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Module 6: Data Link Layer

Introduction to Networks v7.0 (ITN)

Rizoan Toufiq,

Assistant Professor,

Department of Computer Science & Engineering

Rajshahi University of Engineering & Technology.

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Module Objectives

Module Title:

Data Link Layer

Module Objective:

Explain how media access control in the data link layer supports communication across networks.

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Module Objectives

Section	Topic Title	Topic Objective
6.1	Purpose of the Data Link Layer	Describe the purpose and function of the data link layer in preparing communication for transmission on specific media.
6.2	Topologies	Compare the characteristics of media access control methods on WAN and LAN topologies.
6.3	Data Link Frame	Describe the characteristics and functions of the data link frame.

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6.1 �Purpose of the Data Link Layer

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Purpose of the Data Link Layer�The Data Link Layer

Responsible for communications between end-device Network Interface Cards (NIC).
Allows upper layer protocols to access the physical layer media and encapsulates Layer 3 packets (IPv4 and IPv6) into Layer 2 Frames.
Performs error detection and rejects corrupts frames.

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Purpose of the Data Link Layer�IEEE 802 LAN/MAN Data Link Sublayers

IEEE 802 LAN/MAN standards are specific to the type of network (Ethernet, WLAN, WPAN, etc).

The Data Link Layer consists of two sublayers:

Logical Link Control (LLC)

Communicates between the networking software at the upper layers and the device hardware at the lower layers.

Media Access Control (MAC).

The MAC sublayer is responsible for data encapsulation and media access control.

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Purpose of the Data Link Layer�IEEE 802 LAN/MAN Data Link Sublayers

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Purpose of the Data Link Layer�Providing Access to Media

Packets exchanged between nodes may experience numerous data link layers and media transitions.

At each hop along the path, a router performs four basic Layer 2 functions:

Accepts a frame from the network medium.
De-encapsulates the frame to expose the encapsulated packet.
Re-encapsulates the packet into a new frame.
Forwards the new frame on the medium of the next network segment.

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Purpose of the Data Link Layer�Data Link Layer Standards

Data link layer protocols are defined by engineering organizations:

Institute for Electrical and Electronic Engineers (IEEE).
International Telecommunications Union (ITU).
International Organizations for Standardization (ISO).
American National Standards Institute (ANSI).

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Check Your Understanding

Answer: (b)

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Check Your Understanding

Answer: LLC, MAC

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Check Your Understanding

Answer: Provide the method to get the frame on and off the media

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Check Your Understanding

Answer:

Accepts a frame from medium

De-encapsulates the frame

Re-encapsulates the packet into a new frame

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Check Your Understanding

Answer:

Media Sharing, Topology

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Check Your Understanding

Answer: IEEE

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6.2 �Topologies

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Topologies�Physical and Logical Topologies

The topology of a network is the arrangement and relationship of the network devices and the interconnections between them.

There are two types of topologies used when describing networks:

Physical topology – shows physical connections and how devices are interconnected.
Logical topology – identifies the virtual connections between devices using device interfaces and IP addressing schemes.

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Topologies�Physical and Logical Topologies

Physical topology

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Topologies�Physical and Logical Topologies

Logical topology

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Topologies�WAN Topologies

There are three common physical WAN topologies:

Point-to-point
Hub and spoke
Mesh

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Topologies�WAN Topologies

There are three common physical WAN topologies:

Point-to-point – the simplest and most common WAN topology. Consists of a permanent link between two endpoints.

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Topologies�WAN Topologies

There are three common physical WAN topologies:

Hub and spoke – similar to a star topology where a central site interconnects branch sites through point-to-point links.

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Topologies�WAN Topologies

There are three common physical WAN topologies:

Mesh – provides high availability but requires every end system to be connected to every other end system.

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Topologies�Point-to-Point WAN Topology

Physical point-to-point topologies directly connect two nodes.
The nodes may not share the media with other hosts.
Because all frames on the media can only travel to or from the two nodes, Point-to-Point WAN protocols can be very simple.

