OSI Data Link Layer

© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public 1Version 4.0

OSI Data Link Layer

Network Fundamentals – Chapter 7

2© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Objectives Explain the role of Data Link layer protocols in data transmission.

Describe how the Data Link layer prepares data for transmission on network media.

Describe the different types of media access control methods.

Identify several common logical network topologies and describe how the logical topology determines the media access control method for that network.

Explain the purpose of encapsulating packets into frames to facilitate media access.

Describe the Layer 2 frame structure and identify generic fields.

Explain the role of key frame header and trailer fields including addressing, QoS, type of protocol and Frame Check Sequence.


Data Link Layer – Accessing the Media So far in this class, you have learned about the following layers:

– 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 communication data so that it can travel across internetworks from the originating host to a destination host.

For network layer packets to be transported from source host to destination host, they must traverse different physical networks.

These physical networks can consist of different types of physical media such as copper wires, microwaves, optical fibers, and satellite links.

The services defined by the OSI data link layer prepares network layer packets for transmission and controls access to the physical media.


Data Link Layer – Accessing the Media



The data link layer performs two basic services:–Allows the upper layers to access the media using techniques such as framing.

–Controls how data is placed onto the media and is received from the media using techniques such as media access control (MAC) and error detection



In any given exchange of network layer packets, there can be numerous data link layer and media transitions.

That is, the packet can be repackaged into different frames as it passes across different media.

At each hop along the path, an intermediary device, usually a router, processes the frame as follows:

1. Accepting the frame from a medium

2. Decapsulating the frame into a packet

3. Constructing a new frame appropriate for the next media

4. Forwarding the packet inside the new frame across the next segment of the physical network


Controlling Transfer Across Local Media The technique used for getting the frame on and off

media is called the media access method or MAC method.

For the data to be transferred across a number of different media, different MAC methods might be required during the course of a single communication.

The media access control methods described by the data link layer protocols define the processes by which network devices can access the network media and transmit frames in diverse network environments.


Controlling Transfer Across Local Media An adapter is used to make the connection to the

network. For example, network interface card (NIC) is used to connect host to the LAN media. The adapter manages the framing and media access method.


Controlling Transfer Across Local Media

The packet is received with an encapsulation used by the LAN technology and reencapsulates the packet into a frame supported by the protocol used in the WAN


Creating a Frame The data link layer frame includes the following

elements:

• Data: The packet from the network layer

• Header: The control information, such as addressing, located at the beginning of the PDU

•Trailer: The control information added to the end of the PDU


Creating a Frame


Protocols & Standards The functional protocols and services are described by engineering

organizations (such as IEEE, ANSI, and ITU) and communications companies.

Engineering organizations set open standards and protocols.

Communications companies can set and use proprietary.

Engineering organizations include

■ International Organization for Standardization (ISO, http://www.iso.org)

■ Institute of Electrical and Electronics Engineers (IEEE, http://www.ieee.org)

■ American National Standards Institute (ANSI, http://www.ansi.org)

■ International Telecommunication Union (ITU, http://www.itu.org)


Protocols & Standards


Media Access Control Techniques The method of MAC used depends on

•Media sharing: If and how the nodes share the media

•Topology: How the connection between the nodes appears to the data link layer


MAC for Shared Media

Medium with multiple nodes.

At any one time, there can be a number of devices attempting to send and receive data using the network media.

The two basic MAC methods for shared media are as follows:

■ Controlled: Each node has its own time to use the medium.

■ Contention-based: All nodes compete for the use of the medium


MAC for Shared Media


MAC for Shared Media Controlled Access for Shared Media

In controlled access method, network devices take turns, in sequence, to access the medium = scheduled access or deterministic.

Device turn, but does not need to access, the opportunity passes to the next device in line.

When one device is placing a frame on the media, no other device can do so until the frame has arrived at the destination and has been processed by the destination.

Well ordered and provides predictable throughput, but can be inefficient because of the ‘waiting’.


MAC for Shared Media Controlled Access for Shared Media


MAC for Shared Media Contention-Based Access for Shared Media

Allow any device to try to access the medium whenever it has data to send = nondeterministic.

To prevent complete chaos on the media, these methods use a carrier sense multiple access (CSMA) process to first detect whether the media is carrying a signal.

When the device attempting to transmit sees that the media is busy, it will wait and try again after a short time period. If no carrier signal is detected, the device transmits its data..



