Post on 31-Dec-2015
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.