chapter 7 overview-last

8/2/2019 Chapter 7 Overview-Last

1/55

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

OSI Data Link Layer

Network Fundamentals Chapter 7


2/55

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 transmissionon network media.

Describe the different types of media access control methods.

Identify several common logical network topologies and describehow the logical topology determines the media access controlmethod for that network.

Explain the purpose of encapsulating packets into frames tofacilitate media access.

Describe the Layer 2 frame structure and identify generic fields.

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


3/55


Data Link Layer Accessing the Media Describe the service the Data Link Layer provides as it

prepares communication for transmission on specificmedia


4/55


TWO NOTES

It is important to understand the meaning of the words medium and mediawithin the context of this chapter.

Here, these words refer to the material that actually carries the signalsrepresenting the transmitted data. Media is the physical copper cable,

optical fiber, or atmosphere through which the signals travel.

In this chapter media does not refer to content programming such asaudio, animation, television, and video as used when referring to digitalcontent and multimedia.

A physical network is different from a logical network. Logical networksare defined at the Network layer by the arrangement of the hierarchicaladdressing scheme.

Physical networks represent the interconnection of devices on acommon media. Sometimes, a physical network is also referred to as a

network segment.


5/55


7.1

The Data Link layer performs two basic services:

Allows the upper layers to access the media usingtechniques such as framing

Controls how data is placed onto the media and isreceived from the media using techniques such as

media access control and error detection


6/55


Data Link Layer Accessing the Media Describe why Data Link layer protocols are required to

control media access

See the animation in Section 7.1.1


7/557 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Data Link Layer Accessing the Media Describe the role of framing in preparing a packet for

transmission on a given media




A packet is received from and directed to an upper layer protocol,in this case IPv4 or IPv6, that does not need to be aware of which

media the communication will use.

The media access control methods described by the Data Linklayer protocols define the processes by which network devices canaccess the network media and transmit frames in diverse networkenvironments.

At intermediary devices such as a router, where the media typecould change for each connected network, different physicalinterfaces on the router are used to encapsulate the packet intothe appropriate frame, and a suitable media access controlmethod is used to access each link.

As the router processes frames, it will use Data Link layer servicesto receive the frame from one medium, decapsulate it to the Layer3 PDU, re-encapsulate the PDU into a new frame, and place theframe on the medium of the next link of the network.



7.1.3 Data Link Layer - Creating a Frame

Data - The packet from the Network layer

Header - Contains control information, such as addressing, and

is located at the beginning of the PDU

Trailer - Contains control information added to the end of the PDU



7.1.3 Data Link Layer - Creating a Frame

Fields at the end of the frame form the trailer. These fields are used for error detection and

mark the end of the frame.

When data travels on the media, it isconverted into a stream of bits, or 1s and 0s.

If a node is receiving long streams of bits,

how does it determine where a frame starts

and stops or which bits represent the address



Data Link Layer Accessing the Media Describe the role the Data Link layer plays in linking the

software and hardware layers



7.1.4 Data Link Layer - Connecting Upper LayerServices to the Media

The Data Link layer is embodied as a physical entity,such as an Ethernet network interface card (NIC),

which inserts into the system bus of a computer and

makes the connection between running softwareprocesses on the computer and physical media.

The NIC is not solely a physical entity, however.Software associated with the NIC enables the NIC to

perform its intermediary functions of preparing data fortransmission and encoding the data as signals to besent on the associated media.




To support a wide variety of network functions, the Data Link layeris often divided into two sublayers:

The upper sublayer defines the software processes that provide

services to the Network layer protocols. The lower sublayer defines the media access processes

performed by the hardware.

Separating the Data Link layer into sublayers allows for one type offrame defined by the upper layer to access different types of media

defined by the lower layer.

The two common LAN sublayers are:

Logical Link Control

Media Access Control



Data Link Layer Accessing the Media

Identify several sources for the protocols and standardsused by the Data Link layer

Unlike the upper layer

protocols, which areimplemented mostly in

software such as the host

operating system or specific

applications,

Data Link layer processesoccur both in software and

hardware.



7.2 Media Access Control Techniques

Regulating the placement of data frames onto themedia is known as media access control.

The protocols at the Data Link layer define the rules foraccess to different media.

Some media access control methods use highly-controlled processes to ensure that frames are safelyplaced on the media.

These methods are defined by sophisticated protocols,which require mechanisms that introduce overheadonto the network.



