Ch. 15 LAN Overview

1. Ch. 15 LAN Overview

2. Definition of a LAN • A communication network that provides interconnection of a variety of data communicating devices within a small area.

3. 15.1 Topologies and Transmission Media • Key Elements of a LAN • Topology • Transmission Media • Layout • Medium access control

4. 15.2 Topologies and Transmission Media(p.2) • Bus and Tree Topologies (Fig. 15.1) • 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.15.2

5. 15.2 Topologies and Transmission Media(p.3) • Ring Topology (Fig. 15.3) • Network consists of a set of repeaters joined by point-to-point links in a closed loop. • The links are unidirectional, and data circulates around the ring in one direction. • Each station is attached to a repeater, and frames are inserted onto the ring.

6. 15.2 Topologies and Transmission Media (p.4) • Star Topology • 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.

7. 15.2 Topologies and Transmission Media (p.5) • Choice of Topology • Depends on reliability, expandability, and performance. • Choice of Media • Depends on capacity, reliability, type of data supported, environmental scope.

8. 15.2 LAN Protocol Architecture • Fig. 15.4 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.

9. 15.2 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.

10. 15.2 LAN Protocol Architecture (p.3) • Logical Link Control (LLC) Layer • Provide an interface to higher layers and perform flow and error control. • Fig. 15.5 LAN protocols in context.

11. 15.2 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.

12. 15.2 LAN Protocol Architecture (p.5) • Logical Link Control (cont.) • LLC PDU (Fig. 15.6) • 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)

13. 15.2 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).

14. 15.2 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.

15. 15.2 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.

16. 15.2 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.

17. 15.2 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

18. Problem 15.3 • 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

19. Problem 15.3 (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.

20. Problem 15.3 (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.

21. Problem 15.3 (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

22. Problem 15.3(p.5) • c. Ring Topology • Total circular length is 2D, with the two stations a distance D apart. • Acknowledgement occurs with the circulation of the packet past the destination station, back to the source station. • There are N repeaters, each introduces a delay of one bit time (1/B).

23. Problem 15.3 (p.6) • c. Ring Topology (cont.) • Assume similar overhead as in part b. • RingPropTime=2D/v + N/B • TcycleRing=tframe+RingPropTime • TtotalRing=NoPa(TcycleRing) • ThroughputRing=L/TtotalRing

24. 15.3 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.

25. 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.

26. Functions of a Bridge • See Fig. 15.7 • 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.

27. 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

28. Bridge Protocol Architecture • The IEEE 802 committee has produced specifications for bridges. • These devices are called MAC-level relays. • Fig. 15.8 illustrates the architecture and operation.

29. Routing with Bridges • Figure 15.9 illustrates the concept of alternate routes. • Three Strategies • Fixed Routing • Spanning Tree (IEEE 802.1) • Source Routing (IEEE 802.5)

30. 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.

31. Routing with Bridges (p.3) • The Spanning Tree Approach • Three mechanisms • Frame Forwarding • Address Learning • Loop Resolution

32. 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.

33. 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.

34. 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. 15.10. • 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.

35. 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.

36. 15.4 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. 15.11.) • Hubs are usually placed in a wiring closet. • Stations are about 100 meters away, using twisted pair, or 500 meters with optical fiber.

37. 15.4 Hubs and Switches (p.2) • Layer 2 Switches (Fig. 15.12) • 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.

38. 15.4 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.

39. 15.5 Virtual LANS • Figure 15.13, page 469 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.

40. VLANS (p.2) • Broadcasting is also possible using a broadcast address. • One approach to efficient transmission—partition the LAN into separate broadcast domains. • Figure 15.14 illustrates the use of a router for partitioning a LAN—IP addresses are used for routing—this may not be efficient either.

41. 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 15.15 illustrates a VLAN Configuration.

42. VLANS (cont.) • From a business view, the VLAN provides the ability to be physically dispersed while maintaining its group identity.

43. 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

44. 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.

45. 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.

46. 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.