Ch. 15 LAN Overview

Definition of a LAN

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

15.1 Topologies and Transmission Media

• Key Elements of a LAN– Topology– Transmission Media– Layout– Medium access control

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

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.

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.

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.

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.

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.

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.

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.

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)

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).

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.

15.2 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.

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.

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

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

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.

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.

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

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).

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

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.

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. 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.

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. 15.8 illustrates the architecture and operation.

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)

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. 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.

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.

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.

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.

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.

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.

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.

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.

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.