CS 6551 - UNIT 2 PPT

1. RMK College of Engineering and Technology
CS 6551
Computer Networks
Department
of
Electronics and Communication Engineering

2. Unit 2
Media Access
and
Internetworking
Prepared by
Jai Ganesh S
Asst.Professor - ECE

3. Syllabus
• Media access control
• Ethernet (IEEE 802.3)
• Wireless LAN’s
• Bluetooth (IEEE 802.15)
• Wi – Fi (IEEE 802.11)
• Switching and Bridging
• Basic internetworking
• IP
• CIDR
• ARP
• DHCP
• ICMP

4. Introduction
• The data link layer is further classified in to 2 sub layers
• Logical Link Control sub layer (LLC)
• Media Access Control Sub layer (MAC)
• The computer society of IEEE started a project called 802 to set the standards to
enable intercommunication among various devices.

5. OSI Model and IEEE Standard

6. Ethernet – IEEE 802.3

7. Ethernet Cable and Port

8. Ethernet – Over view
• The topics that are to be discussed in the upcoming sessions are as follows:
• Evolution
• Physical properties
• Frame format
• Addressing
• Transmitter algorithm

9. Ethernet – Evolution – Why?

10. Ethernet - Evolution
• Ethernet is an dominant Local Area Networking Technology developed in the year 1976 at
Xerox’s Palo Alto Research Centre (PARC)

11. Evolutions of Ethernet
• It has gone through 4 generations
• Standard Ethernet – 10Mbps
• Fast Ethernet – 100mbps
• Gigabit Ethernet – 1Gbps
• Ten Giga bit Ethernet – 10Gbps

12. 1. Physical Properties
• Ethernet segments were originally built
using co axial cables that runs up to 500
meters.
• Modern ethers cables are replaced by
twisted pair cables (CAT 5 )
• The structure of the physical Ethernet is
shown in the figure

13. 2. Frame Format
• Ethernet Frame contains 7 fields.

14. PREAMBLE:
• 7 BYTES containing alternate 0’s and 1’s
• This alerts the receiving system about the incoming frame and helps in synchronization
• This is added at physical layer. Not a part of frame.
SFD (Start Frame Delimiter):
• This segment is 1 byte long.
• This indicates the start of the frame.
• The last 2 bits (11) alerts the receiver that the next field is destination address.
Destination Address (DA):
• It is of 6 bytes.
• Contains the physical address of the destination host or stations.
Source Address (SA):
• It is of 6 bytes.
• Contains the physical address of the Source host or stations.
Length or Type:
• Used for 2 purpose
• Ethernet (Type) – To define the upper layer protocol.
• IEEE (Length) – To specify the number of bytes in the data field.
Data & Padding:
• It can be a minimum size from 46 bytes to maximum of 1500 bytes.
• It carries data encapsulated from the upper layer
Cyclic Redundancy Check :
• This field contains Error Detection Information.

15. 3. Addressing
• Each station on the network
has its own Network Interface
Card (NIC).
• This NIC Provides the 6 Byte
Physical Address.

16. Representation of Addresses
47:20:1B:2E:08:EE

17. Classification of Addresses
• The addresses are broadly classified in to 3 categories.
Unicast – It defines only one recipient. One to one communication
Multicast – It defines a group of recipients. One to Many Communication.
Broadcast – It is a special case of Multicast addressing. One to ALL
Communication.

18. Classification of Addressing

19. Identification of Addressing Modes
• The addresses are identified based on the Last Bit of the First Byte
• If ‘0’ – Unicast Address
• If ‘1’ – Multicast Address
• If All Bytes are ‘1’ – Broadcast Address

20. Examples
• 4A:30:10:21:10:1A
This is a unicast address because A in binary is 1010
• 47:20:1B:2E:08:EE
This is a multicast address because 7 in binary is 0111
• FF:FF:FF:FF:FF:FF
This is a broadcast address because all digits are F’s

