitted. This limitation is necessary because collision detection relies on the transmitting station receiving a portion of its own transmitted signal to identify a collision.
22. Token Ring Technique
Token Ring is a standard for ring networks using baseband signaling and a token-passing media access method. It typically uses twisted-pair cabling and supports transmission rates of 4 Mbps and 16 Mbps.
23. Tokens in Token Ring
In Token Ring, a special bit pattern called a token circulates the ring. The token indicates whether the network is free (e.g., pattern 11111111) or busy (e.g., pattern 11111110). Only the station holding the token can transmit data.
25. Token Bus Technique
Token Bus is a standard for bus networks using broadband signaling and a token-passing media access method.
26. Polling Access Method
In polling, a central station controller sends a message to each station in a predetermined order, granting permission to transmit. If a station has no data to send, it sends a control message back to the controller, indicating its operational status.
27. Variations of Polling
Polling has several variations to optimize network characteristics and performance. For instance, some implementations use separate channels for control information and data. In large networks with infrequent transmissions, stations might pass control directly to the next station instead of always responding to the controller, improving efficiency.
30. Network Type Associated with DQDB
DQDB (Distributed Queue Dual Bus) is an access method commonly used in metropolitan area networks (MANs).
31. Contention-Based Access Methods
Three examples of contention-based access methods are:
- Aloha
- CSMA
- CSMA/CD
32. Reservation-Based Access Methods
Three examples of reservation-based access methods are:
- Bitmap
- AARC (Adaptive Allocation Reservation Control)
- IFFO (Interleaved Frame Fluctuation Ordering)
33. Round-Robin Access Methods
Three examples of round-robin access methods are:
- Token Ring
- Token Bus
- Polling