A computer network connects autonomous devices like computers, printers, and other devices to exchange data. Nodes are connected by communication channels like copper wires, fiber optics, or wireless links. A distributed system builds a software system on top of a network so that the existence of multiple computers is transparent to the user. To be effective, networks must meet performance, reliability, and security criteria. Common network applications include electronic messaging, directory services, information services, electronic data interchange, and teleconferencing. Networks can be classified based on their topology like mesh, star, tree, bus, and ring configurations. Local area networks connect devices within a single building or campus.

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UNIT I
DATA NETWORK FUNDAMENTALS
A Network is a set of devices (node) connected by a media links. A node can be a
computer, printer, or any other device capable of sending and receiving data generated by other
nodes on the network. The links connecting the devices are often called communication channels.
Computer network ​is to mean an interconnected collection of autonomous computers.
Two computers are said to be interconnected if they are able to exchange information. The
connection may be a copper wire, fiber optics, microwaves, and communication satellites can
also be used. A network is a large computer with remote printers and terminals.
A Distributed system ​is a software system build on top of a network. In a distributed
system, the existence of multiple autonomous computers is transparent to the user. The user can
type a command to run a program and it runs. It gives a high degree of cohesiveness and
transparency.
NETWORK HIERARCHY
To be effective and efficient, a network must meet a number of criteria. They are
Performance, reliability and security.
1.​Performance: ​It can be measured by transit time and response time. The performance of a
network depends on a number of factors, including the number of users, the type of transmission
medium, the capabilities of the connected hardware, and the efficiency of the software.
2. ​Reliability: ​Network reliability is measured by frequency of failure, the time it takes a link to
recovery from a failure, and the network’s robustness in a catastrophe.
3. ​Security: ​Network security issues include protecting data from unauthorized access and
viruses.
APPLICATIONS OF COMPUTER NETWORKS
Computer networks have become an indispensable part of business, industry, and entertainment.
Some of the network applications in different fields are the following:
Marketing and sales: ​computer networks are used extensively in both marketing and sales
organization. Marketing professionals use them to collect, exchange, and analyze data relating to
customer needs and product development cycles. Sales applications include teleshopping, which
uses order-entry computers or telephones connected to an order processing network, and on-line
reservation services for hostels, airlines.
Financial services: ​Today’s financial services are totally dependent on computer networks.
Application include credit history searches, foreign exchange and investment services, and
electronic funds transfer, which allows a user to transfer money without going into a bank.
Manufacturing: ​Computer networks are used today in many aspects of manufacturing,
including the manufacturing process itself. Two applications that use networks to provide
essential services are computer- assisted design and computer-assisted manufacturing, both of
which allow multiple users to work on a project simultaneously.
Electronic messaging: ​Probably the most widely used network application is electronic mail.
Directory services: ​Directory services allow lists of files to be stored in a central location to
speed worldwide search operations.
Information services: ​Network information services include bulletin boards and data banks. A
worldwide web site offering the technical specifications for a new product is an information
services. ​Electronic data interchange: ​EDI allows business information to be transferred
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2. without using paper.​Teleconferencing: ​teleconferencing allows conferences to occur without the
participants being in the same place. Applications include simple text conferencing, voice
conferencing and video conferencing.
Cellular telephone: ​In the past, two parties wishing to use the services of the telephone
company had to be linked by a fixed physical connection. Today’s cellular networks make it
possible to maintain wireless phone connections even while travelling over long distances.
Cable television: ​Future services provided by cable television networks may include video on
request, as well as the same information, financial, and communications services currently
provided by the telephone companies and computer networks.
NETWORK ATTRIBUTES
It is broadly classified into three types,
1. Broadcast networks.
2. Point-to-point networks.
3. Multipoint' networks.
1. ​Broadcast network ​have a single communication channel that is shared by all the machines on
the network. Short messages, called packets is sent by any machines are received by all the
others. An address field within the packet specifies for whom it is intended. Upon receiving a
packet, a machine checks the address field. If the packet is intended for itself, it processes the
packet; if the packet is intended for some other machine, it is just ignored.
2. ​Point -to-point network ​provides a dedicated link between two devices. The entire capacity of
the channel is reserved for transmission between those two devices.
3. Mostly they use an actual length of wire or cable to connect the two ends. Often Multiple routes,
of different lengths are possible, so routing algorithms play an important role in point-to-point
networks, a. ​A multipoint network ​is one in which more than two specific devices share a single
link. In a multipoint environment, the capacity is shared, either spatially or temporally.
