Chapter 7 The Data Link Layer
It is the role of the OSI Data Link layer to prepare Network layer packets for transmission and to control access to the physical media. Frame. A PDU at the Data Link layer is called a frame. Node. A node is a device n a network. Media. The media are the physical means used to carry data signals. Network. A network is two or more devices connected to a common medium.

The Data Link later performs two basic services: * Allows the upper layers to access the media using techniques such as framing * Controls how data is places onto the media and is received from the media using techniques such as media access control and error detection 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 frame and then forwards the packets in anew frame. The headers of each frame are formatted for the specific medium that it will cross.
Upper Layer Access to Media The Data Link layer effectively insulates the communication processes at the higher layers from the media transitions that may occur end-to-end. It is responsible for controlling the transfer of frames across the media.
Layer 2 protocols specify the encapsulation of a packet into a frame and the techniques for getting the encapsulated packet on and off each medium. The technique used for getting the frame on and off media is called the media access control method. 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.

3 Network Layer

HEADER Packet TRAILER (Data) Frame

1 Physical Layer The Data Link layer prepares a packet for transport across the local media by encapsulating it with a header and a trailer to create a frame.
Data Link layer frame includes:
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

HEADER Packet TRAILER (Data)

Frame Addressing Type Control DATA Error Frame
Start Detection Stop
1 0 1 0 0 1 1 1 0 0 1 1 0 0 1 1 1 0 0 1 0 0 1

A specific bit pattern denotes Another specific bit pattern the start of the frame. denotes the end of the frame.

Formatting Data for Transmission
Start and stop indicator fields – the beginning and end limits of the frame
Naming or addressing fields
Connecting Upper Layer Services to the Media
The Data Link layer links the software Physical devices devoted to the Data Link layer and hardware layers. have both hardware and software components.

7 Application
6 Presentation
5 Session Implemented in
4 Transport software
3 Network
2 Data Link Implemented in
1 Physical hardware

Data Link prepares the Network layer packets for transmission across some form of media, be it copper, fiber, or the atmosphere.
Data Link Sublayers
A packet is encapsulated into a frame. LOGICAL LINK CONTROL - Frames the Network layer packet 2 Data Link Layer - Identifies the Network layer protocol MEDIA ACCESS CONTROL - Addresses the frame - Marks the beginning and ending of the frame

To support a wide variety of network functions, the Data Link layer is often divided into two sublayers: an upper sublayer and a lower sublayer. The upper sublayer defines the software processes that provide services to the Network layer protocols. The lower sublayer define the media access processes performed by the hardware.
Placing Data on the Media No control at all would result in many collisions. Collisions cause corrupted frames that must be resent. Method that enforce a high degree of control prevent collisions, but the process has high overhead. Methods that enforce a low degree of control have low overhead, but there are more frequent collisions.
Regulating the placement of data frames onto the media is known as media access control.
Media access control is the equivalent of traffic rules that regulate the entrance of motor vehicles onto a roadway.
There are two basic media access control methods for shared media:
Controlled – each node has its own time to use the medium. When using the m=controlled access method, network devices take turns, in sequence, to access the medium. This method is also known as scheduled access or deterministic. When one device places a frame on the media, no other device can do so until the frame has arrived at the destination and has been processed by the destination.
Contention-based – all nodes compete for the use of the medium. Also referred to as non-deterministic, contention-based methods allow any device to try to access the medium whenever it has data to send. To prevent compete chaos on the media, these methods use a Carrier Sense Multiple Access (CSMA) process to first detect if the media is carrying a signal. If a carrier signal on the media from another node is detected, it means that another device is transmitting. When the device attempting to transmit sees that the media is busy, it will wait and try again after a short time period. If no carrier signal is detected, the device transmits its data. In CSMA/Collision Detection (CSMA/CD) the device monitors the media for the presence of a data signal. If a data signal is absent, indicating that the media is free the device transmits the data. If signals are then detected that show another device was transmitting at the same time, all devices stop sending and try again later. In the CSMA/CD access method, all network devices that have messages to send must listen before transmitting. If a device detects a signal from another device, it waits for a specified amount of time before attempting to transmit. When there is no traffic detected, a device transmits its message. While this transmission is occurring, the device continues to listen for traffic or collisions on the LAN. After the message is sent, the devices returns to its default listening mode. The messages propagate across the media until they encounter each other. At that point, the signals mix and the messages are destroyed, a collision has occurred. After the delay has expired on a device, the device goes back into the “listening before transmit” mode. Media access control protocols for non-shared media require little or no control before placing frames onto the media.
Full Duplex and Half Duplex
In point-to-point connections, the Data Link later has to consider whether the communication is half-duplex or full-duplex. In full-duplex communication, both services can transmit and receive on the media at the same time. Half-duplex communication means that the devices can both transmit and receive on the media but cannot do so simultaneously. The topology of a network is the arrangement or relationship of the network devices and the interconnections between them. Network topologies can be viewed at the physical level and the logical level. 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. 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.
POINT-TO-POINT TOPOLOGY
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 successfully flow across the link from each node simultaneously, it is a full-duplex-link.
Logical Point-To-Point Networks
The end nodes communicating in a point-to-point network can be physically connected via a number of intermediate devices. In some cases, the logical connection between nodes forms what is called a virtual circuit. A virtual circuit is a logical connection created within a network between two network devices. A logical multi-access topology enables a number of nodes to communicate by using the same shared media. Data from only 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.
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 the frame to the next node. This allows a ring use a controlled media access control technique 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 that node. In a ring, all nodes around the ring- between source and destination node examine the frame. In a fragile environment, more controls are needed to ensure delivery. The header and trailer fields are larger as more control information is needed. Greater effort needed to ensure delivery = higher overhead – slower transmission rates. In a protected environment, we can count on the frame arriving at its destination. Fewer controls are needed, resulting in smaller fields and smaller frames. Less effort needed to ensure delivery = lower overhead = faster transmission rates.
Each data link frame type has three basic parts:
- Header
- Data
- Trailer

Header Data FCS STOP FRAME
Start Frame Address Type/Length

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 Device addresses at this layer are referred to as physical addresses (MAC Address). Data Link layer addressing is contained within the frame header and specifies the frame destination node on the local network. The frame header may also contain the source address of the frame. Point-to-point topologies, with just two interconnected nodes, do not require addressing. Once on the medium, the frame has only one place it can go.
The Data Link layer address or mac address or physical address is only used for local delivery. Because ting and multi-access topologies can connect many nodes on a common medium, addressing is required for these typologies. When a frame reaches each node in the topology, the node examines the destination address in the header to determine if it is the destination of the frame.
DATA LINK LAYER 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) Frame Field name Preamble Destination Source Type Data Frame Check Sequence Size 8 bytes 6 bytes 6 bytes 2 bytes 46-1500 bytes 4 bytes Ethernet Protocol for LANs Ethernet is the most widely used LAN technology and supports data bandwidths of 10, 100, 1000, or 10,000 Mbps. Shared media requires that the Ethernet frame header use a data Link layer address to identify the source and destination nodes. As with most LAN protocols, this address is referred to as the MAC address of the node. An Ethernet MAC address is 48 bits and is generally represented in hexadecimal format. WIRELESS PROTOCOL for LANS In a wireless environment, the environment requires special considerations. There is no definable physical connectivity; therefore, external factors may interfere with data transfer and it is difficult to control access. To meet these challenges, wireless standards have additional controls.