Network Fundamentals: Ch7 - Data Link Layer

Suez Canal University – Faculty of Computers & Informatics - Cisco Local Academy
Network Fundamentals
Abdelkhalik Elsaid Mosa
abdu.elsaid@yahoo.com
http://abdelkhalik.staff.scuegypt.edu.eg/

Introduction
• The Application layer provides the interface to the user.
• The Transport layer is responsible for dividing and managing
communications between the processes running in the two end
systems.
• The Network layer protocols organize our communication data so
that it can travel across internetworks from source to destination
 Packets do not have a way to directly access different media.
• OSI Data Link layer prepares Network layer packets for
transmission and controls access to the physical media.
• The Data Link layer provides a means for exchanging data over a
common local media, hop to hop delivery.

Data Link Layer terms and services
• Frame: The Data Link layer PDU.
• Node: The Layer 2 notation for a device on a network.
• Media/medium (physical): The physical means for the transfer
of information between two nodes.
• Network (physical): Two or more nodes connected to a common
medium, also called “Network segment”.
• The Data Link layer performs two basic services:
1. Allows the upper layers to access the media using techniques
such as Framing.
2. Controls how data is placed onto and received from the media
using techniques such as Media Access Control and error
detection.

Upper Layer Access to media

Creating a frame

Connecting upper layer services to media

Data Link Layer sublayers
• Separating the Data Link layer into sublayers allows for one type of frame
defined by the upper layer to access different types of media defined by the
lower layer.
IEEE 802.2
IEEE 802.3

Placing data on the media
• Media Access Control “equivalent to traffic rules”: Regulating the placement
of data frames onto the media. MAC techniques define if and how the nodes
share the media.
• The method of media access control used depends on
1. Media sharing: If and how the nodes share the media.
2. Topology: How the connection between the nodes appears to the Data Link layer.

Media Access control for shared media
• Media access control methods for shared media:
1.Controlled “scheduled access or deterministic”: Each node has
its own time to use the medium.

Media Access control for shared media
• Media access control methods for shared media:
2. Contention-based ” non-deterministic”: All nodes compete for
the use of the medium.

Media Access control for non-shared media
• Media access control protocols for non-shared media require little
or no control before placing frames onto the media.

Logical topology vs. physical topology
• Physical Topology: is an arrangement of the nodes and the
physical connections between them.
• Logical Topology: is the way a network transfers frames from
one node to the next.
 The Data Link layer "sees" the logical topology of a network
when controlling data access to the media.
 The logical topology influences the type of network framing
and media access control used.
• Logical and physical topologies typically used in networks are
1. Point-to-Point
2. Multi-Access
3. Ring

Point to Point Topology
• Point-to-Point topology: connects two nodes directly/ Indirectly together.
• The media access control protocol can be very simple.
• The end nodes communicating in a point-to-point network can be physically
connected via a number of intermediate devices.

Multi-Access Topology
• A logical multi-access topology enables a number of nodes to
communicate by using the same shared media.
• Data from only one node can be placed on the medium at any
one time.
• CSMA/CD or CSMA/CA Access Control methods can be used.
• Token passing methods can also be used.

Ring Topology
• In a logical ring topology, each node in turn receives a frame. If
the frame is not addressed to the node, the node passes the
frame to the next node.
• Uses a controlled media access control called token passing.

Data Link Layer Protocols – The frame
• There are many different Data Link layer protocols that describe
Data Link layer frames.

Addressing Requirements
• Data Link layer addressing depends on the logical topology.

Data Link Layer Protocols – The frame
• The L2 protocol used depends on the logical topology of the
network and the implementation of the Physical layer.
• Ex: Ethernet , PPP, HDLC, Frame Relay, ATM.
• The Layer 2 protocol used for a particular network topology is
determined by the technology used to implement that topology.
These technologies are
1. LAN Technology
2. WAN Technology
• These technologies are determined by the size of the network,
the number of hosts and the geographic scope, and the services
to be provided over the network.

Ethernet Protocol for LANs
• A family of networking technologies , IEEE 802.2 and 802.3.
• Define both the Layer 2 protocols and the Layer 1 technologies.
• Provides unacknowledged connectionless service over a shared
media using CSMA/CD as the media access methods.

Point-to-Point Protocol for WANs
• PPP is a protocol used to deliver frames between two nodes.
• PPP standard is defined by RFCs.

Wireless Protocol for LANs
• 802.11, Wi-Fi, is an extension of the IEEE 802 standards. It uses
the same 802.2 LLC and 48-bit addressing scheme.
• Contention-based system using CSMA/CA media access process.
• uses Data Link acknowledgements to confirm that a frame is
received successfully.

Follow data through an Internetwork
1
2

3
4

5
6

Data Link Layer of Router B gets data of the media7
8

Data Link Layer of Router A gets data off the media
Data Link Layer of Router B puts data on the media9
10

Data Link Layer of Router A puts data on the media
11
12

Physical Layer transports data across the media13
14

15
16

17

Network Fundamentals: Ch7 - Data Link Layer

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