Chap 5 Datalink Flashcards
(66 cards)
1
Q
Link Layer
A
- providing reliable data transmission over a physical link between adjacent network nodes
- i.e two devices on the same (LAN).
- intermediary between the Network Layer and Physical Layer
2
Q
Frame
A
- unit of data at the Link Layer
- encapsulate the data received from the Network Layer for transmission over the physical medium.
3
Q
frame components
A
header, payload (data), and trailer
4
Q
Media Access Control (MAC) Address
A
- unique identifier assigned to each network interface card (NIC) or network interface controller (NIC) within a network.
- typically expressed as a 12-digit hexadecimal number
- used for addressing and identifying devices at the Link Layer
5
Q
Ethernet
A
- Link Layer protocol for wired (LANs).
- defines the standards for frame formatting, addressing, MAC, error detection (CRC), and collision detection (CSMA/CD)
6
Q
Wi-Fi (IEEE 802.11)
A
A set of Link Layer protocols for wireless (WLANs) defined by the IEEE 802.11 standard.
govern wireless communication, including frame transmission, MAC addressing, channel access (CSMA/CA), and security mechanisms (WPA, WPA2, etc.).
7
Q
Switch
A
- A network device operating at the Link Layer
- forwards data frames between network devices within the same LAN based on MAC addresses.
- use MAC address tables to learn and maintain the association between MAC addresses and the corresponding switch ports
8
Q
Hub
A
- network device that operates at the physical layer
- forwards data received on one port to all other ports.
- Unlike switches, hubs do not have intelligence to filter/forward data based on MAC addresses.
9
Q
NIC (Network Interface Card)
A
- hardware component installed in a device to connect it to a network.
- contain a physical layer interface (PHY) and a Link Layer interface (MAC)
- are assigned a unique MAC address.
10
Q
Media Access Control (MAC) Sublayer
A
- A sublayer of the Link Layer responsible for controlling access to the physical medium
- implements protocols for media access control, collision detection, and error detection
11
Q
Frame Addressing
A
- process of assigning source and destination MAC addresses to frames for communication between devices on the same network segment.
- Frames are addressed using the MAC address of the destination device.
12
Q
Collision Domain
A
- network segment in which data collisions can occur.
- In Ethernet networks, each connected device and the connecting cables constitute a collision domain.
- Switches help reduce collision domains by providing dedicated communication paths between devices
13
Q
Functions of the Link Layer
A
- Error Detection and Correction:
- Encapsulation: data into frames.
- Addressing:
- Media Access Control (MAC):
- Flow Control:
- Frame Handling:
14
Q
Error Detection and Correction
A
- Data can be corrupted during transmission due to signal attenuation or noise, leading to transmission error.
- Error detection and correction are implemented at either the data link layer or the transport layer
15
Q
Importance of Error Detection
A
- Disrupted communication:
- System malfunctions:
- Incorrect information:
- Improved Data Integrity:
- Reliable Communication:
- Reduced Errors:
- Faster Troubleshooting:
16
Q
Factors Contributing to Error
A
- Electrical noise: Interference on cables.
- Radio Frequency (RF) interference:
- Hardware malfunctions:
- Software bugs:
17
Q
Bit Error
A
- Also called a single bit error.
- only one bit in the data unit has been changed (e.g., a ‘0’ to ‘1’ or vice versa)
18
Q
Burst Error
A
- two or more bits in the data have been changed.
- simultaneous alteration of multiple consecutive bits within a data unit.
- often occur due to channel noise or interference.
19
Q
Error Detection (Redundancy)
A
To detect errors, extra bits, called redundant bits, are sent along with the data
20
Q
Error Correction Handling
A
Error correction can be handled in two ways:
- The receiver can have the sender retransmit the entire data unit.
- The receiver can use an error correcting code which automatically corrects certain errors
21
Q
Error Detection Techniques
A
- Vertical redundancy check (VRC) also known as parity check.
- Longitudinal redundancy check (LRC).
- Checksum.
- Cyclic redundancy check (CRC).
22
Q
Benefits of Error Detection
A
- Improved Data Integrity:
- Reliable Communication:
- Reduced Errors:
- Faster Troubleshooting:
23
Q
Vertical Redundancy Check (VRC)
A
- Also known as Parity Checking.
- involves adding an extra bit (parity bit) to each data unit.
- The parity bit is set to 1 or 0 based on whether the number of 1s in the data unit is odd or even
- The receiver checks the parity; a mismatch indicates an error
24
Q
Performance of VRC
A
- Basic and easy to implement.
- Can only detect single-bit errors.
- Can detect burst errors only if the number of errors is odd.
- Identifies errors but doesn’t provide the capability to fix them; retransmission is needed
25
Longitudinal Redundancy Check (LRC)
* A block of bits is organized in rows and columns (2-dimensional parity).
* The parity bit is calculated for each column and sent along with the data.
* The block of parity bits acts as the redundant bits.
