Computer Networks 1: Link Layer

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the primary purpose of the error detection and correction (EDC) bits?

To protect data from corruption (correct)

To increase data transmission speed

To enhance processing power of the link layer

To reduce the need for redundant data

What does a larger EDC field generally provide?

Faster error correction on the receiving end

Increased redundancy without affecting performance

More robust detection and correction capabilities (correct)

Lower overall data transmission size

Which of the following is NOT a feature typically associated with the link layer?

Addressing and ARP

Routing between different networks (correct)

Multiple access protocols

Error detection

Why might error detection at the link layer need additional focus?

It has unique challenges and demands not seen in upper layers. Signup and view all the answers

What type of errors might error detection mechanisms miss?

Some errors, though it rarely occurs Signup and view all the answers

Which protocol is related to managing data flow and addressing in local area networks?

ARP Signup and view all the answers

What does each layer of the network architecture typically handle?

Transmission and integrity of data Signup and view all the answers

What may result from a bit-error prone link in relation to error detection?

A higher likelihood of false negatives in error detection Signup and view all the answers

What is the primary function of single bit parity in data transmission?

Detect single bit errors Signup and view all the answers

How does two-dimensional bit parity improve upon single bit parity?

It can correct single bit errors Signup and view all the answers

In the context of even parity, what condition must be met for the parity bit?

The total number of 1's must be even Signup and view all the answers

What is the main goal of the Internet checksum?

To detect errors in transmitted segments Signup and view all the answers

What action does the receiver take regarding the checksum field in the received segment?

Compute the checksum of the received segment Signup and view all the answers

If the computed checksum equals the checksum field value upon receipt, what does this indicate?

No errors were detected Signup and view all the answers

What method is used to compute the checksum in the UDP segment?

Addition (one's complement sum) of the segment content Signup and view all the answers

Which parity scheme can detect errors but not correct them?

Single bit parity Signup and view all the answers

What is the primary responsibility of the link layer?

Transferring datagrams from one node to an adjacent node. Signup and view all the answers

Which layer-2 packet format encapsulates the datagram?

Frame Signup and view all the answers

What do MAC addresses in frame headers identify?

The source and destination nodes. Signup and view all the answers

Which of the following is NOT a service of the link layer?

Congestion control across multiple networks. Signup and view all the answers

Which option describes a situation where different link protocols are used?

WiFi providing the first link followed by Ethernet. Signup and view all the answers

What analogy is used to explain the function of the link layer?

Transportation modes for travelers between cities. Signup and view all the answers

What is the outcome of error detection and correction in link layer services?

It ensures the integrity of data delivered over the link. Signup and view all the answers

What does the term 'multiple access' in link layer services refer to?

The ability of one device to share a channel with multiple devices. Signup and view all the answers

Which of these best describes a local area network (LAN)?

A network that connects a limited geographical area, usually within one building. Signup and view all the answers

Which component of the datagram’s journey is analogous to a routing algorithm?

The travel agent managing connections. Signup and view all the answers

What is a primary benefit of Slotted ALOHA regarding transmission?

It can allow a single active node to transmit continuously at full rate. Signup and view all the answers

What is the primary characteristic of channel partitioning MAC protocols?

Each node is assigned exclusive use of a channel piece. Signup and view all the answers

What is a significant drawback of the Slotted ALOHA protocol?

It can lead to wasted slots due to collisions. Signup and view all the answers

In the context of Slotted ALOHA, what does the term 'max efficiency' refer to?

The highest fraction of slot usage for successful transmissions. Signup and view all the answers

In a TDMA protocol, what happens to unused slots?

They are ignored and not used in the next round. Signup and view all the answers

What mathematical expression determines the probability of success for a node in Slotted ALOHA?

Np(1-p)N-1 Signup and view all the answers

How does FDMA allocate resources to nodes?

By assigning each station a fixed frequency band. Signup and view all the answers

What is the main feature of random access protocols?

Nodes transmit only when they have data to send. Signup and view all the answers

What is a crucial step in maximizing efficiency in the Slotted ALOHA protocol?

