© 2007 Cisco Systems, Inc. All rights reserved.Cisco Public ITE PC v4.0 Chapter 1 1 OSI Data Link Layer Network Fundamentals – Chapter 7

ITE PC v4.0 Chapter 1 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.

ITE PC v4.0 Chapter 1 3 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Data Link Layer – Accessing the Media  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 placed onto the media and is received from the media using techniques such as media access control and error detection

ITE PC v4.0 Chapter 1 4 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Terms specific to this layer:  Frame: The Data Link layer PDU  Node: The Layer 2 notation for network devices connected to a common medium  Media /Medium: The physical means for the transfer of information between two nodes  Network (physical): Two or more nodes connected to a common medium

ITE PC v4.0 Chapter 1 5 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Data Link Layer – Accessing the Media  Describe why Data Link layer protocols are required to control media access

ITE PC v4.0 Chapter 1 6 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Controlling Transfer across Local media  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.

ITE PC v4.0 Chapter 1 7 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Data Link Layer – Accessing the Media

ITE PC v4.0 Chapter 1 8 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Data Link Layer – Standards

ITE PC v4.0 Chapter 1 9 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Creating a Frame  The description of a frame is a key element of each Data Link layer protocol. Data Link layer protocols require control information to enable the protocols to function.  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

ITE PC v4.0 Chapter 1 10 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Data Link Sublayers  Logical Link Control  Media Access Control The upper sublayer defines the software processes that provide services to the Network layer protocols. Identifies which Network layer protocol is being used for the frame. E.g IP, IPX The lower sublayer defines the media access processes performed by the hardware.

ITE PC v4.0 Chapter 1 11 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Conti…  Typical field types include: Start and stop indicator fields - The beginning and end limits of the frame Naming or addressing fields Type field - The type of PDU contained in the frame Quality - control fields A data field -The frame payload (Network layer packet)

ITE PC v4.0 Chapter 1 12 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Placing data on the Media  Regulating the placement of data frames onto the media is known as media access control.  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

ITE PC v4.0 Chapter 1 13 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Media Access Control for Shared Media There are two basic media access control methods for shared media:  Controlled - Each node has its own time to use the medium  Contention-based - All nodes compete for the use of the medium also known as NON Deterministic  CSMA/CD and CSMA/CA (Detail in Chp9)

ITE PC v4.0 Chapter 1 14 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Media Access Control Techniques  Full Duplex and Half Duplex  Logical Topology Vs Physical Topology  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. (Detail in next Chapters)  Logical topology is the way a network transfers frames from one node to the next. This arrangement consists of virtual

ITE PC v4.0 Chapter 1 15 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Media Access Control Techniques  Contrast logical and physical topologies

ITE PC v4.0 Chapter 1 16 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Media Access Control Addressing and Framing Data  Describe the purpose of encapsulating packets into frames to facilitate the entry and exit of data on media

ITE PC v4.0 Chapter 1 17 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Fields in 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 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

ITE PC v4.0 Chapter 1 18 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Conti…  Frame Check Sequence: The Frame Check Sequence (FCS) field is used to determine if errors occurred in the transmission and reception of the frame. To ensure that the content of the received frame at the destination matches that of the frame that left the source node, a transmitting node creates a logical summary of the contents 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. The FCS is used to detect the error, but not every protocol will support correcting the error.  Stop Frame: It indicates the end of frame.

ITE PC v4.0 Chapter 1 19 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Media access control addressing and framing data  Data Link layer address is only used for local delivery.  If the packet in the frame must pass onto another network segment, the intermediate device - a router - will decapsulate the original frame, create a new frame for the packet, and send it onto the new segment.

ITE PC v4.0 Chapter 1 20 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Different types of protocols 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)  Each protocol performs media access control for specified Layer 2 logical topologies. E.g LAN technology, WAN techonology

ITE PC v4.0 Chapter 1 21 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Summary

ITE PC v4.0 Chapter 1 22 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public