This Lecture. BUS Computer Facilities Network Management. Information Sharing. Local Area Network

Transcription

1 This Lecture US omputer Facilities Network Management Local rea Network (LN). LN applications. LN topologies: us, Tree, Ring, Faculty of Information Technology Monash University Star. LN protocol architecture (802 model: M, LL). ridges. Faculty of Information Technology 2 Faculty of Information Technology Local rea Network Information Sharing collection of computers and computing devices, generally located in a building, connected so information and communication between computers can occur. Optimised for a limited distance. The scope of a LN is small. Owned, used and operated by a single organisation. Shared resources: Information consistent, restricted, better control. Hardware efficient, cost-effective use of expensive devices. Software better installation, configuration, management. May provide access to the organisation s mainframe computer. 3 Faculty of Information Technology Refers to having users who access the same data files, exchange information. Types of information an enterprise might want to locate centrally and share or control access include: Inventory. ompany letterhead and letter style. Sales contact information. ompany procedure manuals. Sensitive financial records. Employee record. ompany memos. Different level of access control can be defined. 4 Faculty of Information Technology

2 Hardware Resource Sharing Software Resource Sharing Refers to many computers sharing the same hardware resources. Hardware devices that networked computers can share. Printers. Plotters. Fax modems. Scanners. Hard disks. D-ROMs. Tape backup units. lmost any devices that can be attached to a computer. n alternative to purchasing a single copy of software for each user is to install the software on the network for all to use. entral installation and configuration. Easy maintenance. In most cases, not all users would need to access the software package simultaneously, allowing a reduced number of licenses to be purchased. Exact restrictions on network usage need to checked in software package documentation. 5 Faculty of Information Technology 6 Faculty of Information Technology LN rchitecture LN Topology Topology - how the stations are connected. Transmission medium - what type of medium is used. Medium ccess ontrol - what rules/algorithms are used to control the shared medium. Topology defines how nodes/stations are connected and refers to the physical layout of the network. Typical topology: us: all nodes connect to a common medium. Tree: generalisation of a bus with branching and no loops. Star: all nodes are joined to a central node. Ring: nodes form a ring by point-to-point links to adjacent neighbours. 7 Faculty of Information Technology 8 Faculty of Information Technology

3 LN Topology us and Tree Topologies Headend Tap Flow of data (a) us Terminating resistance Repeater (c) Ring entral Hub, Switch, or Repeater (b) Tree (d) Star 9 Faculty of Information Technology Multipoint medium. Transmission propagates throughout the medium. Terminator absorbs frames at end of the medium. Heard by all the stations. Need to identify target station: Each station has unique address. Full duplex connection between a station and its tap: llows for transmission and reception. Need to regulate transmission: To avoid collisions. To avoid hogging. Data transmitted in small blocks - frames. 10 Faculty of Information Technology Frame Transmission on a us LN us and Tree Topologies transmits frame addressed to Frame is not addressed to ; ignores it copies frame as it goes by 11 Faculty of Information Technology Example: Ethernet 10base2 ( baseband coaxial up to 200m). dvantages: Simple, reliable in very small network, easy to use. Requires the least amount of cable. Is easy to extend. arrel connector or repeater. Disadvantages: Heavy traffic can slow a bus considerably. Difficult to troubleshoot: able break or loose connection can cause reflection and bring down the whole network. Each barrel connector weakens the signal. 12 Faculty of Information Technology

4 Ring Topology Frame Transmission on a Ring LN Repeaters joined by point-to-point links in a closed loop: Receive data on one link and retransmit on another. (a) transmits frame addressed to Links unidirectional - one direction only. Stations attach to the repeaters. Data is transmitted in frames: (b) Frame is not addressed to ; ignores it irculate frames past all stations. Destination recognises the address and copies the frame. Frame circulates back to the source where it is removed. (c) copies frame as it goes by Media access control determines when a station can insert a frame. (d) absorbs returning frame 13 Faculty of Information Technology 14 Faculty of Information Technology Ring Topology Star Topology Good for very high speed links over long distances. dvantages: Every station gets equal access to the medium, no station can monopolise the network. Fair sharing allows the network to degrade gracefully as more users are added. Disadvantages: Failure of one station on the ring can affect the whole network. Difficult to troubleshoot. dding or removing station disrupts the network. Each station is connected directly to a common central node: Usually via two point-to-point links. Two alternatives for operation: entral node can broadcast to all stations (shared hub): Transmission is broadcast to every station. ly a star, but logically a bus. Only one station can transmit at a time. entral node can act as a frame switch: Transmission is sent directly to the destination station. 15 Faculty of Information Technology 16 Faculty of Information Technology

5 Frame Transmission on a Star LN Hubs and Switches roadcast Shared medium hub: entral hub. Hub retransmits incoming signal to all outgoing lines. Only one station can transmit at a time. The frame is broadcast to all; receives Switching With a 10Mbps LN, total capacity is 10Mbps. Switched LN hub: Hub acts as switch. transmits a frame addressed to Incoming frame switches to appropriate outgoing line. Unused lines can also be used to switch other traffic. With two pairs of lines in use, overall LN capacity is now 20Mbps. The frame is sent directly to 17 Faculty of Information Technology 18 Faculty of Information Technology Hubs and Switches Diagram Star Topology Total capacity up to D (b) Shared medium hub Shared us D 19 Faculty of Information Technology (a) Shared medium bus Total capacity N D (c) Layer 2 switch Uses natural layout of wiring in a building. Good for high data rates to a small number of devices over short distances. dvantage: Easy to modify and add new stations. single station failure does not bring down the whole network. Hub can detect network fault and isolate that station or cable. Star is the most flexible and easiest to diagnose network faults. Intelligent hub provides centralised monitoring and management. an support all cable types. Disadvantages: If the central hub fails, the whole network fails to operate. star network requires more cable. 20 Faculty of Information Technology