21. 4. Transmitter Algorithm
• Ethernet Uses CSMA – CD as the access method.
• CSMA works on the principle of “Listen Before Talk”
i.e., It Senses the Channel Before sending the
Data. This Approach reduces the possibility of
collision of packets

22. CSMA - CD
• CSMA – Carrier Sense Multiple Access
Defines the procedure for sensing the channel before transmission of data
• CD – Collision Detection
Defines the procedure for tracking the collision.
• Lets discuss these terms individually

23. Behaviour of CSMA
• The behaviour of CSMA – CD can be seen in 3 types.
One – Persistent Method
Non – Persistent Method
P – Persistent Method

24. One Persistent Method

25. Non Persistent Method

26. P Persistent Method

27. Functioning of Collision Detection

28. End
of
Ethernet

29. Wireless LAN’s

30. Problems to be addressed in Wireless Scenario

31. 1. Bit Errors due to unpredictable environments

32. 2. Reliability of message as we cannot send it for intended user

33. 3. Power Requirements for transmitting and receiving

34. 4. There are lots of chances for Eavesdropping

35. Spreading Techniques
• Since all the nodes in the WLAN share the same medium, it is mandatory to use the
spectrum effectively and reliably.
• Spread Spectrum is the Technique which is used to achieve the above mentioned
requirements.
• Classifications:
Frequency Hopping Spread Spectrum (FHSS)
Slow Frequency Hopping
Fast Frequency Hopping
Direct Sequence Spread Spectrum (DSSS)

36. Concept of Spread Spectrum

37. 1. Direct Sequence Spread Spectrum
• The idea behind this approach is the generated message signal is
multiplied with an pseudorandom sequence to get the spread signal

39. 2. Frequency Hopping Spread Spectrum
• Frequency Hopping Spread Spectrum (FHSS) is a method of transmitting
radio signals by rapidly switching a carrier among many frequency channels,
using a pseudorandom sequence known to both transmitter and receiver.

40. Frequency Hopping Concept

41. Types
• Fast Frequency Hopping:
• Fast FH changes the carrier frequency several times during transmission of one symbol.
• Slow Frequency Hopping:
• Slow FH transmits one or several symbols on each frequency
• Each timeslot is transmitted on a given carrier frequency; the next slot then changes to a
different frequency

42. FH for Multiple Access

43. Wireless Technologies
• There are several wireless technologies available

44. But Our Syllabus Covers Only 2 Technologies
IEEE 802.11 IEEE 802.15

45. Wi – Fi [IEEE 802.11]
• IEEE 802.11 standards are designed for limited geographical area (Homes, Offices, Colleges
etc.,)
• We are about to discuss
• Physical Properties – (Evolution)
• Problems in Wireless LAN’s
• Hidden Node Problem
• Exposed Node Problem
• Access Method – CSMA CA
• Architecture
• Frame Format
• Addressing Mechanisms

46. 1. Physical Properties
• As discussed in the Ethernet the Wireless standards have also been evolved over Decades.
• The various versions of IEEE802.11 based on their data rate are as follows
• 802.11 a – upto 54 Mbps – 5 GHz
• 802.11 b – up to 11 Mbps – 2.4 GHz
• 802.11 g – up to 54 Mbps – 2.4 GHz
• 802.11 n – up to 600 Mbps – 5 GHz (Note)

47. 2. Problems in WLAN
• There are 2 main Problems in
WLAN. They are,
• Hidden Node Problem
• Exposed Node Problem

48. a. Hidden Node Problem
• Any node present in their respective region can hear all of its respective
transmission
• From figure station C is outside the coverage of station A
• Station A is outside the coverage of station C
• Station B is covered by both A and C. Hence it can hear all communications
from A and C as well.
• A sends data to B. after some time C sends data to B. this leads in collision. (C
does not know that A is transmitting the data to B)
• In this case A and C are hidden Nodes to each other.
• This problem is overcome by using handshaking signals before transmission

49. b. Exposed Node Problem
• Consider the given scenario
• A is transmitting data to B
• C has some data to send to D
• Both the transmissions can happen simultaneously.
• But when C senses the channel it assumes that the channel is
busy and refrains its transmission to D
• This results in wastage of channel efficiency
• This problem cannot be solved by RTS – CTS Method.

