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Communication and Computer Networks The Lecturer: Ahmed Badri 6-Data Link Layer : For Network layer packets to be transported from source host to destination host, they must traverse different physical networks. These physical networks can consist of different types of physical media such as copper wires, microwaves, optical fibers, and satellite links. Network layer packets do not have a way to directly access these different media. 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. 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 As with each of the OSI layers, there are 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 (physical) - The physical means for the transfer of information between two nodes • Network (physical) - Two or more nodes connected to a common medium 1 Communication and Computer Networks The Lecturer: Ahmed Badri 6-1 Data Link Sub layers : 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. 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. Separating the Data Link layer into sublayers allows for one type of frame defined by the upper layer to access different types of media defined by the lower layer. Such is the case in many LAN technologies, including Ethernet. The two common LAN sublayers are: 2 Communication and Computer Networks The Lecturer: Ahmed Badri 6-1-1 Logical Link Control Logical Link Control (LLC) places information in the frame that identifies which Network layer protocol is being used for the frame. This information allows multiple Layer 3 protocols, such as IP and IPX, to utilize the same network interface and media. there are two functions in this sub layer : 1-Greating the 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 3 Communication and Computer Networks The Lecturer: Ahmed Badri • Which nodes will communicate next 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. Unlike the other PDUs that have been discussed in this course, the Data Link layer frame includes: • Data - The packet from the Network layer • Header - Contains control information, such addressing, and is located at the beginning of the PDU • Trailer - Contains control information added to the end of the PDU Framing breaks the stream into decipherable groupings, 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. Typical field types include: • Start and stop indicator fields - The beginning and end limits of the frame • Naming or addressing fields: Indicates the upper layer service contained in the frame • Type field -The type of PDU contained in the frame( it is IPv4) •Quality - control fields (ex: priority) • A data field -The frame payload (Network layer packet) Fields at the end of the frame form the trailer. These fields are used for error detection and mark the end of the frame. For example for frame address is : This address is call MAC address (media access control address) 4 Communication and Computer Networks The Lecturer: Ahmed Badri 2-Identify several sources for the protocols and standards used by the Data Link layer 5 Communication and Computer Networks 6 The Lecturer: Ahmed Badri