Chapter 7 Notes

Chapter 7 – Data Link Layer Prepares network data for the physical network  Accessing the Media ➢ Performs 2 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 ➢ Terms:  Frame – Data Link Layer PDU  Node – Layer 2 notion for network devices connected to a common medium (a device on a network)  Media/Medium (physical) – physical means for the transfer of information between 2 nodes  Network (physical) – 2 or more nodes connected to a common medium ➢ Data link layer protocols govern how to format a frame for use on different media ➢ Different protocols may be in use for different media  Point-to-Point Protocol (PPP)  Ethernet  High-Level Data Link Control (HDLC)  Frame Relay  Asynchronous Transfer Mode (ATM) ➢ At each hop along the path  An intermediary device accepts frames from one medium  Decapsulates the frame and forwards the packets in a new frame  Headers of each frame are formatted for the specific medium that it will cross ➢ Media access control method – technique used for getting frame on and off media  Media access control methods described by DLL protocols define the processes by which network devices can access the network media and transmit frames in diverse network environments ➢ Creating a Frame  DLL protocols require control info to enable the protocols to function. Info 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  DLL frame includes: • Data – packet from the network layer • Header – contains control information (addressing) & is located at beginning of PDU • Trailer – contains control info added to the end of the PDU  When data travels on media, converted into bits. Field types include:

• Start and stop indicator fields – beginning & end limits of the frame • Naming or addressing fields • Type field – type of PDU contained in frame • Quality – control fields • Data field – frame payload (Network layer packet) ➢ DLL links the software and hardware layers ➢ Physical devices devoted to the DLL have both hardware and software components ➢ Data link sublayers  Upper sublayer defines software processes that provide services to the Network layer protocols  Lower defines the media access processes performed by the hardware 2 Common LAN sublayers • Logical Link Control (LLC) ♦ Places info in the frame that identifies which Network layer protocol is being used for the frame ♦ Allows layer 3 protocols to utilize same network interface & media • Media Access Control (MAC) ♦ Provides DLL addressing & delimiting of data according to physical signaling requirements of medium and type of DLL protocol in use ➢ Organizations define open standards & protocols that apply to DLL  International Organization for Standardization (ISO) • HDLC (High Level Data Link Control)  Institute of Electrical and Electronics Engineers (IEEE) • 802.2 (LLC) • 802.3 (Ethernet) • 802.5 (Token Ring) • 802.11 (Wireless)  American National Standards Institute (ANSI) • Q.922 (Frame Relay Standard) • Q.921 (ISDN Data Link Standard) • HDLC (High Level Data Link Control)  International Telecommunications Union (ITU) • 3T9.5 • ADCCP (Advanced Data Communications Control Protocol) ➢ DLL Processes occur in both software and hardware  Media Access Control Techniques ➢ Media Access Control – regulating the placement of data frames onto the media ➢ Methods  No control at all = collisions • Cause corrupted frames that must be resent  Methods that enforce a high degree of control prevent collisions • Process has high overhead  That enforce a low degree of control have low overhead • More frequent collisions 

➢ Method of MAC used depends on:  Media sharing – if & how nodes share the media  Topology – how connection between nodes appears to DLL ➢ Two basic media access control methods:  Controlled – each node has own time to use the medium (token ring, FDDI) • Network devices take turns • Scheduled access / deterministic • No collisions • Can be insufficient because a device has to wait its turn before using medium  Contention-based – all nodes compete for use of medium (Ethernet, Wireless) • Devices try to access medium whenever it has data to send • Carrier Sense Multiple Access (CSMA) ♦ Prevent chaos ♦ First detect if media is carrying a signal ♦ CSMA/Collision Detection ➢ Ethernet networks ➢ Monitors media for presence of a data signal ➢ If media is free, device transmits data ♦ CSMA/ Collision Avoidance ➢ Wireless networks ➢ Examines media for presence of data signal ➢ If media is free, notification of intent to use is sent, device sends data ➢ Media Access Control for Non-shared media  Half Duplex • Communication means that devices can both transmit & receive on media, but not simultaneously  Full Duplex • Botch devices can transmit & receive on the media at the same time ➢ Topology – arrangement or relationship of network devices and interconnections between them  Physical topology – arrangement of the nodes and the physical connections between them  Logical topology – the way a network transfers frames from one node to the next  The physical topology of a network will most likely not be the same as the logical topology  Media access method used by DL protocol is determined by logical pointto-point topology  Logical & Physical typically used in networks are: • Point-to-Point ♦ Connects 2 nodes directly together ♦ If data can only flow in 1 direction at a time, it is operating at halfduplex link ♦ If data can flow across the link from each node simultaneously; fullduplex link

