module 2 lan,data link layer

MODULE 2 MCA-402 Computer Networks ADMN 2012-‘15

Dept. of Computer Science And Applications, SJCET, Palai

LOCAL AREA NETWORK -LAN

Uses of LAN As LAN provides the capability to route data between devices connected to a common network within a

relatively limited distance, numerous benefits can accrue to users of the network.

Peripheral sharing: Peripheral sharing allows network users to access color laser printers and

other devices. Users of a LAN can obtain access to resources that would be too expensive to justify

for each individual workstation user.

Common software access: The ability to access data files and programs from multiple

workstations can substantially reduce the cost of software and shared access to database

information allows network users to obtain access to updated files on a real-time basis.

Internet access: Instead of supporting individual dial access to the Internet you could install a router

and leased line to an Internet Service Provider (ISP) that is a fraction of the cost associated with a

large number of individual Internet dial network accounts

Attributes of a LAN

i. Transmission technology

ii. Signalling methods

iii. Transmission medium

iv. Access Control

v. Topologies

i. Transmission Technology

Two Transmission Technologies used by LANs are :

a. Broad cast: A single communication channel is shared by all the machines on the network

b. Point to Point: consists of many connections between individual pairs of machines.Packets have to follow

multiple routes of different lengths

ii. Signalling Methods

Two signaling methods used by LANs are :

a. Broadband: The bandwidth of the transmission medium is subdivided by frequency to form two or

more sub-channels

b. Base-Band: only one signal is transmitted at any point in time

iii. Transmission Medium

Transmission medium can be classified into 4 from physical cable connection to wireless systems.

a. Twisted Pair: consists of two individual insulated copper wires physically twisted together to

minimize unwanted electromagnetic signals from interfering with or radiating from

the pair

b. Coaxial cables: consists of a central copper core surrounded by a layer of insulating material.

c. Optical fiber cables: contains glass fiber and signals are transmitted in the form of light pulses.

d. Wireless transmission

iv. Access control

Represents how the devices get permission to communicate on the network.

Some of the access methods primarily employed in local area networks are :

Polling

Token-passing

Slotted ring

CSMA/CD



Switching

v. Topology

Refers to the way in which the end points, or stations, attached to the network are interconnected.

The common topologies for LANs are:

i. Bus

ii. Tree

iii. Ring

iv. Star

LAN PROTOCOL ARCHITECTURE

The LAN protocol architecture consists of layering of protocols that contribute to the basic functions of a

LAN

The standardized LAN protocol architecture encompasses 3 layers

i. Physical layer

ii. Medium Access control layer (MAC)

iii. Logical Link control

IEEE 802 REFERENCE MODEL

The layers of OSI Reference Model can be classified as Network Support Layers and User support

layers

LAN protocols are concerned with the network support layers, mainly Physical and Data Link Layer

IEEE 802 Reference Model is a modified reference model suitable for LANs built by IEEE.

It corresponds to the lower 2 layers of OSI reference model

Here the data link layer has been divided into two sub layers: logical link control (LLC) and media

access control (MAC)

Fig 2.1 IEEE 802 Protocol Layers Compared to OSI Model



Physical layer

includes functions such as :

i. Encoding/decoding of signals

ii. Specification of the transmission medium and the topology

Logical Link Layer

Acts as the interface between the Network layer and the MAC sub layer

Functions:

i. Error Control

ii. Flow Control

iii. Sequencing & User Addressing Functions

LLC standard is common to all LAN’s and offers three types of services for controlling the exchange

of data between two users.

i. Unacknowledged connectionless service: does not involve any of the flow- and error control

mechanisms and delivery of data is not guaranteed.

ii. Connection-mode service: A logical connection is set up between two users exchanging data, and

flow control and error control are provided.

iii. Acknowledged connectionless service: datagram are to be acknowledged, but no prior logical

connection is set up.