Fig. 1 Multipoint Network
TOPOLOGY OF DATA NETWORKS
It refers to the way a network is laid, either physically or logically. Two or more devices
connect to a link; two or more links form a topology. The topology of a network is the geometric
representation of the relationship of all the links and linking devices to each other. There are five
basic topologies are mesh, star, tree, bus, and ring.
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Fig.2 Topology
Mesh Topology
In mesh topology, every device has dedicated link carries traffic only between the two
devices it connects. A fully connected mesh network has n(n - l)/2 physical channels to link n
devices. To accommodate that many links, every device on the network must have n-1
input/output (I/O) ports.
Advantages:
Each connection can carry its own data load, thus eliminates the traffic problems.
A mesh topology is robust.it is privacy or security.It makes fault identification and fault
isolationeasy.
Disadvantages:
1. Installation and reconfiguration are difficult.
2. The sheer bulk of the wiring can be greater than the available space can accommodate.
3 The hardware required to connect each link can be expensive.
Star Topology
In a star topology, each device has a dedicated point-to-point link only to a central
controller, usually called a hub. The devices are not directly linked to each other. The controller
acts as an exchanger. If one device wants to send data to another, it sends the data to the
controller, which then relays the data to the other connected device.
Advantages
It is less expensive than a mesh topology.It is easy to install and reconfigure.
A star topology is robust. It makes fault identification and fault isolation.
Disadvantage More cabling is required in a star topology.
Tree Topology
A tree topology is a variation of a star. As in a star, nodes in a tree are linked to a central
hub that controls the traffic to the network. Only the majority of devices connect to a secondary
hub that in turn is connected to the central hub.
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4.
The central hub in the tree is an active hub. An active hub contains a repeater, which is a
hardware device that regenerates the received bit patterns beforesending them out. A passive hub
provides a physical connection between the attached devices.
Advantages
1. It allows more devices to be attached to a single central hub and can increase the distance a
signal can travel between devices.
2. It allows communications from different computers.
Disadvantages​: ​It requires more cabling.
Bus Topology
A bus topology is a multipoint topology. One long cable acts as a backbone to link all
the devices in the network. Nodes are connected to the bus cable by drop lines and taps. A drop
line is a connection running between the device and the main cable. A tap is a connector that
either splices into the metallic core.
Advantages
Easy to install.It requires less cabling. In this topology redundancy is eliminated.
Disadvantages
It is difficult to reconfiguration and fault isolation. It is difficult to add new devices.
A fault or break in the bus cable can stops all transmission.The damaged area reflects signals
back in the direction of origin, creating noise in both directions.
Ring Topology
In a ring topology, each device has a dedicated point-to-point line configuration only
with the two devices on either side of it. A signal is passed along the ring in one direction, from
device to device, until it reaches its destination. Each devise in the ring incorporates a repeater.
When a device receives a signal intended for another device, its repeater regenerates the bits and
passes them along.Advantages : It is easy to install and reconfigure, Fault isolation is simplified.
Disadvantages of ring topology: It is a unidirectional traffic. A break in the ring can disable the
entire network.
Hybrid topology​:
A network combines several topologies as subnetworks linked together in a larger
topology. One department of a business may have decided to use a bus topology while another
department has a ring. The two can be connected to each other via a central controller in a star
topology.
CATEGORIES OF NETWORK​:
The three primary categories are Local area network (LAN), Metropolitan area network
(MAN), Wide area network (WAN). It is usually privately owned network. Within a single
building or campus of up to a few kilometers in size. They are widely used to connect personal
computers and workstations in company offices and factories to share resources and exchange
information. LAN’s are distinguished from other kinds of network by three characteristics, (i)
their size, (ii) their transmission technology and (iii) their topology.
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Fig.4 Local Area Network
A common example is engineering workstations or accounting PCs. LAN’s often use a
transmission Technology consisting of a single cable to which all the machines are attached, like
the telephone company Party lines once used in rural areas. Traditionally, LAN’s have data rates
4 to 16 Mbps range. Today, However, speeds are increasing and can react 100 Mbps with
gigabit systems in development. Various Topologies are possible for broadcast LAN’s. Most
commonly used are bus, ring, and star.
Metropolitan Area Network (MAN)
Fig. 5 Metropolitan Area Network
It is basically a bigger version of a LAN and normally uses similar technology. It might
cover a group of nearby corporate offices or city and might be either private or public.
A MAN can support both data and voice, and might even be related to the local cable
television network. A MAN just has one or two cables and many telephone companies provide a
popular MAN service called switched multi- megabit data services (SMDS).
Wide Area Network (WAN)
It provides a long-distance transmission of data, voice, image and video information over
large geographical areas that may comprise a country, or a continent or even the whole world. It
contains a collection of machines intended for running user programs.
