Tài liệu Module 7- Data Link Layer CCNA Exploration 4.0 pptx

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Module 7/- Data Link Layer

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Overview

Upon completion of this chapter, you will be able to:

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.faeilitate media access

Describe the Layer 2 frame structure and identify generic fields

Explain the role of key frame header and trailertfields,»including addressing, QoS, type of protocol, and Frame Check Sequence

Hoc vién mang Bach khoa - Website: www.bkacad.com

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Data Link Layer — Accessing the Media

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Supporting & Connecting to Upper Layer Services

A PDU at the Data

Link layer is called

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The Data Link layer provides a means for exchanging data over a common local 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.olaced ơnto the

fnedia and Is received from the

media using techniques such as

media access control and error detection

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Supporting & Connecting to Upper Layer Services

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The Data Link Layer

Data link layer protocols Different protocols may be

govern how to format a in use for different media

frame for use on different

media,

At each hop along the path, an intermediary device accepts frames from one medium, decapsulates the rame and then forwards the packets in a new frame The

2 headers of each frame are formatted for the specific

medium that it will cross

e The Data Link layer effectively insulates the communhication*processes at the higher layers from the media transitions that mayoccurend-to-end

° 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

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Controlling Transfer across Local Media

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‘ ‹ ——ễễễ —————

¬Y —— wAN Header WAN Trailer

WAN Header WAN Treiber ee

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Creating a Frame

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Data Link Layer Services

° Control information may tell:

— Which nodes are in communication with each other:

— When communication between individual nodes begins and when it ends

— Which errors occurred while the nodes communicated

— Which nodes will communicate next

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Creating a Frame

Formatting Data for Transmission

Packet

Frame Addressina Start ressing | lype ÍT Quality Ki DATA L rror Detection D : E rame Stop S

frame of the frame

e Framing breaks the stream into decipherable greupings, with control information inserted in the header and trailer as values in different fields

° This format gives the physical signals a structure that can be received by nodes and decoded into packets at the destination

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Connecting Upper Layer Services to the Media

components

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e The Data Link layer exists as a connecting layer between the software

orocesses of the layers above it and the Physical layer below it

° As such, it prepares the Network layer packets for transmission across some form of media, be it copper, fiber, or the atmosphere

Hoc vién mang Bach khoa - Website: www.bkacad.com 10

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Connecting Upper Layer Services to the Media

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Data Link Sublayers

[ 3 Network Layer |

HEADER Packet TRAILER A packet is encapsulated into a frame

LOGICAL LINK CONTROL

«= Frames the Network layer packet

= Identifies the Network layer protocol

< 2 Data Link Layer PN

MEDIA ACCESS CONTROL

» Addresses the frame

* Marks the beginning and ending ofthe frame

— 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

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Standards

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Standards for the Data Link Layer

ISO: HDLC (High Level Data Link Control)

802.2 (LLC), IEEE: 802.3 (Ethernet)

802.5 (Token Ring) 802.11 (Wireless LAN)

Q.922 (Frame Relay Standard) ITU: Q.921 (ISDN Data Link Standard)

HDLC (High Level Data Link Control)

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Media Access Control Techniques

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Placing Data on the Media

Media Access Control Methods

No control

No control at all would result

in many collisions | Collisions cause corrupted FRAME —

frames that must be resent Shared Media : FRAME

Methods that enforce a high Take turns

degree of control prevent

collisions, but the process =

> “———————— FRAME

Methods that enforce a low FRAME 2 ——— |

degree of control have low Shared Media 31 |

overhead, but there are FRAME

more frequent collisions là

»

° Regulating the placement of data frames onto the media\is known’as media access

control

e The method of media access control 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

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Media Access Control for Shared Media

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We need rules for how to

e There are two basic media access control methods forshared media:

— Controlled - Each node has its own time tose thexmedium

— Contention-based - All nodes compete for the use of the medium

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Controlled

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> Controlled Access * Only one station transmits at a time * Token Ring

* Devices wishing to transmit must wait their * FDDI turn

* No collisions

* Some deterministic networks use token passing

e When using the controlled access method, network devices.take turns, in

sequence, to access the medium This method _is\also known as scheduled

access or deterministic

e Although controlled access is well-ordered and provides predictable

throughput, deterministic methods can be inefficient because a device has to wait for its turn before it can use the medium

