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WMU CS 5550 - Data Link

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Data Link ProtocolsTCP/IP Suite and OSI Reference Model• The TCP/IP protocol stack does not define the lower layers of a complete protocol stack• In this lecture, we will address how the TCP/IP protocol stacks interfaces with the data link layerData Link Layer The main tasks of the data link layer are: Transfer data from the network layer of one machine to the network layer of another machine Convert the raw bit stream of the physical layer into groups of bits (“frames”)NetworkLayerData LinkLayerPhysicalLayerNetworkLayerData LinkLayerPhysicalLayerTwo types of networks at the data link layer Broadcast Networks: All stations share a single communication channel Point-to-Point Networks: Pairs of hosts (or routers) are directly connected Typically, local area networks (LANs) are broadcast and wide area networks (WANs) are point-to-pointBroadcast Network Point-to-Point NetworkLocal Area Networks Local area networks (LANs) connect computers within a building or a enterprise network Almost all LANs are broadcast networks Typical topologies of LANs are bus or ring or star We will work with Ethernet LANs. Ethernet has a bus or star topology.Bus LAN Ring LANMAC and LLC In any broadcast network, the stations must ensure that only onestation transmits at a time on the shared communication channel The protocol that determines who can transmit on a broadcast channel are called Medium Access Control (MAC) protocol The MAC protocol are implemented in the MAC sublayer which is the lower sublayer of the data link layer The higher portion of the data link layer is often called Logical Link Control (LLC)Logical LinkControlMedium AccessControlData LinkLayerto Physical Layerto Network LayerIEEE 802 Standards IEEE 802 is a family of standards for LANs, which defines an LLCand several MAC sublayersEthernet Speed: 10Mbps -10 Gbps Standard: 802.3, Ethernet II (DIX) Most popular physical layers for Ethernet: 10Base5 Thick Ethernet: 10 Mbps coax cable 10Base2 Thin Ethernet: 10 Mbps coax cable 10Base-T 10 Mbps Twisted Pair 100Base-TX 100 Mbps over Category 5 twisted pair 100Base-FX 100 Mbps over Fiber Optics 1000Base-FX 1Gbps over Fiber Optics 10000Base-FX 1Gbps over Fiber Optics (for wide area links)Bus TopologyEthernet 10Base5 and 10Base2 Ethernets has a bus topology Starting with 10Base-T, stations are connected to a hub in a star configurationStar TopologyHubEthernet Hubs vs. Ethernet Switches An Ethernet switch is a packet switch for Ethernet frames  Buffering of frames prevents collisions.  Each port is isolated and builds its own collision domain An Ethernet Hub does not perform buffering: Collisions occur if two frames arrive at the same time.HighSpeedBackplaneCSMA/CDCSMA/CDCSMA/CDCSMA/CDCSMA/CDCSMA/CDCSMA/CDCSMA/CDOutputBuffersInputBuffersCSMA/CDCSMA/CDCSMA/CDCSMA/CDCSMA/CDCSMA/CDCSMA/CDCSMA/CDHub SwitchEthernet and IEEE 802.3: Any Difference?  There are two types of Ethernet frames in use, with subtle differences: “Ethernet” (Ethernet II, DIX) An industry standards from 1982 that is based on the first implementation of CSMA/CD by Xerox. Predominant version of CSMA/CD in the US.802.3: IEEE’s version of CSMA/CD from 1985. Interoperates with 802.2 (LLC) as higher layer.Difference for our purposes: Ethernet and 802.3 use different methods to encapsulate an IP datagram.Ethernet II, DIX Encapsulation (RFC 894)802.3 MACdestinationaddress6sourceaddress6type2data46-1500CRC408002IP datagram38-149208062ARP request/reply28PAD1008352RARP request/reply28PAD10IEEE 802.2/802.3 Encapsulation (RFC 1042)802.3 MACdestinationaddress6sourceaddress6length2DSAPAA1SSAPAA1cntl031org code03type2data38-1492CRC4802.2 LLC 802.2 SNAP- destination address, source address:MAC addresses are 48 bit- length:frame length in number of bytes- DSAP, SSAP: always set to 0xaa- Ctrl:set to 3- org code:set to 0- type fieldidentifies the content of thedata field- CRC:cylic redundancy check08002IP datagram38-149208062ARP request/reply28PAD1008352RARP request/reply28PAD10Dial-Up AccessAccessRouterModemsPoint-to-Point (serial) links Many data link connections are point-to-point serial links: Dial-in or DSL access connects hosts to access routers Routers are connected by high-speed point-to-point links Here, IP hosts and routers are connected by a serial cable Data link layer protocols for point-to-point links are simple: Main role is encapsulation of IP datagrams No media access control neededData Link Protocols for Point-to-Point links SLIP (Serial Line IP)  First protocol for sending IP datagrams over dial-up links (from 1988) Encapsulation, not much else PPP (Point-to-Point Protocol):• Successor to SLIP (1992), with added functionality• Used for dial-in and for high-speed routers HDLC (High-Level Data Link) :• Widely used and influential standard (1979)• Default protocol for serial links on Cisco routers• Actually, PPP is based on a variant of HDLCPPP - IP encapsulation The frame format of PPP is similar to HDLC and the 802.2 LLC frame format: PPP assumes a duplex circuit Note: PPP does not use addresses Usual maximum frame size is 1500Additional PPP functionality In addition to encapsulation, PPP supports: multiple network layer protocols (protocol multiplexing) Link configuration Link quality testing Error detection Option negotiation Address notification  Authentication The above functions are supported by helper protocols:  LCP  PAP, CHAP NCPPPP Support protocolsLink management: The link control protocol (LCP) is responsible for establishing, configuring, and negotiating a data-link connection. LCP also monitors the link quality and is used to terminate the link.Authentication: Authentication is optional. PPP supports two authentication protocols: Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP).Network protocol configuration: PPP has network control protocols (NCPs) for numerous network layer protocols. The IP control protocol (IPCP) negotiates IP address assignments and other parameters when IP is used as network layer.Switched networks Some data link technologies can be used to build complete networks, with their own addressing, routing, and forwarding mechanisms. These networks are often called switched networks. At the IP


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