50. 3. Access Method
• In CSMA – CD the station senses the channel only for that instant and decides the idleness of the
channel.
• In this protocol the station, before sending the data senses that , is there any other transmissions from
other stations for a particular PERIOD.
• These periods are measured by a special timers called as Network Allocation Vector (NAV)
• The operation remains as same as the CSMA – CD except the waiting time. There are 2 timers involved in
this process.
 DISF – Distributed Coordinated Function Inter Frame Space.
 SIFS – Short Inter Frame Space.

51. Important Timers in CSMA - CA
• If station finds that the medium is continuously idle for Distributed Coordinated
Function Inter Frame Space (DIFS) duration, it is then permitted to transmit a frame.
If the channel is found busy during the DIFS interval, the station should defer its
transmission.
• Short Inter Frame Space (SIFS), is the amount of time in micro seconds required for a
wireless interface to process a received frame and to respond with a response frame

52. Flow diagram of CSMA - CA
• Before sending a frame the source station senses the channel for a
period of DIFS.
• If the channel is found idle for the entire period of DIFS then the
sender sends the control frame RTS “Request To Send”
• The receiver takes some time for processing the received frame
and to respond this time is referred to as SIFS. Then the receiver
responds with the control frame CTS “Clear To Send”
• Other stations are not allowed to sense the channel during this
period

53. Flow Chart of
CSMA - CA

54. Wireless Services
• IEEE 802.11 standard defines 2 types of services they are
• Basic Service Set – BSS
• Extended Service Set – ESS

55. Basic Service Set (BSS)
• BSS is made up of stationary or mobile stations and an
optional central base station known as Access Point (AP)
• BSS without AP is called as Ad hoc Networks
• BSS with AP is called as Infrastructure Network

56. Extended Service Set
• ESS is made up of more BSS and are
connected to a “Distributed Systems”
• The Distributed systems connects the AP’s
in the BSS
• There are 2 types of stations
Mobile Stations
Stationary Stations

57. 4. Architecture of 802.11

58. MAC Sublayer
• The MAC sublayer contains 2 protocols
• Distributed Coordination Function – DCF
• Point Coordination Function – PCF
• These 2 protocols are related with the access methods of the standard

59. i. Distributed Coordination Function
• It uses CSMA – CA as the access method
• CSMA – CD cannot be used for wireless scenario for 3 major reasons
• For collision detection the station must send the data packet and receive the collision signal at the
same time. This increases the bandwidth requirement.
• Collision may not be detected due to hidden node problem.
• Due to large distance between the stations fading occurs. This prevents the station at one end
from hearing a collision at other end.

60. ii. Point Coordination Function
• These are the access methods that are well suited for the infrastructure network. (Not
for Ad hoc Networks)
• They are specially designed for the time sensitive applications.
• The AP performs polling method to allow data transmission by the stations.
• This uses 2 timers as like CSMA – CA
• PIFS – Same as DIFS
• SIFS – Same as SIFS

61. 5. Frame Format
• The MAC layer frame contains 9 fields.

62. Type of Frames
• There are 3 types of Frames
• Management Frame:
• These frames are used for initial communications between the stations and access points
• Control Frame:
• These frames are used for accessing the channel and acknowledging the frames.
• It can be an RTS frame, CTS Frame or ACK Frame
• Data Frames:
• These frames are used for carrying the data and control information

