data link layer

© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE I Chapter 6 1

Data Link Layer

Network Fundamentals – Chapter 7

© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 2

Objectives Identify Data Link layer protocols in data transmission.

Identify 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.

Identify purpose of encapsulating packets into frames to facilitate media access.

Identify Layer 2 frame structure and generic fields.

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


Data Link Layer – Accessing the Media Prepares Network layer packets for transmission on

media via encapsulation

Control access to the physical media

Allows exchange data over a common local media


Data Link Layer – Accessing the Media Data Link layer protocols are required to control media

access

WAN layer 2 protocols HDLC, SLIP, PPP

LAN layer 2 protocols IEEE 802.3, FDDI and Token Ring


Data Link Layer – Accessing the Media Network layer packets are encapsulated into layer 2 frames Frames contain fields that allow the delivery of the data across the WAN or LAN link to be managed Typical fields include: 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


Data Link Layer – Accessing the Media To support a wide variety of network functions, the Data Link layer is often divided into two sublayers: an upper sublayer and an

lower sublayer. Logical Link Control

The upper sublayer defines the software processes that provide services to the Network layer protocols. Media Access Control

The lower sublayer defines the media access processes performed by the hardware.


Data Link Layer – Accessing the Media Standards used by the Data Link layer


Media Access Control Techniques The need for controlling access to the media


Media Access Control Techniques Controlled Access (Token Ring, FDDI)

Contention Based Access (CSMA/CD and CSMA/CA)Carrier Sense Multiple Access / Collision Detection

Carrier Sense Multiple Access / Collision Avoidance


Media Access Control – Duplex and Non Shared Media Full Duplex Transmit and Receive simultaneously (can

double Bandwidth)

Half Duplex Transmit or Receive in turns


Media Access Control Topologies A logical topology is the way a network transfers frames from one node to the next. 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. The logical topology influences the type of network framing and media access control used.

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.


Media Access Control Topology Logical topology example

Type of Layer 2 frame include:

PPP,HDLC, ATM ,FRAME-RELAY etc


Media Access Control Topology Physical point-to-point topology only 2 nodes non multi-

access, non shared

Type of Layer 2 frame include:

PPP,HDLC, ATM ,FRAME-RELAY etc


Media Access Control Topology Physical multi-access topology Contention based

access, Bus topology, Collisions can occur


Media Access Control Techniques Physically resembles a ring topology, media access is

controlled typically by tokens passing between nodes


Media Access Control Addressing and Framing Data

Independence of upper layer protocols is ensured by layer 2 encapsulation. More controls or frame fields are required for fragile environments

GPRS (General Packet Radio Service)

IEEE layer 2 Framing


Media access control addressing and framing data

Frame header contains the control information specified by the Data Link layer protocol for the specific logical topology and media used

Source and Destination MAC address Upper layer data

Error checkingPreamble Start of Frame

End of Frame

Upper layer Service or Protocol i.e. IP or other

layer 3 protocol


Media access control addressing and framing data Data Link layer address is only used for local delivery.

Addresses at this layer have no meaning beyond the local network. Compare this to Layer 3, where addresses in the packet header are carried from source host to destination host regardless of the number of network hops along the route.


Media access control addressing and framing data The frame check sequence is used for error checking.

The source calculates a number based on the frame’s data and places that number in the FCS field. The destination then recalculates the data to see if the FCS matches. If they don’t match, the destination discards the frame.


Labs

7.5.2- Frame Examination (Capturing using Wireshark, inspecting Frame fields, Source, Destination MAC address, Upper layer protocol, FCS)

7.6.1- Skills Integration Challenge Data Link Layer issues (IP subnetting, Configuring using Packet Tracer)

data link layer

Education

cisco systems

data access

media access control

media access processes

physical media

media accesswan layer

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