Module 7: the data link layer. application layer provides interface to user. transport layer evides and manages communications between processes running on end systems. network layer protocols allow trafic to travel across network. datalink layer sits between network and physical layers. 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. Datalink layer uses media access methods and error detection techniques. Frame is the pdu for this layer. Node is the term for stations at this layer. physical network as aposed to logical network. Physical network is how cables fibres wireless are set up, Logical network is how network is arranged using hirarcical addresses. data link allows upper layers to put data onto network without concerning themselves with the media used and characteristics. Data Link Layer protocols govern how to format a frame for different media. media access control controls how frames get on and off media. A device has a card or interface that handles the media access control and framing. Frames are created and formatted differently for each media. Frame has header, data and trailer. Frames can contain: 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 Control - Flow control services A data field -The frame payload (Network layer packet) Data link is broken into 2 sub layers, LLC and Mac. This is done so that different types of frame can travel on the same media example ethernet ipx and tcp/ip. Logical Link Control sublayer: - Frames the Network Layer packet. - Identifies the Network Layer protocol. Media Access Control sublayer: - Addresses the frame. - Marks the beginning and end of the frame. Datalink layer not defined by rfcs. Engineering organizations that define open standards and protocols that apply to the Data Link layer include: International Organization for Standardization (ISO) Institute of Electrical and Electronics Engineers (IEEE) American National Standards Institute (ANSI) International Telecommunication Union (ITU) Media control techniques: Sets rules to how media is shared if it is shared as all and rules for getting frames onto the media. Control and contention. deterministic and nondeterministic. Devices take turns and are arbitrated. non-deterministic methods such as csma/cd or csma/ca. If 2 packets get sent at once collision occurs, data corrupted packets need to be resent. if too many devices collisions are 2 numerous and throughput decreases. csma/cd listens to media looks for signal if no signal is seen device transmits, if 2 devices transmit then devices wait random time and try again. If 15 retransmits fail frame iEthernet Frame is dropped. uses csma/cd, wireless uses csma/ca. Token ring, fddi etc use deterministic methods to transmit data. Point to point links do not require control rules generally speaking. Define full and half duplex. If medium is full duplex no arbitration necessary. Logical verses physical topology. Discuss bus, star ring and mesh. point to point can be connected together using a virtual circuit and packets going through intermediate devices but logical topology still point to point. Multi-access toopology needs media control method. Discuss token passing. Depending on type of environment amount of overhead and error checking differs. Frame header contains information specific to environment, physical addresses etc. Example of fields in frame header are: 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 Physical addresses are used at data-link layer, these addresses are not hirarcical. Physical addresses are only relevant to the network segment they are on when packet goes through router physical addresses are removed and re-added. Trailers are used for error detection and end of frame markers amung other things. Cyclic redundancy check. Examples of data-link protocols: Ethernet Point-to-Point Protocol (PPP) High-Level Data Link Control (HDLC) Frame Relay Asynchronous Transfer Mode (ATM) *** Lans use high bandwidth small geographic area, Wans use lower bandwidth technologies. Ethernet standardized by 802.2 and 802.3 standards. speeds 10-10,000 mbps. extremely popular. Ethernet frames used across all different types of ethernet lans. Mac address 48 bits long, usually expressed in hexadecimal, 24 bits for manifacturer id and 24 bits for specific network card. Ethernet does not acknowledge frames but it does check them. Wireless networks have different protocols even though mac addresses are used no notion of connection as such. Frames are acknowledged on wireless.