63. Frame Control (FC) :
• Defines the type of the frame and some other control information.
• It has 11 sub fields.
Protocol Version :
• It defines the version of the protocol.
Type :
• It defines the type of information.
• 00 – management frame
• 01 – control frame
• 10 – data frame
Sub Type :
• It defines the sub type of each frame.
• 1011 – RTS Frame
• 1100 – CTS Frame
• 1101 - Acknowledgement
More Flag :
• When this bit is “1” it indicated more fragments are to be received
Retry :
• When this bit is “1” it indicates the retransmitted frame
Pwr Mgt (Power Management) :
• When this bit is “1” it indicates station is in power management Mode
More Data:
• When this bit is “1” it indicates station has more data to send
WEP (Wired Equivalent Privacy):
• When this bit is “1” it indicates encryption is implemented
RSVD (Reserved):
• This is reserved for future use
D (Duration):
• This field defines the duration of the transmission used to set the NAV timers
• This is used as the ID for control frames
Addresses:
• There are 4 address fields
• The values in the address fields depends on the values of “to DS” and “From DS” sub fields
SC (Sequence Control):
• This field defines the sequence number of the frames for flow control.
Frame Body :
• This field can be between 0 bytes to 2312 bytes.
• This field contains the actual information based on the values of the sub fields
FCS (Frame control Sequence):
• This field carries the 32 bit CRC for error detection and Correction.

64. 6. Addressing Mechanisms
• The values of the 4 Address fields will be decided based on the values of
“To DS” and “From DS”
Address 1:
• This field is always the address of next device
Address 2:
• This field is always the address of previous device
Address 3:
• This field is always the address of final destination if it is not defined by address 1
Address 4:
• This field is always the address of Original Source if it is not defined by address 2
ADDRESS REPRESENTS
1 Address of Next Device
2 Address of Previous device
3 Address of Final Destination
4 Address of Original Source

65. Understanding Addressing
1 2 3

66. Case 1
• In this case the frame is not going to DS (To DS =0)
also not coming from DS (From DS =0)
• The frame is moving from one station to another
station of same BSS without passing through DS

67. Case 2
• In this case the frame is not coming from DS (From
DS =0) and the frame is going to DS (To DS =1)
• The frame comes from a station and going to the AP

68. Case 3
• In this case the frame is coming from DS (From DS
=0) and the frame is going to DS (To DS =1)
• The frame comes from AP of one BSS and going to
the AP of another BSS

69. Case 4
• In this case the frame is coming from a DS (From
DS =1) and the frame is not going to DS (To DS =0)
• The frame comes from AP and going to the station

70. End
of
Wi - Fi

71. Bluetooth – IEEE 802.15

72. Bluetooth
• A Bluetooth LAN is an Ad hoc Network.
• The various applications of Bluetooth are short range communications and
implementing various sensor networks
• Topics Discussed:
• Architecture
• Bluetooth Layers
• Frame Format

73. 1. Architecture
• Bluetooth defines 2 types of architecture
• Piconets:
• It can have maximum of 8 stations out of which 1 station acts as
primary station and others acts as secondary stations.
• All the secondary stations are synchronised with the primary
station
• Scatternets:
• When 2 or more piconets combined together then they are
referred to as scatternet.

74. 2. Bluetooth Layers

75. Radio Layer
• This layer is roughly equivalent to the physical layer.
• This also defines 3 other parameters
• Operating Band
• It is operated in 2.4 GHz ISM Band
• FHSS
• It uses FHSS method. It hops 1600 times per second
• Modulation
• It uses GFSK (Gaussian FSK) – Version of FSK as a modulation Technique

76. Base Band Layer
• This layer is roughly equivalent to MAC Sub Layer.
• This layer defines the following parameters
• Access Method
• It uses TDD – TDMA as the access mechanism
• Physical Link
• There are 2 types of link that can be created between the primary and secondary stations
• Synchronous Connection Oriented Link (SCL)
• This link is used when avoiding latency is more important than any reliable delivery.
• No retransmission of frames occurs during failure
• They are mainly used in audio communications
• Asynchronous Connectionless Link (ACL)
• This link is used when reliable delivery is more important than any latency.
• Frames are retransmitted on the event of failure.