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Topologies�LAN Topologies

End devices on LANs are typically interconnected using a star or extended star topology.
Star and extended star topologies are

easy to install,
very scalable and
easy to troubleshoot.

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Topologies�LAN Topologies

Early Ethernet and Legacy Token Ring technologies provide two additional topologies:

Bus – All end systems chained together and terminated on each end.
Ring – Each end system is connected to its respective neighbors to form a ring.

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Half-duplex communication

Only allows one device to send or receive at a time on a shared medium.
Used on WLANs and legacy bus topologies with Ethernet hubs.

Topologies�Half and Full Duplex Communication

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Full-duplex communication

Allows both devices to simultaneously transmit and receive on a shared medium.
Ethernet switches operate in full-duplex mode.

Topologies�Half and Full Duplex Communication

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Contention-based access

All nodes operating in half-duplex, competing for use of the medium. Examples are:

Carrier sense multiple access with collision detection (CSMA/CD) as used on legacy bus-topology Ethernet.
Carrier sense multiple access with collision avoidance (CSMA/CA) as used on Wireless LANs.

Controlled access

Deterministic access where each node has its own time on the medium.
Used on legacy networks such as Token Ring and ARCNET.

Topologies�Access Control Methods

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Contention-based access

Topologies�Access Control Methods

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Controlled access

Topologies�Access Control Methods

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CSMA/CD

Used by legacy Ethernet LANs.
Operates in half-duplex mode where only one device sends or receives at a time.
Uses a collision detection process to govern when a device can send and what happens if multiple devices send at the same time.

CSMA/CD collision detection process:

Devices transmitting simultaneously will result in a signal collision on the shared media.
Devices detect the collision (the signal amplitude is higher than normal on the media).
Devices wait a random period of time and retransmit data.

Topologies�Contention-Based Access – CSMA/CD

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CSMA/CA

Used by IEEE 802.11 WLANs.
Operates in half-duplex mode where only one device sends or receives at a time.
Uses a collision avoidance process to govern when a device can send and what happens if multiple devices send at the same time.

CSMA/CA collision avoidance process:

When transmitting, devices also include the time duration needed for the transmission.
Other devices on the shared medium receive the time duration information and know how long the medium will be unavailable.

Topologies�Contention-Based Access – CSMA/CA

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Check Your Understanding

Answer: Logical topology

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Check Your Understanding

Answer: WAN

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Check Your Understanding

Answer: Extended star

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Check Your Understanding

Answer: Half-duplex

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Check Your Understanding

Answer: CSMA/CD

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6.3 �Data Link Frame

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Data Link Frame�The Frame

Data is encapsulated by the data link layer with a header and a trailer to form a frame.
A data link frame has three parts:

Header
Data
Trailer

The fields of the header and trailer vary according to data link layer protocol.
The amount of control information carried with in the frame varies according to access control information and logical topology.

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Data Link Frame�Frame Fields

Field	Description
Frame Start and Stop	Identifies beginning and end of frame
Addressing	Indicates source and destination nodes
Type	Identifies encapsulated Layer 3 protocol
Control	Identifies flow control services
Data	Contains the frame payload
Error Detection	Used for determine transmission errors

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Data Link Frame�Layer 2 Addresses

Also referred to as a physical address.
Contained in the frame header.
Used only for local delivery of a frame on the link.
Updated by each device that forwards the frame.

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Data Link Frame�Layer 2 Addresses

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Data Link Frame�LAN and WAN Frames

The logical topology and physical media determine the data link protocol used:

Ethernet
802.11 Wireless
Point-to-Point (PPP)
High-Level Data Link Control (HDLC)
Frame-Relay

Each protocol performs media access control for specified logical topologies.

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Check Your Understanding

Answer: Header, trailer

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Check Your Understanding

Answer: To determine whether the frame experienced transmission errors

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Check Your Understanding

Answer: Des NIC add, Sou NIC Add, Sou IP Add, Des IP Add

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Check Your Understanding

Answer: 802.11, Ethernet, PPP

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