7.2 Media Access Control Techniques

The method of media access control used dependson:

* Media sharing - If and how the nodes share themedia

* Topology - How the connection between the nodes

appears to the Data Link layer



7.2.2 Media Access Control for Shared Media

There are two basic media access control methodsfor shared media:

* Controlled - Each node has its own time to use themedium

* Contention-based - All nodes compete for the useof the medium




7.2.2

Controlled Access for Shared Media

Network devices take turns, in sequence, to access themedium. This method is also known as scheduledaccess or deterministic.

If a device does not need to access the medium, theopportunity to use the medium passes to the nextdevice in line.

When one device places a frame on the media, noother device can do so until the frame has arrived at thedestination and has been processed by the destination.


20/55



7.2.2

Contention-based Access for Shared Media

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

This is called a data collision. If this occurs, the data sentby both devices will be corrupted and will need to beresent.

The two commonly used methods:

CSMA/Collision Detection (CSMA/CD)

CSMA/Collision Avoidance (CSMA/CA)



Media Access Control Techniques

Explain the necessity for controlling access to themedia




Identify two media access control methods for sharedmedia and the basic characteristics of each


24/55



Define Full Duplex and Half Duplex as it relates toMedia Access Control for non-shared media


25/55


7.2.4

The physical topology is an arrangement of the nodes and thephysical connections between them. The representation of how themedia is used to interconnect the devices is the physical topology.

A logical topology is the way a network transfers frames from onenode to the next.

This arrangement consists of virtual connections between thenodes 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 whencontrolling data access to the media.


26/55


7.2.4

The physical or cabled topology of a network will mostlikely not be the same as the logical topology.

Logical topology of a network is closely related to

the mechanism used to manage network access.

Logical and physical topologies typically used innetworks are:

* Point-to-Point* Multi-Access

* Ring


27/55



Describe the purpose of a logical topology and identifyseveral common logical topologies


28/55



Identify the characteristics of point-to-point topologyand describe the implications for media access whenusing this topology


29/55


7.2.5 Point-to-point Topology

A point-to-point topology connects two nodes directly together, asshown in the previous figure.

In data networks with point-to-point topologies, the media accesscontrol protocol can be very simple.

All frames on the media can only travel to or from the two nodes.

The frames are placed on the media by the node at one end and takenoff the media by the node at the other end of the point-to-point circuit.

In point-to-point networks,

if data can only flow in one direction at a time, it is operating as a half-duplex link.

If data can successfully flow across the link from each nodesimultaneously, it is a full-duplex link.


30/55



Contrast logical and physical topologies

The logical point-to-point connection

betweentwo nodes may not necessarily be between

two physical nodes at each end of a single physical link.


31/55


7.2.5 Point-to-point Topology: Logical Point-to-Point Networks

As shown in these two figures, the source anddestination node may be indirectly connected to eachother over some geographical distance.

In some cases, the logical connection between nodesforms what is called a virtual circuit.

A virtual circuit is a logical connection created within anetwork between two network devices.


32/55


7.2.6 Multi-Access Topology

Data from only one node can be placed on the mediumat any one time.

Every node sees all the frames that are on the medium,

but only the node to which the frame is addressedprocesses the contents of the frame.

A Data Link media access control method is required toregulate the transmission of data and thereby reduce

collisions between different signals.

The media access control methods used by logicalmulti-access topologies are typically CSMA/CD orCSMA/CA.


33/55



Identify the characteristics of multi-access topology anddescribe the implications for media access when usingthis topology

Play the animation in 7.2.6


34/55


7.2.7 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 theframe to the next node.

This allows a ring to use a controlled media access controltechnique called token passing.

Nodes in a logical ring topology remove the frame from the ring,examine the address, and send it on if it is not addressed for thatnode.

There are multiple media access control techniques that could be

used with a logical ring, depending on the level of controlrequired. For example, only one frame at a time is usually carriedby the media.

Remember that the Data Link layer "sees" a logical ring topology.The actual physical cabling topology could be another topology.


35/55



Identify the characteristics of ring topology and describethe implications for media access when using thistopology

Play the animation

In Section 7.2.7


36/55


7.3.1 Data Link Layer Protocols - The

FrameThere are many different Data Link layer protocols that describe DataLink layer frames, each frame type has three basic parts:

Header

Data

Trailer

All Data Link layer protocols encapsulate the Layer 3 PDU within thedata field of the frame

There is no one frame structure that meets the needs of all data

transportation across all types of media.

As shown in the following figure, depending on the environment, theamount of control information needed in the frame varies to match themedia access control requirements of the media and logical topology.


37/55


Media Access Control Addressing and Framing Data

Describe the purpose of encapsulating packets intoframes to facilitate the entry and exit of data on media


38/55



Describe the role of the frame header in the Data Linklayer and identify the fields commonly found inprotocols specifying the header structure

Different Data Link layer protocols may use different fields from those

mentioned. Because the purposes and functions of Data Link layer

protocols are related to the specific topologies and media, each protocol has to be

examined to gain a detailed understanding of its frame structure.


