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 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 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 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 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 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 –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 ) –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 ) –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 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 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.