7Reminder of encapsulation/decapsulation Data Link HeaderIP HeaderTCP HeaderHTTP HeaderData Link TrailerDataData Link HeaderData Link HeaderData Link TrailerData Link TrailerIP PacketIP PacketData Link HeaderData Link HeaderData Link TrailerData Link TrailerIP PacketIP PacketData Link HeaderData Link HeaderData Link TrailerData Link TrailerIP PacketIP PacketData Link HeaderIP HeaderTCP HeaderHTTP HeaderData Link TrailerData
8Data Link Frame1.2.The Data Link layer provides a means for exchanging data over a common local media.The Data Link layer performs two basic services:1. Allows the upper layers to access the media using techniques such as framing2. Controls how data is placed onto the media and is received from the media.
9Data Link FrameThe 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.The Data Link layer frame includes:Data – Layer 3 (IP Packet or other Layer 3 information)Header - Control information, such addressingTrailer - Control information, such as error detection
11Data Link Frame Typical field types may include: Start and stop indicator fieldsAddressing fieldsType field - The type of PDU contained in the frameQuality - control fieldsData field -The frame payload (Network layer packet)Not all protocols include all of these fields.
12Data Link Layer Data Link layer Connects the Network Layer with the Physical LayerNetwork Layer and above is software (IP, TCP, HTTP, etc.)Physical layer is implemented in hardware (converting bits to a transmission signal)Data Link layer is implemented in both:SoftwareHardwareData Link Layer prepares Network Layer packets for transmission across some form of media, be it copper, fiber, or the atmosphere.
13Data Link Sublayers Data Link layer has two sublayers (sometimes): Logical Link Control (LLC) – Software processes that provide services to the Network layer protocols.Frame information that identifies the Network layer protocol.Multiple Layer 3 protocols, (IP and IPX) can use the same network interface and media.Media Access Control (MAC) - Media access processes performed by the hardware.Provides Data Link layer addressing and framing of the data according to the protocol in use.
14Data Link StandardsTCP/IP standards are defined by Internet Engineering Task Force (IETF).Data Link layer protocols are defined by:ISO - International Organization for StandardizationIEEE - Institute of Electrical and Electronics EngineersITU - International Telecommunication UnionANSI - American National Standards Institute
15Media Access ControlMedia Access Control - Regulates the placement of data frames onto the media.The method of media access control used depends on:Media sharingDo more than two nodes share the media?If so, how? (Switches, hubs, etc.)
16Serial vs multi-access Point-to-PointPoint-to-Point networksOnly two nodes/30 subnets are commonProtocols: PPP, HDLC, Frame RelayMulti-access networks (LANs)Multiple nodesSubnets mask range depends upon the number of hosts (nodes)Protocols: Ethernet, (wireless), Frame Relay Multipoint
17Duplex Transmissions Simplex Transmission: One way and one way only. One way streetHalf-duplex Transmission: Either way, but only one way at a time.Two way street, but only one way at a time (land slide).Ethernet hubs use half-duplexFull-duplex Transmission: Both ways at the same time.Two way streetEthernet switches use full-duplexMost serial links are full-duplex
18Physical TopologyHubThe physical topology is an arrangement of the nodes and the physical connections between them.SwitchSerial Connections
19Logical TopologyA logical topology - The way a network transfers frames from one node to the next.Defined by Data Link layer protocols.Media Access Control used.Type of network framing
20Point-to-Point topology A point-to-point topology connects two nodes directly together.The media access control protocol can be very simple.Frames from one devices are for the device at the other end.Point-to-point topologies, with just two interconnected nodes, do not require special addressing.
21Logical Point-to-Point Networks Point-to-point networks may include intermediate devices.No affect on logical topology.The logical connection (in some cases) may be a virtual circuit.A virtual circuit is a logical connection created within a network between two network devices.The two nodes exchange the frames with each other.Data Link Destination address is the device at the other end of the virtual circuit.
22Multi-access Topology A logical multi-access topology - Enables a number of nodes to communicate by using the same shared media.“Data from only one node can be placed on the medium at any one time.”(This is only true when using CSMA/CD (hubs), NOT true with switches or wireless)Every node “may” see all the frames that are on the medium.Data Link Destination Address denote which device the frame is for.
23Multi-access Addressing 2222444466663333555566662222Multi-access networks require an address to specifically identify the destination.
24Media Access ControlThe media access control methods used by logical multi-access topologies are typically:CSMA/CD - HubsCSMA/CA - WirelessToken passing – Token RingLater
25Ring Topology Token Passing media access control Each node in turn receives a frame.If data link destination address is not for that device, passes frame to next node.
26Data Link FrameNo one frame structure meets the needs of all data transportation across all types of media.Depending on the:EnvironmentAmount of control information neededTopology
27Data Link Frame Fields Data Link frame header fields may include: Start Frame field - Indicates the beginning of the frameSource and Destination address fields - Indicates the source and destination nodes on the mediaPriority/Quality of Service field - Indicates a particular type of communication service for processingType field - Indicates the upper layer service contained in the frameLogical connection control field - Used to establish a logical connection between nodesPhysical link control field - Used to establish the media linkFlow control field - Used to start and stop traffic over the mediaCongestion control field - Indicates congestion in the media
28Framing- The Trailer The signals on the media could be subject to: InterferenceDistortionLossThis would change the bit values that those signals represent.The trailer is used to determine if the frame arrived without error.Error detection.The Frame Check Sequence (FCS) field is used to determine if errors occurred in the transmission and reception of the frame.
29Cyclic Redundancy Check Cyclic redundancy check (CRC) is commonly used.Sending node includes a logical summary of the bits in the frame.Receiving node calculates its own logical summary, or CRC.Compares the two CRC values.Equal – Accepts the frameDifferent – Discards the frame
30Bandwidth LAN typically uses a high bandwidth technology Supporting large numbers of hostsWANHigh bandwidth technology is usually not cost-effective across large geographic areas (cities or multiple cities, for example).The cost typically results in lower bandwidth capacity.Note: This is relative and the need for high bandwidth on WANs is increasing due to video, voice, and other applications.
31Ethernet Protocol for LANs Ethernet is a family of networking technologies that are defined in the IEEE and standards.Uses 48 bit addressing (Ethernet MAC addresses) for Source and DestinationMore next week!
32Point-to-Point Protocol for WANs Point-to-Point Protocol (PPP) is a protocol used to deliver frames between two nodes.PPP can be used on various physical media, including:Twisted pairFiber optic linesSatellite transmission
33Wireless Protocol for LANs is an extension of the IEEE 802 standards.It uses the same 48-bit addressing scheme as other 802 LANs.Contention-based system using a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA)
34Chapter 7 Data Link Layer CIS 81 Networking FundamentalsRick GrazianiCabrillo CollegeLast Updated: 4/27/2008