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Transcript
Ch. 11 LAN Overview Definition of a LAN • A communication network that provides interconnection of a variety of data communicating devices within a small area. 11.1 Bus and Star Topologies • Key Elements of a LAN – – – – Topology Transmission Media Layout Medium access control 11.1 Bus and Star Topologies • Bus and Tree Topologies – Bus • All stations are attached directly to the media. – Tree • The media is a branching cable with no closed loops. • The tree starts at the “headend” and branches out from there. – Each station must have an address and access is controlled (multipoint configuration.)—Fig.11.1 11.1 Bus and Star Topologies • Star Topology(Fig. 11.2) – Each station is connected to a common central node using two point-to-point links. – Received frames can either be "broadcast" or "switched" to a particular link. 11.2 LAN Protocol Architecture • Fig. 11.3 IEEE 802 vs. OSI Reference Model. • Physical Layer – – – – Encoding/decoding of signals. Preamble generation/removal (for synchronization). Bit transmission/reception. IEEE 802 also specifies the transmission medium and topology. 11.2 LAN Protocol Architecture (p.2) • Medium Access Control (MAC) Layer – Assemble data into a frame with address and error-detection fields. – Disassemble frames, perform address recognition and error detection – Govern access to the LAN transmission medium. 11.2 LAN Protocol Architecture (p.3) • Logical Link Control (LLC) Layer – Provide an interface to higher layers and perform flow and error control. • Fig. 11.4 LAN protocols in context. 11.2 LAN Protocol Architecture (p.4) • Logical Link Control – Specifies the mechanisms for addressing and the control of the data exchange. – Operation and format are based on HDLC. – Three Services • Unacknowledged connectionless service. • Connection-mode service. • Acknowledged connectionless service. 11.2 LAN Protocol Architecture (p.5) • Logical Link Control (cont.) – LLC PDU (Fig. 11.5) • Destination Service Access Point (1 octet) – 7 bits for the address. – One bit to indicate if it is a group address or not. • Source Service Access Point (1 octet) – 7 bits for the address. – One bit is used to indicate if it is a command or response. • LLC Control Field (1 or 2 octets) – Similar to HDLC control field. • Information Field (variable length) 11.2 LAN Protocol Architecture (p.6) • Differences between LLC and HDLC – LLC uses asynchronous balanced mode to support connection-mode service (type 2 operation). – LLC supports and unacknowledged connectionless service using the unnumbered information PDU (type 1 service). – LLC supports an acknowledged connectionless service by using two new unnumbered PDUs (type 3 operation.) – LLC permits multiplexing (using LSAPs). 11.2 LAN Protocol Architecture (p.7) • Medium Access Control – MAC protocols control access to the transmission medium in some type of orderly and efficient manner. – Access control could be centralized or distributed. – Centralized schemes tend to be simpler and avoid various "distributed control" problems, but performance and reliability can be a concern. 11.2 LAN Protocol Architecture (p.8) • Medium Access Control (cont.) – Synchronous Techniques • Specific capacity is dedicated to a connection, such as with circuit-switching, FDM, and TDM. • Generally do not work well in LANs. 11.2 LAN Protocol Architecture (p.9) • Medium Access Control (cont.) – Asynchronous techniques--capacity is allocated in a dynamic fashion. • Round Robin--each station is given a turn to transmit. • Reservation--a station wishing to transmit "reserves" slots of "time". • Contention--all stations "contend" for the medium. 11.2 LAN Protocol Architecture (p.10) • Medium Access Control (cont.) – Generic MAC Frame Format--Fig. 15.6 • MAC Control Field • Destination MAC Address • Source MAC Address • LLC PDU • CRC Problem 11.3 • Consider the transfer of a file containing one million 8-bit characters from one station to another. What is the total elapsed time and effective throughput for the following cases? • a. Circuit-Switched LAN – TtotalSwitch=S + L/B+tprop – ThroughputSwitch= L/TtotalSwitch Problem 11.3 (p.2) • b. Bus Topology – – – – – – – D--distance between stations. B--data rate (use R bps if you wish.) P--packet size. Header is 80 bits. Information field is P-80. Acknowledgement is 88bits. v=200 m/microsecond. Problem 11.3 (p.3) • b. Bus Topology (cont.) – Assume that each packet is acknowledge before the next is sent (stop-and-wait.) – Let NoPa= the number of packets. – NoPa= L/(P-80), rounded up (assuming fixed length packets and L is the number of inoformation bits in the message.) – There will be NoPa cycles needed to transfer the entire message. Problem 11.3 (p.4) • b. Bus Topology (cont.) – Ignore additional overhead--then tframe=P/B. – Also let tprop= D/v and tack=88/B. – Then TcycleBus=tframe +tprop+tack+tprop (ignoring processing delays.) – Thus, TtotalBus=NoPa (TcycleBus) – ThroughputBus=L/TtotalBus 11.3 Bridges • Bridges were originally used to interconnect LANs using the same physical and MAC protocols. • Eventually, bridges were developed that interconnected LANs with different MAC protocols. • In general, bridges are simpler than routers. Bridge Operation • Why use a bridge, instead of simply operating as one large LAN? – Reliability--bridges can be used to partition a large LAN environment. – Performance--in general, as stations are added to a LAN, the performance decreases. – Security--different types of traffic with different security needs can be kept on physically separate media. – Geography--two LANs in different locations can be bridged using point-to-point communications. Functions of a Bridge • See Fig. 11.6 • The bridge reads all frames transmitted on network A, accepting those addressed to B. • Frames accepted are transmitted on B. • The same is done for B-to-A traffic. Design Considerations • 1. The bridge makes no modifications to the content or format of the frames it receives. • 2. The bridge should contain enough buffer space to meet peak demands. • 3. The bridge must contain addressing and routing