LLC Protocol Data Unit (PDU)

which contains 4 fields

Fig 2.2 LLCPDU

Destination Service Access Point (DSAP), Source Service Access Point (SSAP) : address fields

which specify the destination and source users of LLC.These identify the network protocol entities

which use the link layer service

LLC control field: describes type of PDU(U,I and S) and includes other information such as

sequencing and flow control information

Medium Access Control

It is lower sub-layer of data link layer closer to the physical layer

Key parameters of MAC technique is where and how

a. Where: refers to whether control is exercised in a centralized or distributed fashion.

Central: A controller is designated that has the authority to grant access to the network.

Distributed: The stations collectively perform a MAC function to determine the order in which

stations transmit.

b. How: is constrained by the topology and competing factors like cost, performance, and

complexity.

Basic functions of MAC Sub Layer are:

i. Media Access Control

ii. Error Detection

iii. Station Addressing

Defines two medium access control techniques specific for each LAN.



i. Synchronous techniques: a specific capacity is dedicated to a connection.

ii. Asynchronous: allocate capacity in an asynchronous (dynamic) fashion, more or less in response to

immediate demand.

Asynchronous techniques can be classified into three:

i. Round Robin

ii. Reservation

iii. Contention

i. Round Robin

Each station in turn is given the opportunity to transmit.

When it is finished, relinquishes its turn, and the right to transmit passes to the next station in logical

sequence.

Control of sequence may be centralized or distributed.

Efficient when many stations have data to transmit over an extended period of time

ii. Reservation

Time on the medium is divided into slots.

A station wishing to transmit reserves future slots for an

extended or even an indefinite period.

Reservations may be made in a centralized or distributed

fashion.

iii. Contention

Useful for bursty type traffic

No control is exercised to determine whose turn it is.

Stations send data by taking risk of collision (with others’ packets).however they understand

collisions by listening to the channel, so that they can retransmit.

Efficient under light or moderate load, bad under heavy load

Round-Robin and Contention techniques are the most commonly used in LANs.

MAC- Frame Format

The MAC layer is responsible for performing functions related to medium access and for transmitting

the data.

Fig 2.3 MAC Frame

MAC Control: contains any protocol control information needed for the functioning of the MAC

protocol

Destination MAC Address: The destination physical attachment point on the LAN for this frame.

Source MAC Address: The source physical attachment point on the LAN for this frame.

LLC PDU: The LLC data from the next higher layer.

CRC: The Cyclic Redundancy Check field

IEEE 802 STANDARDS

LAN standards proposed by the IEEE committee have the following goals in mind:

• To promote compatibility

• Implementation with minimum efforts



• Accommodate the need for diverse applications

For the fulfillment of the above mentioned goals, the committee came up with a bunch of LAN

standards collectively known as IEEE 802.The various standards differ at the physical layer & MAC sub-

layer but are compatible at the data link layer.

Fig 2.4 IEEE 802 standards

MULTIPLE ACCESS PROTOCOLS

A MAC (Media Access Control) protocol is a set of rules to control access to a shared

communication medium among various users.

Fig 2.5 Multiple access protocols

Random Access

In random access or contention methods, no station is superior to another station and none is assigned

the control over another.



Two features give this method its name:

First, there is no scheduled time for a station to transmit.

Second, no rules specify which station should send next. Stations compete with one another to access

the medium

ALOHA

When the user simply transmits a frame, there are chances of collision

The user could simply retransmit, but this would not help, other user involved in the collision will

also retransmit, resulting in another collision

One way to avoid this is to wait a random amount of time before retransmitting which forms the basis

of ALOHA

There are two versions of ALOHA: pure and slotted.

Pure ALOHA

The system is working as follows:

1. Let users transmit whenever they have data to be sent.

2. Collisions will occur.

3. Using a feedback mechanism to know about the status of frame.

4. The collided frames will be destroyed.

5. Retransmit the destroyed frame.

The number of collisions rises rapidly with increased load.

After a maximum number of retransmission attempts Kmax station must give up and try later.

