Data Link Layer
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Transcript of 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.
© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 3
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
© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 4
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
© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 5
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
© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 6
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.
© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 7
Data Link Layer – Accessing the Media Standards used by the Data Link layer
© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 8
Media Access Control Techniques The need for controlling access to the media
© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 9
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
© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 10
Media Access Control – Duplex and Non Shared Media Full Duplex Transmit and Receive simultaneously (can
double Bandwidth)
Half Duplex Transmit or Receive in turns
© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 11
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.
© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 12
Media Access Control Topology Logical topology example
Type of Layer 2 frame include:
PPP,HDLC, ATM ,FRAME-RELAY etc
© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 13
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
© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 14
Media Access Control Topology Physical multi-access topology Contention based
access, Bus topology, Collisions can occur
© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 15
Media Access Control Techniques Physically resembles a ring topology, media access is
controlled typically by tokens passing between nodes
© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 16
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
© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 17
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
© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 18
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.
© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 19
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.
© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 20
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)