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Transcript
Data Link Issues Relates to Lab 2. This module covers data link layer issues, such as local area networks (LANs) and point-to-point links, Ethernet, and the Point-to-Point Protocol (PPP). 1 Data 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 using hop by hop transmission over single links. • Convert the raw bit stream of the physical layer into groups of bits (“frames”) Network Layer Network Layer Data Link Layer Data Link Layer Physical Layer Physical Layer 2 Types of Single Segment Networks • There are two types of communication networks: – Broadcast Networks: All stations share a single communication channel – Point-to-Point Networks: Pairs of hosts (or routers) are directly connected Broadcast Network Point-to-Point Network • Typically, local area networks (LANs) are broadcast and wide area networks (WANs) are point-to-point 3 Local Area Network • Local area networks (LANs) typically connect computers within a building or a campus • 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 (Wifi) topology. Bus LAN Ring LAN 4 MAC and LLC Data Link Layer • In any broadcast network, the stations must ensure that only one station transmits at a time on the shared communication channel • The protocol that determines who can transmit on a broadcast channel is called the Medium Access Control (MAC) protocol • The MAC protocol is implemented in the MAC sublayer which is the to Network Layer lower sublayer of the data link layer Logical Link Control • The MAC is physical layer/topology Medium Access dependent Control • The higher portion of the data link to Physical Layer layer is often called the Logical Link Control (LLC) 5 IEEE 802 Standards • IEEE 802 is a family of standards for LANs, which defines an LLC and several MAC sublayers IEEE 802 standard IEEE Reference Model 802.1 Logical Link Control 802.2 802.6 802.5 802.4 802.3 Medium Access Control Physical Layer Higher Layer Data Link Layer Physical Layer 6 Ethernet • Speed: • Standard: 10-1000 Mbps 802.3, Ethernet II (DIX) • Most popular physical layers for Ethernet: • • • • • 10Base-T 10Base2 100Base-TX 100Base-FX 1000Base-FX 10 Mbps Twisted Pair Thin Ethernet: 10 Mbps thin coax cable 100 Mbps over Category 5 twisted pair 100 Mbps over Fiber Optics 1Gbps over Fiber Optics 7 Bus Topology • 10Base5 and 10Base2 Ethernets have a bus topology Ethernet 8 Star Topology • With 10Base-T and higher bit rates, stations are connected to a hub in a star configuration. • Wifi, basically wireless Ethernet, is also a star, Multiple stations connected to one Access Point (AP). Hub 9 Ethernet 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. Hub Switch CSMA/CD CSMA/CD CSMA/CD CSMA/CD CSMA/CD CSMA/CD CSMA/CD CSMA/CD CSMA/CD CSMA/CD CSMA/CD CSMA/CD HighSpeed Backplane CSMA/CD CSMA/CD CSMA/CD CSMA/CD Input Buffers Output Buffers 10 Ethernet II, (RFC 894) 1. Minimum Ethernet Frame size (legacy) has to be 64bytes 2. Minimum Ethernet data payload = 64 – 6 – 6 – 2 - 4 = 46bytes 3. Padding is used to fill up the data field if the IP datagram is less than 46 bytes 802.3 MAC Ethernet destination address source address type data CRC 6 6 2 46-1500 4 0800 IP datagram 2 38-1492 0806 ARP request/reply PAD 2 28 10 0835 2 RARP request/reply PAD 28 Padding Padding 10 11