Fig 2.6 flow chart for pure ALOHA



Fig 2.7 Frames in a pure ALOHA network

Suppose L: the average frame length,

R: rate,

X=L/R: frame time

1. Transmit a frame at t=t0 (and finish transmission of the frame at t0+X )

2. If ACK does not come after t0+X+2tprop or detect collision, wait for random time: B

3. Retransmit the frame at t0+X+2tprop+B

Fig 2.8 timing diagram for pure aloha

Vulnerable period: t0-X to t0+X, if any other frames are transmitted during the period, the collision will

occur.

Therefore the probability of a successful transmission is the probability that there is no additional

transmission in the vulnerable period.

Therefore, if a station generates only one frame in this vulnerable time (and no other stations generate a

frame during this time), the frame will reach its destination successfully.

Max channel utilization is 18% - very bad.

Slotted ALOHA

Slotted ALOHA was invented to improve the efficiency of pure ALOHA.

Here, we divide the time into slots and force the station to send only at the beginning of the time slot

If a station misses this moment, it must wait until the beginning of the next time slot.



There is still the possibility of collision if two stations try to send at the beginning of the same time

slot

Fig 2.9 frames in slotted ALOHA

Fig 2.10 timing diagram for slotted ALOHA

Max channel utilization is 37%,doubles Normal ALOHA, but still low

Carrier Sense Multiple Access (CSMA)

A station wishing to transmit first listens to the medium if another transmission is in progress (carrier

sense).

If the medium is in use, station waits

if the medium is idle, station may transmit

Collision probability depends on the propagation delay



Longer propagation delay, worse the utilization

Collisions can occur only when more than one user begins transmitting within the period of

propagation delay.

The vulnerable time for CSMA is the propagation time .

If collision occurs

Wait random time and retransmit

Suppose tprop is propagation delay from one extreme end to the other extreme end of the medium.

When transmission is going on, a station can listen to the medium and detect it.

Fig 2.11 vulnerable period in CSMA

After tprop, A’s transmission will arrive the other end; every station will hear it and refrain from the

transmission, so A captures the medium and can finish its transmission.

Following are some versions of CSMA protocol Based on how to do when medium is busy

i. 1-Persistent CSMA

ii. Non-Persistent CSMA

iii. p-Persistent CSMA

1-persistent CSMA

if the medium is idle, transmit.

if the medium is busy, continue to listen until the channel is sensed idle; then transmit immediately.

If more than one station are sensing, then they will begin transmission the same time when channel

becomes idle, so collision. At this time, each station wait for a random time, and then re-senses the

channel again.

Problem with 1-persistent CSMA is “high collision rate”.

Fig 2.12 flow chart for 1-persistent CSMA



Nonpersistent CSMA

If the channel is busy the station does not continually check it for detecting the end of ongoing

transmission. It waits for a random time then checks the channel. If the channel is idle, sends the

frame.

Fig 2.13 flow chart for non persistent CSMA

P-persistent CSMA

If medium is idle, station transmits with a probability p. otherwise it defers to the next slot with

probability 1-p. the process repeat until either the frame has been transmitted or another station has

begun transmission.

Fig 2.14 flow chart for P-persistent CSMA

Fig 2.15 Behaviour of three persistence methods



CSMA/CD (CSMA with Collision Detection)

Drawback of CSMA: when two frames collide, the medium remains unusable for the duration of

transmission of both damaged frames.

CSMA/CD:

1. If the medium is idle, transmit; otherwise, go to step 2.

2. If the medium is busy, continue to listen until the channel is idle then transmit.

3. if a collision is detected during transmission, transmit a brief jamming signal

4. After transmitting a jamming signal, wait a random amount of time, then attempt to transmit.

5.

Fig 2.16 flow chart for CSMA/CD

CSMA/CD efficiency

tprop = max prop between 2 nodes in LAN

ttrans = time to transmit max-size frame

Efficiency = 1/(1+5 * tprop / ttrans)

For 10 Mbit Ethernet, tprop = 51.2 us, ttrans = 1.2 ms

Efficiency is 82.6%!