Determine the optimal probability p* that maximizes Np(1-p)N-1. Signup and view all the answers

Which of the following best describes the operation of TDMA?

Time is divided into slots for fixed length data transmission. Signup and view all the answers

What happens during collisions in random access protocols?

Nodes must wait for a certain interval before attempting to resend. Signup and view all the answers

In an FDMA system, what do idle frequency bands signify?

Those nodes have no data to send. Signup and view all the answers

Which statement accurately describes the difference between TDMA and FDMA?

TDMA divides time, while FDMA divides frequency. Signup and view all the answers

What is the primary goal of using a cyclic redundancy check (CRC)?

To detect burst errors in data transmission Signup and view all the answers

In the CRC formula, what do the symbols D and G represent respectively?

Data bits and generator bit pattern Signup and view all the answers

What does a non-zero remainder indicate when dividing by G in the context of CRC?

An error has been detected Signup and view all the answers

Which of the following protocols commonly utilizes cyclic redundancy checks for error detection?

Ethernet Signup and view all the answers

In a point-to-point link, what type of connection is established?

A direct connection between two devices Signup and view all the answers

Which of the following is a characteristic of broadcast links?

They involve multiple users sharing a communication medium Signup and view all the answers

What does the 'r' in the CRC formula represent?

The number of CRC bits chosen for error detection Signup and view all the answers

What is a key feature of the CRC technique in error detection?

It requires the receiver to know the generator pattern Signup and view all the answers

In error detection methods like CRC, burst errors are defined as errors in what?

A sequence of bits that all malfunction Signup and view all the answers

Which of the following best describes point-to-point links?

They are characterized by a direct connection between two endpoints Signup and view all the answers

Study Notes

Course Information

Course: CNT 4007 - Computer Networks 1
Topic: Link Layer
Professor: Patrick Traynor
Semester: Fall 2024

Reminders/Notes

Homework 3 due Thursday
Project 3 due next Tuesday
Midterm grading started yesterday and progress is being tracked
Students should keep an eye on the calendar for important dates.

Link Layer and LANs: Our Goals

Understand link layer services (error detection, correction, broadcast channel sharing, multiple access)
Learn about link layer addressing (e.g., Ethernet, VLANs)
Learn about datacenter networks
Implement various link layer technologies

Link Layer, LANs: Roadmap

Introduction to Link Layer
Error detection and correction
Multiple access protocols
Local Area Networks (LANs):
- Addressing (e.g., ARP)
- Ethernet
- Switches
- VLANs
Link virtualization (e.g., MPLS)
Data center networking
Day in the life of a web request

Link Layer: Introduction

Terminology:
- Hosts and routers as nodes
- Communication channels connecting nodes: links (wired, wireless, LANs)
- Layer-2 packet: frame that encapsulates datagram.
Link Layer's responsibility: Transferring datagrams between physically adjacent nodes.

Link Layer: Context

Datagrams transferred by different link protocols over different links (e.g., WiFi to Ethernet)
Each link protocol provides different services (e.g., reliable data transfer may or may not be available)

Link Layer: Services

Framing, link access: encapsulating datagrams into frames and adding headers/trailers
Channel access: relevant when multiple nodes share a common medium. MAC Addresses identify source and destination.
Reliable delivery between adjacent nodes (though not frequently used on links with low error rates).
Wireless links often have high error rates so link-level reliability is needed
Flow control: pacing of data transmission between sending and receiving nodes
Error detection methods: for identifying errors caused by signal attenuation, noise, etc.; receiver signals retransmission or drops frame.
Error correction: identifying and correcting bit errors without retransmission
Half-duplex and full-duplex: with half duplex transmissions are sent simultaneously, but full duplex ones aren't

Where is the Link Layer Implemented?

