1.
Computer Networks -LAN
Deepak John
Department Of MCA,SJCET-Pala

2.
Local Area Network

3.
Attributes Of A LAN
i. Transmission technology
ii. Signalling methods
iii. Transmission medium
iv. Access Control
v. Topologies

4.
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

5.
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

6.
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

7.
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

8.
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

9.
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)

10.
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

11.
 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.

12.
LLC Protocol Data Unit (PDU)
 which contains 4 fields
 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

13.
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.

14.
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.

15.
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

16.
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.

17.
MAC- Frame Format
 The MAC layer is responsible for performing functions related to
medium access and for transmitting the data.
 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

18.
Standard Title Description
802.1 High-level Interface Specification of standards for LAN architecture,
interconnection, management
802.2 Logical Link Control Specification of standards for the LLC layer
802.3 Ethernet(CSMA/CD) Specification of standards for CSMA/CD architectures
802.4 Token Bus Specification of standards for token bus architectures
802.5 Token Ring Specification of standards for token ring architectures
802.6 Metropolitan Area
Networks
Specification of standards for MANs
802.7 Broadband Technical
Advisory Group
Provision of guidance to other groups working on
broadband LANs
802.8 Fiber Optic Technical P i i f id h ki
Advisory Group
Provision of guidance to other groups working on
fiber optic-based LANs
802.9 Integrated Data and Specification of standards for interfaces to ISDN
Voice Networks

19.
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.

20.
RANDOMACCESS
 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

21.
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.

22.
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 rise rapidly with increased load.
 After a maximum number of retransmission attempts Kmax
station must give up and try later.

23.
Frames in a pure ALOHA network

24.
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
First transmission Retransmission
t
t0-X t0 t0+X t +X+2t t0+X+2tprop +B
0+2tprop t0 X Vulnerable Time-out Backoff period: B Retransmission
period if necessary

25.
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 transmissions 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

26.
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

27.
First transmission Retransmission
t
t0 -X t0 t0+X+2tprop t0+X t0+X+2tprop +B
Time-out Backoff period:
B
Retransmission
if necessary
Vulnerable period: t0-X to t0 , i.e., X seconds long
 Max channel utilization is 37%,doubles Normal ALOHA, but still low

28.
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

29.
 Suppose tprop is propagation delay from one extreme end to the other
extreme end of the medium. When transmission is going on, a station
Station A begins
g g ,
can listen to the medium and detect it. Vulnerable period = tprop
A
g
transmission at
t=0
sense sense
Station A
captures
channel A
at t=tprop CSMA random access scheme
sense sense
 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.

30.
 Following are some versions of CSMA protocol Based on how to
do when medium is busy
• 1-Persistent CSMA
• Non-Persistent CSMA
• p-Persistent CSMA

31.
 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
Sense carrier
yes
Busy?
sensing, then they will begin
transmission the same time when
no
Send the frame
h lb idl lli i channel becomes idle, so collision.
At this time, each station wait for a
random time and then re-senses
with probability 1
time, re the channel again.
 Problem with 1-persistent CSMA is
“high collision rate”.

32.
 Nonpersistent
 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.
Sense carrier Wait random
time…
yes
Busy?
no
Send the frame

33.
 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.
Sense carrier
Busy?
yes
no
Send the frame
with p probability p
yp

34.
Behaviour of three persistence methods

35.
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.

36.
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

37.
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 generate a random number that falls within a specified
range of values. which 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

38.
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.

39.
 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.

40.
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:
 Polling
 Reservation
 Token Passing

41.
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

42.
Primary is sending to Secondary Secondary is sending to Primary
 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

43.
 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,…

44.
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.

45.
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.
token

46.
LAN SYSTEMS

47.
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
Ethernet Evolution
Standard
Ethernet
Fast
Ethernet
Gigabit
Ethernet
Ten-Gigabit
Ethernet
100-Gigabit
Ethernet
(10 Mbps) (100 Mbps)

48.
IEEE 802.3 MAC Frame

49.
 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

50.
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

51.
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.

52.
Physical Layer Implementation
 The Standard Ethernet defines several physical layer implementations;
four of the most common, are

53.
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.

54.
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

55.
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).

56.
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.

57.
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.

58.
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.

59.
A network with and without a bridge

60.
 Separating collision domains:
 Collisions domains become much smaller and possibility of collision
is reduced.
 With bridging, lesser number of channels compete for access to the
medium.

61.
Switched Ethernet
cards,  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 allows
faster handling of packets.
 When a station wants to transmit a frame, it outputs a frame to
switch.
 Half duplex

62.
 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.

63.
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

64.
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.

65.
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.

66.
Physical Layer
 Implementation

67.
Topologies
Fast Ethernet Topologies

68.
Gigabit Ethernet
 IEEE 802.3z.
 All config rations of gigabit Ethernet are point to point
configurations point.
 Point-to-point, between two computers or one computer – to –switch.
 Compatible with 100BASE-T and 10BASE-T
MAC Sublayer

69.
 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.

70.
 Carrier Extension tells the hardware to add its own padding bits after
the normal frame to extend the frame to 512 bytes.
RRRRRRRRRRRRR
Carrier Extension
Frame
512 bytes
 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.
Frame Extension Frame Frame Frame
512 bytes
Frame burst

71.
Physical Layer
 Topology.

72.
LAN CONNECTING DEVICES

73.
Connecting Devices
Networking Devices
Internetworking
Devices
Repeaters Bridges Routers Gateways

74.
Network Connecting Devices

75.
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.

76.
 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
Function of repeater

77.
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

78.
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

79.
Types of Bridges
1. Simple Bridge
2. Multiport Bridge
3. Transparent Bridge
Simple Bridge
 The address table must be entered manually
 Whenever a new station is added or removed, the table must
modified.
 Installation and maintenance of simple bridges are time-consuming
and potentially more.
Multiport bridges
 Amultiport bridge can be used to connect more than two LANs.

80.
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.

81.
Learning Bridge

82.
Loop Problem
 multiple paths of bridges and local-
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 .

83.
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).

84.
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

85.
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

86.
The forwarding ports are shown as solid lines, whereas the blocked
ports are shown as dotted lines.
Spanning tree of a network of bridges

87.
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.
 A bridge with many ports may be 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.

88.
Types
Store-and-forward switch
• Accepts frame on input line
• Buffers it briefly,
• Then routes it to appropriate output line
• Delay between sender and receiver
Cut-through switch
• Takes advantage of destination address appearing at beginning of
frame
• Switch begins repeating frame onto output line as soon as it
recognizes destination address

89.
Routers
 Capable of connecting networks of different types
 Routers separate networks into different broadcast domains.
 They use the “logical address” of packets and routing tables to
determine the best path for data delivery.
 RIP(Routing Information Protocol)
98

90.
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.

91.
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.

92.
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.

93.
Connecting Remote LANs
 Consider a situation where 2 LANS are located at some
distance.
 It is possible to connect these LANS by taking a full duplex leased
connection from telephone network operator.
 Use 2 bridges, one at each end of leased connection
 These bridges are called remote bridges
 Useful when a company has several offices with LANs and needs to
connect them.
 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