intelligence. • 4. A bridge may connect more than two LANs. • Note: Bridges can be more complex and have special functionality Bridge Protocol Architecture • The IEEE 802 committee has produced specifications for bridges. • These devices are called MAC-level relays. • Fig. 11.7 illustrates the architecture and operation. Routing with Bridges • Figure 11.8 illustrates the concept of alternate routes. • Three Strategies – Fixed Routing – Spanning Tree (IEEE 802.1) – Source Routing (IEEE 802.5) Routing with Bridges (p.2) • Fixed Routing – A route is selected for each source-destination pair of LANs in the internet. – If alternative routes exist, then the route with the fewest hops in chosen and placed in a routing table. – Widely used; simple and requires minimal processing. – Too limited for a dynamically changing internet. Routing with Bridges (p.3) • The Spanning Tree Approach – Three mechanisms • Frame Forwarding • Address Learning • Loop Resolution Routing with Bridges (p.4) • The Spanning Tree Approach (cont.) – Frame Forwarding • The bridge maintains a forwarding database for each port attached to a LAN. • The database indicates the station addresses for which frames should be forwarded through that port. Routing with Bridges (p.5) • The Spanning Tree Approach (cont.) – Address Learning • When a frame arrives at a particular port, the source address can be checked. • If the source address is not in the database for that port it can be added. • Each time an element is added to the database, a timer can be set. • When the timer expires, then the element will be removed from the database. • If the element is already in the database, the timer is reset. Routing with Bridges (p.6) • The Spanning Tree Approach (cont.) – Spanning Tree Algorithm--Loop Problems • The above procedures work fine when the topology is a tree, but problems occur when alternate routes exist. • Consider Fig. 11.9. – When A transmits to B, both bridges will update their databases and relay the frame. – However, they will receive each others relay and update the databases again. – B then cannot transmit to A. 15.3 Routing with Bridges (p.7) • The Spanning Tree Approach (cont.) – Spanning Tree Algorithm--Some Assumptions • 1.Each bridge is assigned a unique identifier. • 2.There is a special group MAC address that means "all bridges on this LAN". • 3. Each port of a bridge is uniquely identified within the bridge. • These assumptions allow the bridges to exchange routing information in order to obtain a spanning tree. 11.4 Hubs and Switches • Hubs – The active central element of a star layout. – Each station is connected to the hub with two lines, one for transmitting and one for receiving. – The system is essential a logical bus, since a transmission from any one station is transmitted to all other stations. – Multiple levels of hubs are possible (Fig. 11.10.) – Hubs are usually placed in a wiring closet. – Stations are about 100 meters away, using twisted pair, or 500 meters with optical fiber. 11.4 Hubs and Switches (p.2) • Layer 2 Switches (Fig. 11.11) – A shared medium hub (like a shared medium bus) has collisions when more than one station is transmitting at the same time. – A layer 2 switch takes an incoming frame and transmits it only on the destination station’s line. – Two types of switches: • Store-and-Forward--packets are buffered. • Cut-through--headers are read and switching occurs immediately--but no error checking. 11.4 Hubs and Switches (p.3) • Layer 2 switches may function as a multiport bridge--the differences are: – Bridge frames are handled in software, while layer 2 switches have hardware that performs address recognition and frame forwarding. – A bridge handles one frame at a time, while a switch can handle multiple frames at a time. – A bridge uses store and forward operations, while cut-through operations are possible with layer 2 switches. 11.5 Virtual LANS • Figure 11.12,illustrates a typical LAN configuration. • Consider a single MAC frame from X. • Assume that X wants to transmit to Y—the local switch transmits it to Y. • Alternatively, assume that X wants to transmit to W or Z—then the local switch routes the frame accordingly—unicast addressing. VLANS (p.2) • Broadcasting is also possible using a broadcast address. • One approach to efficient transmission— partition the LAN into separate broadcast domains. • Figure 11.13 illustrates the use of a router for partitioning a LAN—IP addresses are used for routing—this may not be efficient either. The Use of VLANs • VLAN logic is implemented in LAN switches and functions at the MAC layer. • A VLAN is a logical subgroup within a LAN that is created by software rather than by physical partitioning. • Figure 11.14 illustrates a VLAN Configuration. VLANS (cont.) • From a business view, the VLAN provides the ability to be physically dispersed while maintaining its group identity. Defining VLANs • A VLAN is a broadcast domain consisting of a group of end stations that are not constrained by their physical locations. • Approaches – Membership by Port Group – Membership by MAC Address – Membership based on Protocol Information Membership by Port Group • Each switch has two types of ports. – Trunk ports will connect switches and end ports will connect workstations to the switch. – A VLAN can be defined by assigning each end port to a particular VLAN • Advantage—easy to configure. • Disadvantage—Network manager must take care of configurations manually. Membership by MAC Address • MAC Addresses on in the hardware network interface cards (NICs). • If a network manager physically moves a machine, the device automatically retains its VLAN membership. • Disadvantage—VLAN membership is assigned initially, which is difficult in large organizations. There is also a problem when docking stations are used—they contain the NICs. Membership Based on Protocol Information • IP addresses can be used to assign VLAN membership. • Or, transport protocol information could be used (or even higher protocol information.) • Advantage—flexible. • Disadvantage—issues related to performane and the processing of MAC addresses and other addressing.