77. L2CAP Layer
• Logical Link Control and Adaptation Layer
• They are roughly equivalent to the LLC sub layer.
• There are 4 specific duties
• Multiplexing
• It accepts the data from the upper layer and delivers them to the lower layers
• Segmentation an Reassembly
• This layer divides the larger packets into segments and adds extra information to define the location of the segment in original
packets
• At the destination this layer reassembles them back.
• Quality of Service (QoS)
• QoS can be defined under any circumstances
• Group Management
• Bluetooth allows data transmission between the group of users
• As similar to multicast

78. 3. Frame Format
• Bluetooth frame can be classified in to 3 types.
• 1 slot frame:
• This frame frequency hops for each slot.
• Duration of this frame is 366 Bits (Note)
• 3 slot frame:
• This frame frequency hops for every 3 slots.
• Duration of this frame is 1616 Bits (Note)
• 5 slot frame:
• This frame frequency hops for every 5 slots
• Duration of this frame is 2866 Bits (Note)

79. General Frame Format
Access Code:
• This field contains a synchronisation bits to identify the primary from various piconets
• The secondary stations uses this bits to synchronise their speed with their primary station
Address:
• This field runs from 000 – 111 to define up to 7 secondary address.
• 000 – represents broadcast address.
Type:
• This field represents the type of data coming form the upper layer
F :
• This field is used for flow control
• When this field is set to “1” it indicates that the buffer is Full
A :
• This field is used for Acknowledgement
S :
• This field is used for Sequence Number
HEC (Header Error Correction) :
• This field is used for error detection (Check Sum)
Data :
• This field carries the actual data.
• The length of this field varies according to the type of the frame (As Mentioned in the
Figure)

80. L2CAP Frame Format
• Length :
• Indicates the size of the data field
• Channel ID:
• This defines the unique identifier for the virtual channel created
• Data and Control:
• Actual data to be sent.
• It can take a maximum value of 65,535 Bytes

81. End
of
Bluetooth

83. Switching & Bridging

84. Introduction
• This section especially deals particularly with “building networks at global level”
• Making all the system to come under 1 network is not practically possible
• Only possible solution is INTERNETWORKING
• One way of interconnecting the networks is by using “Switches” and “Bridges”

85. Switching
• Switch is a mechanism that allows us to interconnect
links to form a larger networks.
• Switch is a multi – input , multi – output device.
• It receives packets from one input port and forwards
them to one or more output ports.
• Switch provides a star topology

86. Primary Function of a Switch
• The primary function of a Switch is to receive incoming packets on one of
its links and to transmit them on some other link. This function is
referred to as “forwarding”

87. Forwarding Approaches
• How does a switch decide which output link to place each packets on ?
• There are 3 basic approaches from which switch knows where to forward.
• Datagram (or) connection less approach
• Virtual circuit (or) connection oriented approach
• Source Routing

88. Common Requirement for all 3 approach
• Forwarding can be done only if the switch knows 2 important parameters
• IP address of all the host connected to it
• This going to be a unique value so no problem.
• The port number on which a particular host is connected
• Can be done in 2 ways
• Numbering each ports (ie. Forward to Seat No : 10)
• Identifying each port by the name of the node connected to it (ie. Forward to Seat on which Ram sat
yesterday)

89. 1. Datagram Approach @ Connection Less Approach
• All the packets are included with enough information about the destination. So
that any switch will know how to take it to the destination
• Each and every switch on the network maintains a table which indicates the
details of connected hosts on all ports. This table is referred to as “Forwarding
Table” or “Switching Table”

91. Drawbacks of Datagram Approach
• Difficult to from a forwarding table for large network
• Not suitable for dynamically changing topologies
• Creates a problem when there are multiple paths available to reach the
destination.