Much better than ALOHA,

simple, and cheap

Efficiency goes to 1 as tprop goes to 0

Goes to 1 as ttrans goes to infinity



Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)

In CSMA/CA once the channel is clear, it again waits for an additional time period before performing

the transmission.

Before sending a frame, source senses the medium

Backoff until the channel is idle.

After the channel is found idle, the station waits for a period of time called the Distributed

Inter Frame Space (DIFS); then the station sends a control frame called Request To Send

(RTS).

After receiving RTS, the destination waits for a period called Short Inter Frame Space (SIFS),

the destination station sends a control frame, called Clear To Send (CTS) to source. This

control frame indicates that the destination station is ready to receive data.

Source sends data after waiting for SIFS

Destination sends ACK after waiting for SIFS.

Fig 2.17 flowchart for CSMA/CA

RTS frame indicates the duration of time that the source needs to occupy the channel.

Stations that are affected by this transmission create a timer called a Network Allocation Vector

(NAV) that shows how much time must pass before these stations are allowed to check the channel

for idleness.



Fig 2.18 signals in CSMA/CA

Exponential Back off Algorithm

used by a transmitting station to determine how long to wait following a collision before attempting

to retransmit the frame

Each station generates a random number that falls within a specified range of values. This determines

the length of time it must wait before testing the carrier. The range of values increases exponentially

after each failed retransmission.

After c collisions, the range is between 0 and 2c – 1, it then waits that number of slot times before

attempting retransmission.

If repeated collisions occur, the range continues to expand, until after 10 attempts when it reaches

1023. After that the range of values stays fixed .If a station is unsuccessful in transmitting after 16

attempts, then gives up if cannot transmit

low delay with small amount of waiting stations

large delay with large amount of waiting stations

CONTROLLED ACCESS

In controlled access, the stations consult one another to find which station has the right to send

A station cannot send unless it has been authorized by other stations.

Three popular control access methods are:

i. Polling

ii. Reservation

iii. Token Passing

i. Polling

Stations take turns accessing the medium

Two models: Centralized and distributed polling

Centralized polling

One device is assigned as primary station and the others as secondary stations

All data exchanges are done through the primary.

If the primary wants to receive data, it asks the secondary's if they have anything to send; this

is called poll function.



If the primary wants to send data, it tells the secondary to get ready to receive; this is called

select function

Polling can be done in order (Round-Robin) or based on predetermined order

Fig 2.19 signals in polling

ACK is the acknowledgment of the secondary's ready status.

Secondary responds either with a NAK frame if it has nothing to send or with data (in the

form of a data frame) if it does

Distributed polling

No primary and secondary.

Stations have a known polling order list which is made based on some protocol.

station with the highest priority will have the access right first, then it passes the access right

to the next station (it will send a pulling message to the next station in the pulling list), which

will passes the access right to the following next station, …

ii. Reservation

A station needs to make a reservation before sending data.

Transmissions are organized into variable length cycles.

Each cycle begins with a reservation interval that consists of (N) minislots. One minislot for each of

the N stations.

When a station needs to send a data frame, it makes a reservation in its own minislot.

By listening to the reservation interval, every station knows which stations will transfer frames, and

in which order.

The stations that made reservations can send their data frames after the reservation frame.

Fig 2.20 reservation scheme



iii. Token Passing

Here the stations in a network are organized in a logical ring.

In this method, a special packet called a token circulates through the ring.

token gives the station the right to access the channel and send its data

When a station receives the token and has no data to send, it just passes the data to the next

station.

Fig 2.21 token ring

Fig 2.22 flow chart for token ring

LAN SYSTEMS

ETHERNET

Ethernet is a dominant physical and data link layer technology for local area networks (LANs).

It is a bus based broadcast network using co-axial cable operating at 10 or 100 Mbps.

It uses a control method called Carrier Sense Multiple Access/Collision Detection (CSMA/CD) to

transmit data.