♦ Intermediary devices do not affect logical topology ♦ Some cases – logical connection between nodes forms a virtual circuit ➢ Logical connection created within a network between 2 network devices • Multi-Access ♦ Enables a number of nodes to communicate by using same shared media ♦ Data from 1 node on medium at any 1 time ♦ CSMA/CD, CSMA/CA ♦ Token passing ➢ A deterministic network method where a “token” is passed around between nodes that authorizes the node to communicate • Ring ♦ Each node receives a frame, if not addressed to node, node passes the frame to next node ➢ Token passing ♦ Nodes remove the frame from ring, examine address & send it on if not addressed for that node ♦ All nodes between source & dest. examine frame  Media Access Control Addressing & Framing Data ➢ The Frame  DLL protocol describes features required for transport of packets across diff. media  Features of protocol integrated into encapsulation of frame  At destination, DLL takes frame off media & framing info is read & discarded In a fragile environment • More controls are needed to ensure delivery • Header and trailer fields are larger – more control info is needed  In a protected environment • Can count on frame arriving at its dest. • Fewer controls needed – smaller fields & frames ➢ Role of the Header  Start Frame field – tells other devices on network that a frame is coming along the medium  Source & Dest. address fields – source and dest. nodes on media  Priority/Quality of Service field – indicates a particular type of communication service for processing  Type field – indicates upper layer service contained in frame (what type of data)  Logical connection control field – establish a logical connection between nodes  Physical link control field – establish the media link  Flow control field – start and stop traffic over media  Congestion control field – indicates congestion in the media 

➢ Where the Frame Goes  DLL address is only used for local delivery  If frame must go to another network • Travels through intermediate device (router) • Decapsulates original frame & creates a new frame for packet • Send onto new segment  Addressing • Point-to-Point (2 nodes) ♦ No address required • Ring & multi-access ♦ Addressing required ➢ Role of Trailer  Used to determine if the frame arrived without error (error detection)  Frame Check Sequence (FCS) field • Used for error checking • Source calculates a number based on the frame’s data and places that # in the FCS field • Dest. recalculates the data to see if FCS matches ♦ If don’t match, dest. deletes frame • Cyclic redundancy check (CRC) – transmitting node creates a logical summary of the contents of the frame  Stop Frame field • Optional field that is used when the length of the frame is not specified in the Type/Length field. • Indicates end of the frame when transmitted ➢ The Frame  Protocol for LANs • Many fields ♦ Preamble – used for synchronization, contains a delimiter to mark end of timing info ♦ Dest. Address – 48 bit MAC address for dest. node ♦ Source Address – 48 bit MAC address for source node ♦ Type – value to indicate which upper layer protocol will receive data after Ethernet process is complete ♦ Data (payload) – the PDU, typically – IPv4 packet, to be transported over media ♦ Frame Check Sequence (FCS) – value used to check for damaged frames  Point-to-Point for WANs (PPP) • Layered architecture • Establishes logical connections – sessions, between 2 nodes • Fields: ♦ Flag – single byte indicates beginning/end of frame – binary sequence 01111110 ♦ Address – single byte contains standard PPP broadcast address

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♦ Control – single byte contains binary seq. 00000011 – transmission of user data in an unsequenced frame ♦ Protocol – 2 bytes identify protocol encapsulated in data field ♦ Data – zero + bytes that contain datagram for the protocol specified in protocol field ♦ FCS – normally 16 bits (2 bytes) – improved error detection Wireless protocol for LANs • 802.11 frame ♦ Protocol Version field - Version of 802.11 frame in use ♦ Type and Subtype fields - Identifies one of three functions and sub functions of the frame: control, data, and management ♦ To DS field - Set to 1 in data frames destined for the distribution system (devices in the wireless structure) ♦ From DS field - Set to 1 in data frames exiting the distribution system ♦ More Fragments field - Set to 1 for frames that have another fragment ♦ Retry field - Set to 1 if the frame is a retransmission of an earlier frame ♦ Power Management field - Set to 1 to indicate that a node will be in power-save mode ♦ More Data field - Set to 1 to indicate to a node in power-save mode that more frames are buffered for that node ♦ Wired Equivalent Privacy (WEP) field - Set to 1 if the frame contains WEP encrypted information for security ♦ Order field - Set to 1 in a data type frame that uses Strictly Ordered service class (does not need reordering) ♦ Duration/ID field - Depending on the type of frame, represents either the time, in microseconds, required to transmit the frame or an association identity (AID) for the station that transmitted the frame ♦ Destination Address (DA) field - MAC address of the final destination node in the network ♦ Source Address (SA) field - MAC address of the node the initiated the frame ♦ Receiver Address (RA) field - MAC address that identifies the wireless device that is the immediate recipient of the frame

♦ Transmitter Address (TA) field - MAC address that identifies the wireless device that transmitted the frame ♦ Sequence Number field - Indicates the sequence number assigned to the frame; retransmitted frames are identified by duplicate sequence numbers ♦ Fragment Number field - Indicates the number for each fragment of a frame ♦ Frame Body field - Contains the information being transported; for data frames, typically an IP packet ♦ FCS field - Contains a 32-bit cyclic redundancy check (CRC) of the frame  Follow Data through an Internetwork 1. Client requests data from web 2. Source application layer initiates data transfer 3. Source transport layer directs session 4. Network layer directs data to the destination host 5. Data link layer puts data onto the media 6. Physical layer transports data across the media 7. Data Link Layer of Router B gets data off the media 8. Network layer of Router B directs toward the destination 9. Data link layer of Router B puts data onto the media 10.Physical layer transports across the media 11.Data link layer of Router A gets data off the media 12.Network layer of Router A directs toward the destination 13.Data link layer of Router A puts data onto the media 14.Physical layer transports data across the media 15.Destination Data Link layer of Server gets data off the media 16.Destination Network layer of server identifies the packet is to the host 17.Destination Transport layer of server identifies the session 18.Destination Application layer server delivers the data