39/55


7.3.2 Framing - Role of the Header

Typical frame header fields include:

Start Frame field - Indicates the beginning of the frame

Source and Destination address fields - Indicates 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 connectionbetween 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


40/55



Describe the role of the frame header in the Data Linklayer and identify the fields commonly found inprotocols specifying the header structure


41/55



Describe the role of addressing in the Data Link layerand identify cases where addresses are needed andcases where addresses are not needed


42/55


7.3.3 Addressing - Where the Frame Goes

Device addresses at this layer are referred to as physicaladdresses.

Data Link layer addressing is contained within the frame headerand specifies the frame destination node on the local network.

The frame header may also contain the source address of theframe.

Unlike Layer 3 logical addresses that are hierarchical, physicaladdresses do not indicate on what network the device is located.

Because the frame is only used to transport data between nodesacross the local media, the Data Link layer address is only usedfor local delivery.


43/55


7.3.3 Addressing - Where the Frame Goes

Addressing Requirements

The need for Data Link layer addressing at this layer dependson the logical topology.

Point-to-point topologies, with just two interconnected nodes,do not require addressing. Once on the medium, the framehas only one place it can go.

Because ring and multi-access topologies can connect many

nodes on a common medium, addressing is required for thesetypologies. When a frame reaches each node in the topology,the node examines the destination address in the header todetermine if it is the destination of the frame.


44/55




45/55


7.3.4 Framing - Role of the Trailer

Data Link layer protocols add a trailer to the end of eachframe.

The trailer is used to determine if the frame arrived withouterror. This process is called error detection.

Error detection is accomplished by placing a logical ormathematical summary of the bits that comprise the framein the trailer.

The Frame Check Sequence (FCS) field is used todetermine if errors occurred in the transmission andreception of the frame.


46/55


7.3.4 Framing - Role of the Trailer

Error detection is added at the Data Link layer because thisis where data is transferred across the media.

The signals on the media could be subject to interference,

distortion, or loss that would substantially change the bitvalues that those signals represent.

To ensure that the content of the received frame at thedestination matches that of the frame that left the source

node, a transmitting node creates a logical summary of thecontents of the frame.

This is known as the cyclic redundancy check (CRC) value.This value is placed in the Frame Check Sequence (FCS)

field of the frame to represent the contents of the frame.


47/55


7.3.5 Data Link Layer Protocols - The Frame

In a TCP/IP network, all OSI Layer 2 protocols work with theInternet Protocol at OSI Layer 3.

However, the actual Layer 2 protocol used depends on thelogical topology of the network and the implementation of thePhysical layer.

Protocols that will be covered in CCNA courses include:

* Ethernet

* Point-to-Point Protocol (PPP)* High-Level Data Link Control (HDLC)

* Frame Relay

* Asynchronous Transfer Mode (ATM)


48/55



LAN Technology

A Local Area Network typically uses a high bandwidth technologythat is capable of supporting large numbers of hosts.

WAN TechnologyHowever, using a high bandwidth technology is usually not cost-effective for Wide Area Networks that cover large geographic areas(cities or multiple cities, for example). The cost of the long distancephysical links and the technology used to carry the signals over those

distances typically results in lower bandwidth capacity.Difference in bandwidth normally results in the use of different

protocols for LANs and WANs.


49/55



Ethernet Protocol for LANs

Ethernet is a family of networking technologies that aredefined in the IEEE 802.2 and 802.3 standards.

Ethernet standards define both the Layer 2 protocols andthe Layer 1 technologies.

Ethernet is the most widely used LAN technology andsupports data bandwidths of 10, 100, 1000, or 10,000Mbps.

Ethernet provides unacknowledged connectionlessservice over a shared media using CSMA/CD as themedia access methods.


50/55



51/55



Point-to-Point Protocol for WANs

Point-to-Point Protocol (PPP) is a protocol used to deliverframes between two nodes.

Unlike many Data Link layer protocols that are defined byelectrical engineering organizations, the PPP standard isdefined by RFCs (from the IETF).

PPP uses a layered architecture. To accommodate thedifferent types of media, PPP establishes logicalconnections, called sessions, between two nodes.

This includes authentication, compression, and multilink(the use of multiple physical connections).


52/55



53/55


Wireless Protocol for LANs (See the Section 7.3.5, part4)

See the animations in Section

7.4 Putting it All Together


54/55


Summary


55/55

chapter 7 overview-last

Documents