Fig 2.23 Ethernet standards

IEEE 802.3 MAC Frame

Fig 2.24 IEEE802.3 MAC frame

Preamble: Alternating 0s and 1s; used for synchronizing; 7bytes (56 bits).

Start Frame Delimiter (SFD): 10101011 indicates the start of the frame. Last two bits alerts that the

next field is destination address.

Destination Address (DA): 6 bytes (48 bits) physical address of destination station(s)

Source Address (SA): 6 bytes (48 bits) physical address of sender

Length/Type: if less than 1500, it indicates the length of data field. If greater than 1536, it indicates

the type of PDU.

Data: 46 to 1500 bytes;

CRC: CRC-32 for error detection

Addressing

Each station on an Ethernet network has its own network interface card( NIC) fits inside and provides

a 6-byte physical address

Eg:06:01:02:01:2C:4B.

First three bytes from left specify the vendor. (Cisco 00-00-0C, 3Com 02-60-8C) and the last 24 bit

should be created uniquely by the company.

A source address is always a unicast address where as destination address can be unicast, multicast,

or broadcast.

Unicast: defines one recipient ,second digit from left is even



Multicast: defines a group of recipients ,Second digit from left is odd

Broadcast : defines a group of all stations in the same LAN ,All ones

The transmission is left-to-right, byte by byte; however, for each byte, the least significant bit is sent

first and the most significant bit is sent last.

Physical Layer Implementation

The Standard Ethernet defines several physical layer implementations; four of the most common, are

Fig 2.25 Ethernet standards

Physical Layer Signaling

Uses Manchester encoding.

At the sender, data are converted to a digital signal using the Manchester scheme; at the receiver, the

received signal is interpreted as Manchester and decoded into data.

Helps synchronize sender and recvr.

Fig 2.26 Manchester encoding

10Base5: Thick Ethernet

Use a bus topology with an external transceiver (transmitter/receiver) connected via a tap to a thick

coaxial cable.

10-Mbps transmission speed and 5 represents 500 meters maximum cable segment length.

The transceiver is responsible for transmitting, receiving, and detecting collisions



Fig 2.27 10base 5 Thick Ethernet

10Base2: Thin Ethernet

Uses a bus topology, but the cable is much thinner and more flexible.

The transceiver is normally part of the network interface card (NIC), which is installed inside the

station.

Can transmit 10 Mbps digital signals over coaxial cable.

More cost effective than 10Base5 because thin coaxial cable is less expensive than thick coaxial.

The length of each segment cannot exceed 185 m (close to 200 m).

Fig 2.28 10 base2 thin Ethernet

10Base-T: Twisted-Pair Ethernet

Uses a physical star topology.

The stations are connected to a hub via two unshielded twisted pair cables; One for transmitting data,

and the other for receiving data

Maximum length of the cable segment can be 100 meters.

Fig 2.29 10 base T twisted pair



10Base-F: Fiber Ethernet

Although there are several types of optical fiber l0-Mbps Ethernet, the most common is called

10Base-F.

It uses a star topology to connect stations to a hub.

The stations are connected to the hub using two fiber-optic cables.

Fig 2.30 10BaseF

BRIDGED ETHERNET

LAN can be divided using bridges. Bridges have two effects on an Ethernet LAN:

• They raise the bandwidth

• They separate collision domains

Raising the Bandwidth

Each network is independent.

Suppose there are 12 stations. And bandwidth is 10 Mbps.

If we divide the network into 2 networks using bridge, each network has a capacity of

10 Mbps.

The 10 Mbps capacity is shared between 7 stations, 6+1(bridge acts as a station in each

segment), not 12 stations.

Fig 2.31 network with bridge and without bridge



Separating collision domains:

Collisions domains become much smaller and possibility of collision is reduced.

With bridging, lesser number of channels competes for access to the medium.

Fig 2.32 separating domains using bridge

SWITCHED ETHERNET

The heart of the system is a switch containing room for typically 4 to 32 plug-in cards, each

containing one to eight connectors that allow faster handling of packets.

When a station wants to transmit a frame, it outputs a frame to switch.