It is possible that the CSMA process will fail and two devices will transmit at the same time.

This is called a collision. If a collision occurs, the data sent by both devices will be corrupted and will need to be re-sent.

CSMA is usually implemented in conjunction with a method for resolving the media contention. The two commonly used methods are

■ CSMA/collision detect (CSMA/CD)

■ CSMA/collision avoid (CSMA/CA)


MAC for Shared Media CSMA/Collision Detect

In CSMA/CD, the device monitors the media for the presence of a data signal.

If a data signal is absent, indicating that the media is free, the device transmits the data.

CSMA/Collision Avoid

In CSMA/collision avoid (CSMA/CA), the device examines the media for the presence of a data signal.

If the media is free, the device sends a notification across the media of its intent to use it. The device then sends the data.


MAC for Nonshared Media Require little or no control before placing frames onto the media.

Have simpler rules and procedures for MAC.

Such is the case for point-to-point topologies

In point-to-point connections, the data link layer has to consider whether the communication is half duplex or full duplex.

Traffic analogy, half duplex = one lane road, and full duplex = two-lane road.

Half-duplex communication means the devices can both transmit and receive on the media but cannot do so simultaneously

In full-duplex communication, both devices can transmit and receive on the media at the same time. The data link layer assumes that the media is available for transmission for both nodes at any time.


MAC for Nonshared Media

Full Duplex


Logical Topology Versus Physical Topology Network topologies can be viewed at the physical level

and the logical level.•The physical topology is an arrangement of the nodes and the physical connections between them. The representation of how the media is used to interconnect the devices is the physical topology.

•A logical topology is the way a network transfers frames from one node to the next. This arrangement consists of virtual connections between the nodes of a network independent of their physical layout. These logical signal paths are defined by data link layer protocols.

The data link layer “sees” the logical topology of a network when controlling data access to the media. It is the logical topology that influences the type of network framing and MAC used.


Logical Topology Versus Physical Topology Logical and physical topologies typically used in

networks are

■ Point-to-point

■ Multiaccess

■ Ring


Logical Topology Versus Physical Topology■ Point-to-point

• A point-to-point topology connects two nodes directly

• Instead of being directly attached, the two end nodes communicating in a point-to-point network can be logically connected through a number of intermediate devices.

• In some cases, the logical connection between nodes forms what is called a virtual circuit. A virtual circuit is a logical connection created within a network between two network devices.


Logical Topology Versus Physical Topology■ Multiaccess

• 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.

• Requires a data link MAC method to regulate the transmission of data and thereby reduce collisions between different signals.


Logical Topology Versus Physical Topology■ Ring

• Each node in turn receives a frame.

• If the frame is not address to the node, the node passes the frame to the next node.

• This allows a ring to use a controlled MAC technique called token passing.

• Nodes in a logical ring topology remove the frame from the ring, examine the destination address of the frame, and send it on if it is not addressed for that node.

• All nodes around ring between the source and destination node examine the frame.

.


Logical Topology Versus Physical Topology Ring


Media Access Control Addressing and Framing Data All data link layer protocols encapsulate the Layer 3

PDU within the data field of the frame.

However, the structure of the frame and the fields contained in the header and trailer vary according to the protocol.

Scenario : For example, we have all types of vehicles from small automobiles to large trucks. Small vehicles are easily maneuverable around crowded urban environments but do not carry many passengers or much cargo.

While large trucks or buses are not maneuverable, they are well suited to carry large payloads on the highway.


Media Access Control Addressing and Framing Data


Framing: Role of the Header

The frame header contains the control information specified by the data link layer protocol for the specific logical topology and media used.

Frame control information is unique to each type of protocol.


Framing: Role of the Header

Typical frame header fields include:■ Start of Frame field: Indicates the beginning of the frame

■ Source and Destination address fields: Indicate the source and destination nodes on the media

■ Priority/Quality of Service field: Indicates a particular type of communication service for processing

■ Type field: Indicates the upper-layer service contained in the frame

■ Logical connection control field: Used to establish a logical connection between nodes

■ Physical link control field: Used to establish the media link

■ Flow control field: Used to start and stop traffic over the media

■ Congestion control field: Indicates congestion in the media


Media Access Control Addressing and Framing Data Cases where addresses are needed and cases where

addresses are not needed


Summary

OSI Data Link Layer

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