* LRC works by adding a check value (checksum) to a block of data
26
LRC Working
1. The sender calculates the LRC based on the data content using a mathematical algorithm.
1. This checksum is appended to the data and transmitted.
1. The receiver performs the same LRC calculation on the received data and compares it with the received checksum.
1. If the values match, data arrived without errors; a mismatch suggests errors, and retransmission might be needed
27
Performance of LRC
* Can detect single-bit, multiple-bit, and burst errors.
* Increases the likelihood of detecting burst errors.
* If 2 bits in one data unit and two bits in the exact same position in another data unit are damaged, LRC will not detect the error.
28
Checksum (Calculation)
* The sender calculates a checksum value for the data by performing a mathematical operation on the data bits.
* generates a fixed-size checksum value that represents a condensed version of the data.
29
Checksum (Appending Checksum)
The calculated checksum value is appended to the end of the data unit, creating a new data unit that includes both the original data and the checksum.
30
Checksum Working (Transmission & Reception)
* Transmission: The sender transmits the data unit with the appended checksum.
* Reception: The receiver receives the data unit and extracts the original data. The receiver then calculates a checksum value for the received data.
31
Checksum Working (Comparison)
The receiver compares the calculated checksum value with the checksum value received from the sender. If the two checksum values match, it indicates successful transmission with no errors. If they do not match, it indicates errors, and the receiver can request retransmission.
32
Benefits of Checksums
* Can detect single-bit, multiple-bit, and burst errors.
* Simple implement.
* Efficiency:
33
Cyclic Redundancy Check (CRC)
error-detecting code.
* It adds a check value to a data block.
* The sender and receiver calculate the CRC value using the same algorithm.
* Any errors during transmission will result in a mismatch between the calculated and received CRC values
34
CRC Working
1. Data is divided into blocks.
1. A CRC polynomial (a mathematical formula) is selected.
1. The CRC value is calculated using the polynomial and a bitwise XOR operation on the data block.
1. The calculated CRC value is appended to the data block.
34
CRC Generation at Sender Site
1. Find the length of the divisor ‘L’.
1. Append L - 1 bits to the original message.
1. Perform binary division operation.
Remainder of the division = CRC.
Note: The CRC must be of l - 1 bits
35
Benefits of CRC
1. Simple and efficient error detection.
1. Widely used and standardized.
1. Can detect a wide range of errors
36
Links
physical or logical connections that allow data transmission between devices.
two main types
1. Point-to-Point Link
1. Broadcast Link.
37
Two Main Types of Links (Point-to-Point, Broadcast)
*
Point-to-Point Link:
* Offers dedicated connections between specific endpoints,
* providing high reliability and security
* less efficient for one-to-many communication.
Broadcast Link:
* Allows efficient communication among multiple nodes
* may suffer from congestion and collisions in shared medium environments
38
Point-to-Point Link
Two nodes are directly connected without any intermediate devices between them.
*
Communication between the two nodes occurs exclusively over this dedicated link.
*
Offers dedicated bandwidth between the connected nodes.
*
Often used in scenarios where a direct, private connection is needed between two specific endpoints
39
Point-to-Point Link Examples
Wired connections: Ethernet cable connecting a computer to a switch or router.
*
Fiber optic cable: Connects data centers or internet service providers (ISPs).
*
Serial cables: Used for console access on network devices or connecting peripherals
40
Point-to-Point Benefits
1. Reliable data transmission with minimal interference.
1. Predictable performance
1. Scalable for connecting individual devices
41
Point-to-Point Drawbacks
Requires more cables for complex network topologies.
*
Limited reach compared to some broadcast technologies
42
Broadcast Link
*
Multiple nodes are connected to the same communication medium or network segment.
*
Any data transmitted by one node on the link is received by all other nodes connected to the same link.
*
Broadcast links facilitate one-to-many communication, where a single transmission can reach multiple destinations simultaneously
43
Broadcast Link Examples
Ethernet hubs: Early Ethernet hubs used a shared collision domain where all devices received all transmissions (Modern switches use point-to-point connections internally).
*
Coaxial cable: Traditionally used for cable TV and early Ethernet networks.
*
Wireless networks (Wi-Fi): Access points broadcast signals that can be received by all devices within range
44
Broadcast Link Benefits
Requires less cabling compared to point-to-point for connecting multiple devices.
*
Easier to set up for simple networks
45
Broadcast Drawbacks
1. Increased potential for collisions if multiple devices try to transmit simultaneously.
1. Lower overall performance.
1. Limited security
46
Multiple Access Protocols
Protocols governing how multiple nodes share a single communication channel.
* Found in wired (Ethernet) and wireless networks (Wi-Fi).
* Aim to minimize collisions and maximize channel utilization
47
Channel Partitioning
The available channel bandwidth or time is divided into fixed-size units (channels or time slots).
* Each device is assigned a specific channel or time slot for transmission, eliminating collisions.
* provides predictable performance and avoids the inefficiencies of contention.
*
Includes Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA).
48
FDMA (Frequency Division Multiple Access)
Divides the total frequency band into sub-bands (channels) assigned to different devices.
