1、OSI Data Link LayerCCNA Exploration Semester 2Chapter 7 M Rajab-2019OSI data link layer OSI model layer 2 TCP/IP model part of Network Access layerApplicationPresentationSessionTransportNetworkData linkPhysicalApplicationTransportInternetNetwork AccessTCP,UDPIPEthernet,WAN technologiesHTTP,FTP,TFTP,
2、SMTP etcSegmentPacketFrameBitsData streamObjectivesExplain 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
3、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 t
4、railer fields,including addressing,QoS,type of protocol,and Frame Check SequenceM Rajab-2019Functions of data link layer Encapsulates packets by adding a frame header and trailer including appropriate addressing.Controls access to the transmission medium.M Rajab-2019Hops There may be a different lay
5、er 2 protocol in use on each hop of a journey.Different media,different types of link,different bandwidths,LAN/WAN affect the choice of protocol.Different protocols have different frames.The router removes the old frame and adds a new header and trailer for the next hop.Data Link Flow Between Medium
6、sData Link Flow Between MediumsData Link Flow Between MediumsM Rajab-2019Layer 2 frame format All protocols have the same general form but there are variations.Creating Layer 2 FrameConnecting upper layer to MediaM Rajab-2019Data Link Sub layersData linkPhysicalNetworkLogical link controlMedia acces
7、s controlAdds layer 2 address.Marks frame start and end.Sets up the frame header and trailer to encapsulate the packet.Identifies network layer protocol.Data Link Sub layersStandardsMedia Access Control MethodsM Rajab-2019Shared medium Needs media access control.If there is no control there will be
8、many collisions and the frames will be destroyed.Physical busStar with hubM Rajab-2019Point to point link Only two devices on the network Full duplex:both can send at the same time,no problem with media access Half duplex:data can only travel one way at a time so one device can send at a time.Simple
9、 media access control.M Rajab-2019Controlled media access Predictable,deterministic.Each device is given a time when it may send,and it most not send at any other time.High overhead.No collisions.Token passing each host in turn gets the token and is allowed to send.E.g.token ring,FDDIControlled medi
10、a accessM Rajab-2019Contention based media access Non-deterministic,first come first served.Each device“listens”and sends when the medium seems to be clear.Low overhead.Collisions occur.Need a way of re-sending lost frames.Becomes inefficient on large networks.E.g.traditional Ethernet.Contention bas
11、ed media accessM Rajab-2019Variation on contention based Traditional Ethernet uses CSMA/CD(collision detection):collisions are allowed and detected,frames sent again.Wi-fi uses CSMA/CA(collision avoidance):when the medium is clear,host sends signal to say it is about to use the medium.It then sends.
12、Quiz True or falseLogical Point-to-pointLogical Point-to-pointMulti Access TopologyM Rajab-2019Ethernet frame Multi-access links.Full addressing.No control field.Same for all Ethernet types/bandwidths.Timing and startAddresses48 bits eachLayer 3 protocolPacketCheck and stopM Rajab-2019PPP frame Point to point links.Minimal addressing.Control mechanisms.StartMinimal addressPacketCheck and stop