92. Characteristics of datagram networks
• They are referred to as “connection less” because they do not check for the
dedicated link to destination before transmission
• A host have no idea about the capability of the network to deliver the packets
• All packets take the same path unless the entries on the forwarding table is
altered. This may overload the path

93. 2. Virtual Circuit Switching
@
Connection Oriented Approach
• This is an another approach which uses a concept of “Virtual Circuit”.
• This approach is referred to as connection oriented approach as it
establishes the virtual circuit between the source and destination

94. Process of Virtual Circuit Approach
• The entire process can be classified in to 2 phase
Connection setup phase Data transfer phase

95. Connection Setup Phase
• In this phase a virtual connection is established in all the switches present between the “Source” and
“Destination”
• This processes is referred to as “defining Connection State”
• Defining connection state has 4 main components
• Incoming interface – on which the packets of this VC arrives the switch
• Incoming Virtual Circuit Identifier – uniquely identifies the incoming VC ID
• Outgoing Interface – on which the packets of this VC leaves the switch
• Outgoing Virtual Circuit Identifier - uniquely identifies the outgoing VC ID

96. 2 approaches for establishing the connection state
Manual Configuration
Automatic Configuration
 In this case connection state is established
by Network Administrator
 Virtual circuit is called “Permanent Virtual
Circuit (PVC)”
 Admin can delete the virtual circuit at any
time
 The process is initiated by the Sending
Node
 Sending node sends a message to the
network requesting to establish a connection
state
 This is referred to as “Signalling”
 The virtual circuit is called “Switched
Virtual Circuit (SVC)”
 Host can delete the VC at any time without
the intervention of admin

97. Establishing Connection State by Network Admin
1. Identify and Create Virtual Circuit from source to destination2. Assigning id for each virtual circuit which is not already used
5
11
7
4
3. Creation of VC table at each switch present between source and destination
3.1 VC table for switch 1
INCOMING
INTERFACE
INCOMING VCI
OUTGOING
INTERFACE
OUTGOING VCI
2 5 1 11
INCOMING
INTERFACE
INCOMING VCI
OUTGOING
INTERFACE
OUTGOING VCI
3 11 2 73.1 VC table for switch 2
3.1 VC table for switch 3
INCOMING
INTERFACE
INCOMING VCI
OUTGOING
INTERFACE
OUTGOING VCI
0 7 1 4INCOMING
INTERFACE
INCOMING
VCI
OUTGOING
INTERFACE
OUTGOING
VCI
2 5 1 11
INCOMING
INTERFACE
INCOMING
VCI
OUTGOING
INTERFACE
OUTGOING
VCI
3 11 2 7
INCOMING
INTERFACE
INCOMING
VCI
OUTGOING
INTERFACE
OUTGOING
VCI
0 7 1 4

98. Establishing Connection State by Sending Node
INCOMING
INTERFACE
INCOMING
VCI
OUTGOING
INTERFACE
OUTGOING
VCI
2 5 1 11
INCOMING
INTERFACE
INCOMING
VCI
OUTGOING
INTERFACE
OUTGOING
VCI
3 11 2 7
INCOMING
INTERFACE
INCOMING
VCI
OUTGOING
INTERFACE
OUTGOING
VCI
0 7 1 4
5
11
7
4
S1 S2 S3
1. Host A sends a setup message into the network with the
destination address of Host B
2. When switch 1 receives the setup message it picks a
VCI1 value and forwards the packet at all ports
3. Switch 1 updates the incoming port number and VCI
chosen
4. Now , Switch 2 receives the setup message. It performs
the same operation as Switch 1 and updates the VC Table
5. Now ,switch 3 receives the setup message and performs
the same operation.
6. Then the setup message is forwarded to Host B. The host
choses the VCI value.
7. At this stage none of the switch knows about the outgoing
interface. This will be updated from the ACK sent by Host B
8. Now, Switch 3 updates its outgoing information on VC
Table and forwards the ACK to Switch 2
9. Switch 2 updates the VC Table and Forwards the ACK to
Switch 1
10. Switch 1 updates the VC Table and forwards the ACK to
Host A
11. By Receiving this ACK Host A knows the Complete
Virtual Path to reach Host B

99. 2. Data Transfer Phase
• Data transfer phase starts after the completion of connection setup phase.
• Host A puts a value of 5 for outgoing VCI in the header of the packet.
• Switch 1 receives any such packets on interface 2 it refers to the VC table and puts the VCI value of 11 and
forwards.
• Similarly the packet reaches Host B.
• Once the data transfer is done Host A can tear down the connection with Host B by sending “Tear Down Signal” to
Host B
• This signal removes all the entries in the switches.