Half duplex

Fig 2.33 Switched Ethernet

All ports on the same card are wired together to form a local on-card LAN.

Collisions on this on-card LAN are detected and handled using CSMA/CD protocol.

One transmission per card is possible at any instant. All the cards can transmit in parallel.

With this design each card forms its own collision domain.

FULL-DUPLEX SWITCHED ETHERNET

Each station is connected to the switch through two links: one to transmit and one to receive.

Increases the capacity of each domain from 10 to 20 Mbps.

no chances of collision, so CSMA/CD is not used



Fig 2.34 full duplexed switched Ethernet

FAST ETHERNET

IEEE 802.3 u.

same frame format, media access, and collision detection rules as 10 Mbps Ethernet

Data transfer rate of 100 Mb/s .

Compatible with Standard Ethernet.

MAC Sub Layer

The only two changes made in the MAC layer are the data rate and the collision domain

A new feature added called Auto negotiation;allows two devices to negotiate the mode or data rate of

operation.

For example, a device with a maximum capacity of 10 Mbps can communicate with a device with a

100 Mbps capacity.

Physical Layer

Implementation

Fig 2.35 Fast Ethernet classification

Fig 2.36 fast Ethernet description



Topologies

Fig 2.37 fast Ethernet topologies

GIGABIT ETHERNET

IEEE 802.3z.

All configurations of gigabit Ethernet are point to point.

Point-to-point, between two computers or one computer – to –switch.

Compatible with 100BASE-T and 10BASE-T

MAC Sublayer

Fig 2.38 gigabit Ethernet access methods

It supports two different modes of medium access: full duplex mode and half duplex mode.

Half duplex is used when computers are connected by a hub. Collision in hub is possible and so

CSMA/CD is required.

Full duplex is used when computers are connected by a switch. No collision is there and so

CSMA/CD is not used.

Carrier Extension tells the hardware to add its own padding bits after the normal frame to extend the

frame to 512 bytes.

Fig 2.39 carrier extension

Frame Bursting allows a sender to transmit a concatenated sequence of multiple frames in a single

transmission. If the total burst is less than 512 bytes, the hardware pads it again.



Fig 2.40 frame bursting

Physical Layer

Topology.

Fig 2.41 gigabit Ethernet topology

Fig 2.42 gigabit Ethernet physical layer implementations

Fig 2.43 gigabit Ethernet physical layer implementation description



LAN CONNECTING DEVICES

Fig 2.44 classification of LAN connecting devices

Fig 2.45 network connecting devices at each layer

REPEATERS

A repeater (or regenerator) is an electronic device that operates on only the physical layer of the OSI

model.

A repeater installed on a link receives the signal before it becomes too weak or corrupted, regenerates

the original pattern, and puts the refreshed copy back on the link.

Fig 2.46 repeater in network



A repeater does not actually connect two LANS; it connects two segments of the same LAN.

A repeater forwards every frame; it has no filtering capability

Fig 2.47 function of repeater

HUBS

Passive Hubs

A passive hub is just a connector which connects the wires coming from different branches

Active Hub

A Hub is a multiport repeater. used to create connections between stations in a physical star topology.

Connection to the hub consists of two pairs of twisted pair wire one for transmission and the other for

receiving.

it copy the received frame onto all other links

Fig 2.48 hub in network

BRIDGES

Bridges operate in both the physical and the data link layers of the OSI model.

Bridges can divide a large network into smaller segments.

When a frame (or packet) enters a bridge, the bridge not only regenerates the signal but checks the

destination address and forwards the new copy only to the segment the address belong.

This is done by a bridge table (forwarding table) that contains entries for the nodes on the LAN

The bridge table is initially empty and filled automatically by learning from frames

movements in the network

A bridge runs CSMA/CD before sending a frame onto the channel



Fig 2.49 bridge in a network

Types of Bridges

1. Simple Bridge

2. Multiport Bridge

3. Transparent Bridge

1. Simple Bridge

The address table must be entered manually

Whenever a new station is added or removed, the table must modify.