*
Example: Imagine multiple radio stations broadcasting on different frequencies
49
TDMA (Time Division Multiple Access)
*
Divides the total time into fixed-size time slots.
*
Each device gets exclusive access to the channel during its assigned slot.
*
Example: Think of taking turns on a microphone during a meeting.
*
Ensures that each node has exclusive access to the medium during its assigned time slot, minimizing collisions and interference.
*
Commonly used in various communication systems, including satellite communication, wireless LANs, and digital cellular networks.
*
Particularly suitable for scenarios where the available bandwidth can be divided into time slots, and users require periodic access to the medium for transmission
50
Random Access
devices compete for access to the shared medium whenever they have data to transmit.
* Collisions can occur if multiple devices attempt to transmit simultaneously.
* mechanism for devices to detect and recover from collisions, minimizing their impact.
*
Examples include Pure ALOHA, Slotted ALOHA, and Carrier Sense Multiple Access (CSMA).
51
Carrier Sense Multiple Access (CSMA)
Devices listen (carrier sense) to see if the channel is busy before transmitting.
Includes variations like CSMA/CD (Collision Detection) and CSMA/CA (Collision Avoidance).
52
CSMA/CD (Collision Detection)
* If the channel is busy, the device waits.
* If a collision occurs during transmission, devices detect it and retransmit after a random delay.
* In CSMA/CD, nodes continue to monitor the channel while transmitting data.
* If a collision is detected, the transmitting node aborts its transmission immediately.
* After aborting, the node enters a backoff period, during which it waits for a random amount of time before attempting to retransmit the data.
53
CSMA/CA (Collision Avoidance)
avoid collisions by sending a request to transmit before actually transmitting data.
* Commonly used in wireless networks.
* deferring their transmissions for random periods of time, based on the current state of the channel.
Nodes may also use techniques such as virtual carrier sensing and Request to Send/Clear to Send (RTS/CTS) handshakes to reserve the channel before transmitting data
54
Pure ALOHA
* contention-based protocol used for multiple access in wireless networks.
* Devices share a single channel to transmit data and Aloha relies on a random access approach to manage this sharing
54
Pure ALOHA Working
*
Devices with data to transmit don't check if the channel is busy before sending.
*
They simply transmit whenever they have a packet ready
55
Collision and Backoff in Pure ALOHA
If multiple devices transmit simultaneously, their data packets collide, corrupting the data.
Upon detecting a collision (usually through lack of acknowledgement from the receiver), devices typically employ a random backoff timer before attempting to retransmit the data packet.
This random delay helps reduce the probability of another collision in the next attempt.
56
Slotted ALOHA
A variation of the Aloha protocol.
* Addresses the collision issue in pure Aloha by introducing time slots.
* Devices are only allowed to transmit data at the beginning of a time slot.
57
Slotted ALOHA Working
When a device has data to send, it waits for the beginning of the next time slot.
* At the slot boundary, the device transmits its data packet.
* If multiple devices transmit in the same slot, a collision occurs, and the data packets are corrupted.
In case of a collision, devices typically employ a random backoff timer before attempting to retransmit the data packet in a subsequent slot.
58
CSMA Working
1. Listen before transmit.
* If channel sensed idle: transmit entire frame.
* If channel sensed busy: defer transmission.
But collisions can still occur with carrier sensing when propagation delay i.e., two nodes may not hear each other’s just-started transmission.
When collision happens entire packet transmission time wasted.
Distance & propagation delay play role in determining collision probability.
59
CSMA/CD Working
*
In CSMA/CD, nodes continue to monitor the channel while transmitting data.
*
If a collision is detected, the transmitting node aborts its transmission immediately.
*
After aborting, the node enters a backoff period before attempting to retransmit.
60
CSMA/CA Working
*
In CSMA/CA, commonly used in wireless networks, nodes use a different strategy to avoid collisions.
*
Instead of detecting collisions after they occur, nodes attempt to avoid collisions by deferring their transmissions for random periods of time, based on the current state of the channel.
61
Multiple Access Protocols Class - Taking Turn
devices share access to the medium in a predetermined order.
A special token/control packet circulates among devices, granting permission to transmit to the device holding the token.
Includes Token passing and Polling.
62
Polling
requires one of the node to be designated as a master node.
The master node polls each of the nodes in Round Robin fashion.
* sends a message to each node saying that it can transmit up to some maximum number of frames.
* determine when a node has finished sending its frame by observing the lack of a signal on the channel.
The polling protocol eliminates collisions.
drawback
* introduces a polling delay
* if the master node fails, the entire channel becomes inoperative.
63
Efficiency Equation (Polling)
*
Tpoll= time for polling
Tt = time required for transmission of data.
Efficiency = Tt / (Tt + Tpoll).
64
token passing
*
A special token circulates among the nodes in the network.
*
Only the node possessing the token is allowed to transmit data.
*
The node that possesses the token can transmit a fixed amount of data or hold onto the token until it has no more data to send.
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After transmitting data or completing its operation, the node passes the token to the next node in a predefined sequence.
*
Token passing is commonly used in Token Bus networks
