11LAN Topologies, Access methods(Week 1, Wednesday 1/10/2007)© Abdou Illia, Spring 20072Learning Objectives Describe LAN topologies Describe major LAN access methods (Ethernet, Token ring, and FDDI)3Network Topologies Physical topology is different than Logical topology Physical topology = Shape of the network How computers connect to each other in the network  Logical topology How actually messages are transmitted in the LAN Three main topologies Bus  Ring  Star Topologies differ in terms of: Cost (both to install and maintain) Performance and Reliability24Bus Topology Built by connecting nodes (PCs or Servers) to a Bus using patch cable Terminators signal the physical end to the segment 10Base2 & 10Base5 Ethernet are the most prevalent bus networks.Patch cable5Advantages of Bus Topology Works well for small networks Relatively inexpensive to implementDisadvantage of Bus Topology Potential for congestion with network traffic6Ring Topology Logical network arrangement Nodes (workstations and servers) attached at points around the ring Data goes around the ring from node to node Implemented using a MAU (Multistation Access Unit)37Advantages of Ring Topology Well-suited for transmitting signals over long distances on a LAN Enables reliable communication8Disadvantages of Ring Topology Expensive Not used as widely as bus topology Fewer equipment options Fewer options for expansion to high-speed communication9Star Topology Most common physical network design Multiple nodes attached to a central wiring point410Advantages of Star Topology Low startup costs Offers opportunities for expansion Most popular topology in use; wide variety of equipment available11Disadvantages ofStar Topology Single point of failure (Hub, etc.) Requires more cable than Bus topology12LAN Access Methods Access Method = Set of rules governing how computers access the network Ethernet IEEE 802.3 specifications Token ring IEEE 802.5 specifications FDDI (Fiber Distributed Data Interface) ANSI X3T9.5 (High-speed variation of token ring)Most Common Access methods used in LANIEEE = Institute of Electrical and Electronic Engineers513Ethernet Many Ethernet (IEEE 802.3) standards Uses CSMA/CD access method for data transmission on a network CSMA/CD* :1) All computers (“carriers”) listen (“sense”) for traffic on the LAN2) If no traffic, computer that wishes to transmit may transmit3) If collision occurs, computers must wait a random amount of time4) The computer with smallest random number send again first.* Carrier Sense Multiple Access with Collision Detection14Ethernet (CSMA/CD) CSMA/CD rules are: Carrier Sense Multiple Access (CSMA)1) If a NIC wishes to transmit, it must listen for traffic If there is no traffic, the NIC may transmit If there is traffic, the NIC must wait until there is no trafficCollision Detection (CD)2) If there is a collision (by two or more stations transmitting at the same time), All NICs stop transmitting and wait for a random amount of time The first NIC that finishes its wait may transmit but only if there is no traffic! If there is traffic, the NIC must wait until there is no traffic Collision Detection (CD)3) If there are multiple collisions, The random wait is increased each time After 16 attempts to transmit, the sending NIC discards the frameSource: Panko (2002:116)15Ethernet (IEEE 802.3) standards802.3CSMA/CD802.3Ethernet802.3uFast Ethernet802.3zGigabit Ethernet10Base-210Base-510Base-T10Broad-36100Base-TX100Base-FX100Base-T41000Base-SX1000Base-LX1000Base-CX1000Base-CX+ New 10Gbps standard10Base-2Speed:10 MbpsSignal type:BasebandtransmissionDistance:185 meters- Speed: 10 Mbps- Signal type: Baseband w coaxial cable- Distance: 185 meters/segment- No more than 30 nodes (computers, printers, etc.) per segment- Nodes must be spaced at least 0.5 meters apart10Base-210Base-5Speed:10 MbpsSignal type:BasebandtransmissionDistance:500 meters- Speed: 10 Mbps- Signal type: Baseband w coax. cable- Distance: 500 meters/segment- No more than 100 nodes per segment- Nodes must be spaced at least 2.5 meters intervals10Base-5Thick Ethernet or ThicknetThin Ethernet or Thinnet616Ethernet (IEEE 802.3) standards10Base-TSpeed:10 MbpsSignal type:BasebandtransmissionMedium:Twisted Pair- Speed: 10 Mbps- Signal type: Baseband- Distance: 100 meters between the Hub/Switch and the node- No more than 1024 nodes per Hub/Switch- Star topology: physical star, logical bus- Uses CAT3 or better UTP with RJ-45 connectors10Base-Thttp://grouper.ieee.org/groups/802/index.htmlhttp://standards.ieee.org/getieee802/802.3.html100Base-TX: Two Twisted-pairs of Category 5 UTP or STP100Base-FX: Fiber-optic cabling using 2-strand cable100Base-T4: Four Twisted-pairs of Category 3,4, or 5 UTP1000Base-CX: Uses special shielded cable called Twinaxial cable1000Base-LX: Uses single-mode Fiber-optic cable (5 km)1000Base-T: Four Twisted-pairs of Category 5 UTP in full-duplex802.3CSMA/CD802.3Ethernet802.3uFast Ethernet802.3zGigabit Ethernet10Base-210Base-510Base-T10Broad-36100Base-TX100Base-FX100Base-T41000Base-SX1000Base-LX1000Base-CX1000Base-CX+ New 10Gbps standard1710Base-2 EthernetNote: 1 meter = 3.28 feet10Base-2 follows a Physical bus topology and a Logical bus topologyNo need for Hub/Switch to implement 10Base-2 or 10Base-5Source: Berg (1998:170-172)1810Base-T Ethernet BridgeNICNICNICNICNICNIC- Speed: 10 Mbps- Signal type: Baseband- Distance: 100 meters between the Hub/Switch and the node- No more than 1024 nodes per Hub/Switch- Star topology: physical star, logical bus- Uses CAT3 or better UTP with RJ-45 connectors10Base-Tor Switch719Ethernet framePreamble: used for synchronization between stationsSOF (Start Of Frame): Signal the receiving station that the next field is comingDestination Address: Receiver’s MAC addressSource Address: Sender’s MAC addressLength: Number of bytes in the Data field (Length used if number < 1518)Type: Type of protocol data being carried in data field (Type used if data field >= 1518 bytes)Pad: Data field must be 36+ bytes. If data field < 36 bytes, padding is added to reach 36 bytesFCS (Frame Check Sequence) or CRC (Cyclical redundancy Check): Prevents transmission error20Token Ring (IEEE 802.5) Developed by IBM in the 1970s Each node connects to a central wiring point called MAU (MultistationAccess Unit) Employs