Installation and maintenance of simple bridges are time-consuming and potentially more.

2. Multiport bridges

A multiport bridge can be used to connect more than two LANs.

3. Transparent Bridges

A transparent, or learning, bridge builds its table of station addresses on its own as it performs its

bridge functions.

The stations are completely unaware of the bridge’s existence.

A transparent bridge must meet three criteria:

1. Frames must be forwarded from one station to another.

2. The forwarding table is automatically made by learning frame movements in the network.

3. Loops in the system must be prevented.

Fig 2.50 transparent bridge table updation



Loop Problem

Multiple paths of bridges and local-area networks (LANs) exist between any two LANs in the

internetwork.

Having more than one transparent bridge between a pair of LAN segments can create loops in the

system.

Bridging Loops Can Result in Inaccurate Forwarding and Learning in Transparent Bridging

Environments.

Fig 2.51 loop problem in network

Step 1. Station-A sends a frame to Station-B. Both the bridges forward the frame to LAN Y and update the

table with the source address of A.

Step 2. Now there are two copies of the frame on LAN-Y. The copy sent by Bridge-a is received by Bridge-

b and vice versa. As both the bridges have no information about Station B, both will forward the frames to

LAN-X.

Step 3. Again both the bridges will forward the frames to LAN-Y because of the lack of information of the

Station B in their database and again Step-2 will be repeated, and so on.

So, the frame will continue to loop around the two LANs indefinitely.

Looping problem Is avoided by using Blocking ports (no frame is send out of these ports).

SPANNING TREE ALGORITHM

IEEE 802.1d

In graph theory, a spanning tree is a graph in which there is no loop

In a bridged LAN, this means creating a topology in which each LAN can be reached from any other

LAN through one path only

A LAN can be depicted as a graph, whose nodes are bridges and LAN segments (or cables), and

whose edges are the interfaces connecting the bridges to the LAN segments

Steps

Every bridge has a built-in ID and the bridge with smallest ID is selected as the root bridge

The algorithm tries to find the shortest path (a path with the shortest cost) from the root bridge to

every other bridge or LAN

The combination of the shortest paths creates the shortest tree

Based on the spanning tree, we mark the forwarding ports and blocking ports

The forwarding ports are shown as solid lines, whereas the blocked ports are shown as dotted lines.



Fig 2.52 Spanning tree of a network of bridges

TWO-LAYER SWITCHES

Two-layer switch performs at the physical and data link layers.

Is a bridge, with many ports and allows better (faster) performance.

Able to allocate a unique port to each station, with each station on its own independent entity.

It makes a filtering decision based on the MAC address of the frame it received.

It can have a buffer to hold the frames for processing.

More than one station transmitting at a time.

It can have a switching factor that forwards the frames faster.

BACK BONE NETWORKS

A backbone network allows several LANs to be connected.

In a backbone network, no station is directly connected to the backbone; the stations are part of a

LAN, and the backbone connects the LANs.

The backbone is itself a LAN that uses a LAN protocol such as Ethernet and each connection to

the backbone is itself another LAN.

The two most common architectures are the bus backbone and the star backbone.

Bus Backbone

In a bus backbone, the topology of the backbone is a bus.

Bus backbones are normally used as a distribution backbone to connect different buildings in an

organization.

Fig 2. 53 bus backbone



Star Backbone

The topology of the backbone is a star; the backbone is just a switch.

Mostly used as a distribution backbone inside a building.

Fig 2.54 star backbone

CONNECTING REMOTE LANS Another common application for a backbone network is to connect remote LANs. This type of backbone

network is useful when a company has several offices with LANs and needs to connect them. The

connection can be done through bridges sometimes called remote bridges. The bridges act as connecting

devices connecting LANs. These bridges have ports that have data rate and signal levels compatible to

telephone network standard. The bridges establish a data link connection through the leased circuit and

then carry out bridge operation

Fig 2.55 Connecting remote LANs with bridges

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