101. End
of
Switching

102. 2. Bridges

103. Bridging

104. Introduction
• Bridges are the devices that are used to interconnect two
Ethernet networks as the length of the cable is limited.
• The usage of bride is shown in the figure.

105. Transparent Bridge
• This is a type of bridge, the hosts does not have any idea about the existence of
the bridge.
• Bridge forwarding table is updated by “Learning Process” when the stations
forward frames among them.
• Forwarding Table can be configured by network admin also. But not suitable
for changing environment

106. Learning Process
1. Initially there will not be any entries in
the forwarding table
2. Table will be updated when the transfer
of Frames occurs
3. When A sends a frame to D : Bridge do
not have any entries for D or A
4. Since the Bridge do not know where D
is. It FLOODS the Frame.
5. By now the Bridge Learns that Host A
is on the LAN Connected at port 16. When E sends a Frame to A
7.Bridge refers to the table and the
location of A is known. So no Flooding.
Packet is sent on Port 1
8. This time the Bridge learns that Host E
is located on the LAN connected at Port 3
and updates the table
9. When B sends frame to C10. Bridge Doesn’t know about the
locations of B or C. Thus it FLOODS the
Packet. Also learns the location of B
11. This process continues until bridge
learns about all the hosts on the network

107. Looping Problem
• Bridges works fine as long as there is no redundant bridges in the system
(More than 1 bridge)
• This redundancy in bridges creates “Looping Problem”
• This problem is illustrated in the following figure.

108. When A Sends a Frame to D. This frame enters
both the Bridges at port 1. B1 and B2 Updates its
table and floods the frame on LAN 2.
Now LAN 2 contains 2 copies of frames. Now
frame sent by the B2 will be received by B1 and
Vice Versa. This time the frame enters at Port 2
of B1 and B2. Thus the Table is updated again
At this stage LAN 1 contains 2 copies of
frame and these frames again enters the
bridges at port 1. Table is updated again.
This Process continues on and on…..

109. Review of Looping Problem

110. Solution for Looping Problem
• This looping problem can be overcome by “Spanning Tree Algorithm”
• This algorithm creates a topology in which each LAN can be reached
from any other LAN through one path only.
• This ultimately removes the LOOP.

111. Spanning Tree Algorithm
Let us consider the given example
network where 4 LAN’s are connected
using 5 Bridges
For understanding and calculating
purpose lets draw an graphical
representation of the given network
Step 1: in order to start the spanning tree
process we need to assign a COST (or)
Metric to each link.
Cost can be assigned based on several
aspects like Minimum hops, Minimum
Delay, or Maximum Bandwidth. This can
be chosen by the choice of Admin.
Here we are considering No. of Hops as a metric.
Assign the cost as mentioned below.
Bridge LAN = 1
LAN  Bridge = 0
Step 2: Compute the Number of Hops from a Root
Node to all other nodes.
The node with least ID will act as root node. We
assume that B1 is the Root Node
If there are multiple path to a node, choose the path
with lowest cost and neglect the remaining paths

112. Before and After Alg.

113. End
of
Bridging

114. Basic
Internetworking

115. Introduction
• By using switches and bridges it is possible to build a large network but it is only up
to certain limit.
• In order to further increase the scalability of network , we connect multiple networks
by a device called as “Router”
• Hence the network layer comes in to picture which takes the responsibility of packet
delivery from source to destination among multiple networks

116. Basic Concepts
• IP – Internet Protocol
• CIDR – Classless Inter Domain Routing
• ARP – Address Resolution Protocol
• DHCP – Dynamic Host Control Protocol
• ICMP – Internet Control Message Protocol

117. Brief Notes first… then Deeper
IP

118. CIDR

119. ARP

120. DHCP

121. ICMP