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—— Contention Based * Stations can transmit at any time * Ethernet

Access * Collisions exist * Wireless

* Mechanisms exist to resolve contention:

* CSMA/CŨD for Ethernet networks

* CSMA/CA for 802.11 wireless networks

° Also referred to as non-deterministic, contention-basedmethods allow any

device to try to access the medium whenevet_ithas.data to send

e To prevent complete chaos on the media, these methods use a Carrier Sense Multiple Access (CSMA) process to first detect if the media is carrying a

signal

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¢ Traditional forms of Ethernet use this method

— CSMA/Collision Avoidance

¢ In CSMA/Collision Avoidance (CSMA/CA), the device examines the media for the presence of a data signal If the-mediaxis tree, the device sends a notification across the media ofvts'intenfto use it The device then sends the data

¢ This method is used by 802.11 wireless networking technologies

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Media Access Control for Non-Shared Media

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Media Access Control for Non-shared media

Only you and |

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We can send We can send

and receive at

wot senna

vane Network | Transmitting

ee FRAME eo BRAM - FRAME - >

-<.- - PRAME -. - .- ERAMER - - - - ERAMEE - - FRAME

Receiving Receiving

e In full-duplex communication, both devices can transmitand receive on the

media at the same time

e The Data Link layer assumes that the media ¡is available for transmission for both nodes at any time Therefore, there is no media arbitration necessary in the Data Link layer

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We can send We can send and receive, and receive,

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Wait until this frame is

received before sending =”

this one Receiving

° Half-duplex communication means that the devices.can both transmit and

receive on the media but cannot do so simultameously

e Ethernet has established arbitration rules for resolving conflicts arising from

instances when more than one station attempts to transmit at the same time

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Logical Topology vs Physical Topology

e The topology of a network is the

interconnections between them

T a e Network topologies can be viewed

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Physical Topologies Topology T 1 † Topology

e The physical topology is an arrangement of the nodesand the physical

connections between them

The representation of how the media is used to interconnect the devices is the physical topology

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° 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

e 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 media access control used

e The physical or cabled topology of a network will most likely not be the same as the logical topology

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Limited to two nodes

° A point-to-point topology connects 2 nodes directly together:

e In data networks with point-to-point topologies, the.media access control

protocol can be very simple

e In point-to-point networks, if data can only flow in-one direction at a time, it is operating as a hali-duplex link If data can successfully flow across the link

from each node simultaneously, it is a full-duplex link

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Point-to-Point Topology

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Logical Point-to-Point Topology

Adding intermediate physical connections may not change the

e A virtual circuit is a logical connection created within a network between two

network devices The two nodes on either end of the-virtual circuit exchange the frames with each other

e Virtual circuits are important logical communication constructs used by some Layer 2 technologies

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Multi-Access Topology

Logical Multi-Access Topology

Data from only one node can be placed on the\medium-at any one time

Every node sees all the frames that are on the medium, but only the node to which the frame is addressed processes the contents of the frame

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Multi-Access Topology

Logical Multi-Access Topology

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° Ina logical ring topology, each node in turn receives @ frame:

e Ifthe frame is not addressed to the node, the-node passes the frame to the

next node This allows a ring to use a controlled media access control

technique called token passing

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Media Access Control Addressing

& Framing Data

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Data Link Layer Protocols- The Frame

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Data Link Layer Protocols - The Frame

In a fragile environment, Greater effort needed to ensure delivery = higher overhead = slower more controls are needed transmission rates

header and trailer fields ow are larger as more control

inf ation is needed

In a protected Less effort needed to ensure delivery = lower overhead = faster

environment, we can count transmission rates

on the frame arriving at its

smaller fields and smaller

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Framing- Role of the Header

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The Role of the Header

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The Type/Length field is an optional field used by some

“||protocols to state either what type of data is coming or possibly the length of the frame

The Start Frame field tells other devices on the network

that a frame is coming along the medium

Tiêu đề	Data Link Layer CCNA Exploration 4.0
Trường học	Bach Khoa Networking Academy
Chuyên ngành	Data Link Layer
Thể loại	lecture slides
Năm xuất bản	2023

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Số trang	64
Dung lượng	2,12 MB