Implemented in each network interface card (NIC) or on chip
Specific to chip: Ethernet, WiFi cards or chips.
Implemented using combination of hardware, software, and firmware. The NIC attaches to host's system buses

Interfaces Communicating

Sending side encapsulates datagram into frame, adds error-checking bits, and handles reliable data transfer and flow control
Receiving side checks for errors, extracts the datagram, and passes it to the upper layer

Link Layer, LANs: Roadmap (Page 11)

Introduction
Error detection and correction
Multiple access protocols
Local Area Networks (LANs): Addressing, ARP; Ethernet; Switches; VLANs
Link virtualization (e.g., MPLS)
Data center networking
A day in the life of a web request

Error Detection

EDC: error detection and correction bits (redundancy)
Data protected by error checking (headers included).
Error detection isn't 100% reliable.
Larger EDC field improves accuracy and correctness

Why?

Repeated discussions of error detection highlight their importance in networking.
Special aspects of link layer requiring additional attention discussed.
Supporting considerations and justifications are provided for the extra effort.

Parity Checking

Single bit parity detects single bit errors (even parity ensures even number of 1's).
Two-dimensional bit parity detects and corrects single bit errors.

Internet Checksum (review)

Goal: Detect errors in transmitted segments.
Sender: treats UDP segment contents as 16-bit integers; finds ones complement sum; puts checksum into UDP checksum field.
Receiver: calculates checksum of received segment; checks if computed checksum equals checksum field value; error is indicated if checksums don't match.

Cyclic Redundancy Check (CRC)

More powerful error-detection coding.
Data bits (D) and generator bit pattern (G) used to calculate CRC bits (R).
<D,R> is divisible by G in modulo 2 if no errors occur

Cyclic Redundancy Check (CRC): Example

Specific example using a 10110111 generator formula to calculate the CRC bits.

Link Layer, LANs: Roadmap (Page 18)

Introduction, error detection and correction
Multiple access protocols
LANs: Addressing, ARP; Ethernet; Switches; VLANs
Link virtualization (e.g., MPLS)
Data center networking
A day in the life of a web request

Multiple Access Links, Protocols

Two types of links: point-to-point (e.g., PPP, Ethernet switch to host) and broadcast (shared wire or medium, e.g., old-fashioned Ethernet, wireless LAN, upstream HFC cable)

Multiple Access Protocols

Single shared broadcast channel. Interference occurs with two or more simultaneous transmissions.
Collision occurs if a receiving node receives more than one signal.
The multiple access protocol distributes the channel allocation among nodes by determining when nodes can transmit.
Sharing must use the channel. No out-of-band channel for coordination.

An Ideal Multiple Access Protocol

Desirable characteristics of an ideal multiple access protocol, a multiple access channel (MAC) operates at R bps

MAC Protocols: Taxonomy

Three broad classes of multiple access protocols (MAC) :
- Channel partitioning (allocating exclusive use of channel parts)
- Random access (allows collisions and recovery)
- Taking turns (nodes take turns)

Channel Partitioning MAC Protocols: TDMA

TDMA: Time Division Multiple Access
Each station accesses channel at fixed time slots.
Unused slots remain idle.

Channel Partitioning MAC Protocols: FDMA

FDMA: Frequency Division Multiple Access
Channel spectrum divided into frequency bands.
Each station gets a fixed frequency band.
Unused transmission time is idle.

Random Access Protocols

When a node has a packet, it transmits at full channel capacity
Random access MAC protocol specifies procedures for collision detection, recovery (e.g., delayed retransmissions), etc.
Examples include ALOHA and variations.

Slotted ALOHA

Assumptions: frames are same size, time divided in equal size slots, synchronized nodes, etc.
Operation: Obtaining frame, transmitting in next slot, handling collisions, retransmission strategy.
Pro/con analysis: considering benefits and drawbacks.

Slotted ALOHA: Efficiency

Efficiency (long run) of successful slots when many nodes transmit many frames.
Maximum efficiency is calculated.

Pure ALOHA

Simple unslotted version, no synchronization.
Collision probability increases without synchronization.
Efficiency is lower compared to the slotted version.

Conclusions

Current topic is only a small piece of broader Link Layer concepts.
Students are progressing well.
Students should improve their reading and participation.
See you Thursday.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

This quiz covers the essential concepts of the link layer in computer networks, including error detection, addressing, and local area networks. Students will also explore multiple access protocols and datacenter networks. Prepare to delve into practical applications and technologies relevant to the link layer.