6 LN Protocol rchitecture IEEE 802 Protocols Only the lower layers of the OSI model are dependent on the network architecture. IEEE 802 reference model:. Logical link control (LL). Media access control (M). OSI Reference Model pplication Presentation Session Transport Network Data Link Medium IEEE 802 Reference Model Upper Layer Protocols ( ) ( ) ( ) Logical Link ontrol Medium ccess ontrol Medium IEEE 802 Layers ompared to OSI Model LL Service ccess Point (LSP) 21 Faculty of Information Technology Scope of IEEE 802 Standards SM /D Other layers Internetworking Logical link control (LL) Token bus Project Token ring Other layers Network Data link OSI Model 22 Faculty of Information Technology IEEE 802 Layers - IEEE 802 Layers - Logical Link ontrol Encoding/decoding of signals. Preamble generation/removal (synchronisation). it transmission/reception. Transmission medium and topology (usually below OSI model). Provide an interface to the higher levels. Perform flow and error control. Transmission of link level PDUs between two stations. Must support multiaccess, shared-medium. Relieved of some link access details by the M layer. ddressing involves specifying source and destination LL users: Referred to as service access points (SP). Typically higher level protocol or network management function. Operation and format is based on HDL. 23 Faculty of Information Technology 24 Faculty of Information Technology

7 IEEE 802 Layers - Media ccess ontrol LN Protocols in ontext ssembly of data into a frame with address and error detection fields. pplication data pplication Layer Disassembly of frame: ddress recognition. TP header TP Layer Error detection. IP header IP Layer Govern access to the transmission medium. Not found in traditional layer 2 data link control. LL header LL Layer For the same LL, several M options may be available. M header M trailer M Layer 25 Faculty of Information Technology TP segment IP datagram LL protocol data unit M frame 26 Faculty of Information Technology Media ccess ontrol synchronous Systems Where: entral - a controller grants access to the network. Greater control (e.g. priority, overrides, guaranteed capacity). Simple access logic at each station. voids problems of co-ordination among peer stations. reates a single point of network failure. Potential bottleneck. Distributed - stations collectively control access. How: Synchronous: Specific capacity dedicated to a connection (allocated timeslot). synchronous: In response to demand (the needs of the stations is unpredictable). 27 Faculty of Information Technology Round Robin: Each station is given the opportunity to transmit (but may decline). Good if many stations have data to transmit over an extended period. Reservation: station reserves future time slots for transmission. Good for stream traffic (e.g. voice, telemetry, bulk file transfer). ontention: Good for bursty traffic (short, sporadic transmissions). No turn based control used, all stations contend for time. Distributed control. Simple to implement and efficient under moderate load. Tends to collapse under heavy load. 28 Faculty of Information Technology

8 M Frame Format ridges M layer receives data from the LL layer (LL PDU). M layer detects errors and discards corrupt frames. LL optionally retransmits unsuccessful frames. M Frame LL PDU M ontrol DSP I/G Destination M ddress I/G = Individual/Group /R = ommand/response Source M ddress LL PDU 1 octet 1 1 or 2 variable SSP LL ontrol Information DSP value /R SSP value LL ddress Fields 29 Faculty of Information Technology R bility to expand beyond single LN. Provide interconnection to other LNs/WNs. Use a bridge or a router... ridge is simpler: onnects similar LNs. Identical protocols for physical and link layers. Minimal processing. May use M format translation in more sophisticated bridges. Router more general purpose: Interconnect various LNs and WNs. overed in higher layers of the protocol stack. 30 Faculty of Information Technology ridges - Operation ridges - Operation Read all frames transmitted on one LN and accept those address to any station on the other LN. Using the M protocol for the second LN, retransmit each frame. Do the same the other way round. LN Station 1 Station 2 Station 10 Frames with addresses 11 through 20 are accepted and repeated on LN ridge LN Frames with addresses 1 through 10 are accepted and repeated on LN 31 Faculty of Information Technology Station 11 Station 12 Station Faculty of Information Technology

9 ridges - Use ridges - Design spects Why use a bridge rather than one large LN? Reliability: fault on the network may disable all devices. Partition network into self-contained units. Performance: Reduce the number of devices on a single length of wire. Improved performance when intranetwork traffic exceeds internetwork traffic. Security: Keep different levels of secure information on separate physical media. Geography: Multiple LNs separated by geographical distances. May use a microwave bridge. 33 Faculty of Information Technology No modification to content or format of frame. Exact bitwise copy of the frame from one LN to the other. Minimal buffering to meet peak demand. ontains routing and address intelligence: Must be able to tell which frames to pass. May be more than one bridge to cross. May connect more than two LNs. ridging is transparent to stations: ppears to all stations on multiple LNs as if they are on one single LN. ridge functions at the M level where station address is. 34 Faculty of Information Technology ridges - onnection of Two LNs Further Reading User LL M t 1 t 8 t 2 t 7 M t 3 t 4 t 5 t 6 LN LN User LL M Stallings, W. Data and omputer ommunications, Prentice Hall. hapter 15. Forouzan,. Data ommunications and Networking, McGraw-Hill. hapters 13 and 14. (a) rchitecture t 1, t 8 User Data t 2, t 7 LL H User Data t 3, t 4, t 5, t 6 M H LL H User Data M T (b) Operation 35 Faculty of Information Technology 36 Faculty of Information Technology