Service and protocols
A Service ​is a set of primitives (operation) that a layer provides to the layer above it.
The service defines what operations the layer is prepared to perform on behalf of its users.
PROTOCOL:
A protocol ​is a set of rules governing the format and meaning of the frames, packets or
messages that are exchanged by the peer entities within a layer. Entities use protocols in order to
implement their service definitions. They are free to change their protocols at will, provided they
do not change the service visible to their users. The key elements of the protocol are ​syntax,
semantics and timing.
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6.
Syntax ​refers to the structure or format of the data, meaning the order in which they are
presented. ​Semantics ​refers to the meaning of each section of bits. ​Timing ​refers to two
characteristics: when data should be sent and how fast they can be sent.
A Standards ​provides a model for development that makes it possible for a product to
work regardless of the individual manufacturer. Standards are essential in creating
andmaintaining an open and competitive market for equipment manufacturers.
They provide guidelines to manufacturer, vendors, government agencies and other service
providers to ensure the kind of interconnectivity necessary in today’s marketplace and in
international communications. Data communication standards fall into two categories, ​de facto
(by fact) and de jure (by law). Dejure​are those that have been legislated by an officially
recognized body.​De facto ​are often established originally by manufacturers to define the
functionality of a new product or technology.
Standards Organisations
Standards are developed through the cooperation of standards creation committees, forums, and
government regulatory agencies.
International organization for standardization (ISO)
American national standards institute (ANSI)
Institute of electrical and electronics engineers (IEEE)
Electronic Industries Association (EIA)
SWITCHING METHODS IN DATA NETWORKS
A switched network consists of a series of interlinked nodes called switches. Switches
are hardware or software devices capable of creating temporary connections between two or
more devices linked to the switch but not to each other.
1. CIRCUITSWITCHING NETWORK
Circuit switching creates a direct physical connection between two devices such as
phones or computers.For example, instead of point-to-point connections between the three
computers on the left to the four computers on the right requiring 12 links, we can use four
switches to reduce the number and the total length of the links.
Computer A is connected through switches I, II, and III to computer D. by moving the levers of
the switches, any computer on the left can be connected to any computer on the right.
A circuit switch is a device with n inputs and m outputs that creates a temporary connection
between an input link and an output link.
The number of inputs does not have to match the number of outputs. An n-by-n folded
switch can connect n lines in full-duplex mode. For example, it can connect n telephones in such
a way that each phone can be connected to every other phone.Circuit switching can use either of
two technologies: space-division switches or time-division switches.
2.PACKET SWITCHING NETWORK
In a packet -switched network, a data are transmitted in discrete units of potentially
variable length blocks called packets.The maximum length of the packet is established by the
network. Longer transmissions are broken up into multiple packets. Each ​n packet contains not
only data but also a header with control information. The packets are sent over the network node
to node.At each node, the packet is stored briefly then routed according to the information in its
header.
There are two popular approaches to packet switching, datagram and virtual circuit.
2.a.​Datagram approach​: In the datagram approach to packet switching, each packet is treated
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7. independently from all others.
1. Even when one packet represents just a piece of a multipacket transmission, the network treats
it as though it exited alone. Packets in this technology are referred to as datagram.
2. The data gram approach can be used to deliver four packets from station A to station X.
3. In this example, all four packets belong to the same message but may go by different paths to
reach their destination.
4. This approach can cause the datagrams of a transmission to arrive at their destination out of
order.
5. It is the responsibility of the transport layer in most protocols to reorder the datagram before
passing them on to the destination port.
6. The link joining each pair of nodes can contain multiple channels. Each of these channels is
capable, in turn, of carrying datagrams either from several different sources or from one source.
Multiplexing can be done using TDM or FDM.
Devices A and B are sending datagrams to devices X and Y. some paths use one channel while
others use more than one. The bottom link is carrying two packets from different sources in the
same direction. The link on the right, however, is carrying datagrams in two directions.
2.b .VIRTUAL CIRCUIT APPROACH
In the ​virtual circuit approach to packet switching, ​the relationship between all
packets belonging to a message or session is preserved. A single route is chosen between sender
and receiver at the beginning of the session.
Fig. 6Virtual Circuit Approach
a When the data are sent, all packets of the transmission travel one after another along that route.
Ttoday, virtual circuit transmission is implemented in two formats: switched virtual circuit
(SVC) aand permanent virtual circuit (PVC).
2.b.1.SWITCHED VIRTUAL CIRCUIT
The switched virtual circuit
(SVC) format is comparable conceptually to
dialup lines in circuit switching. In this method, a
virtual circuit is created whenever it is
needed and exists only for the duration of the
specific exchange. For example,
imagine that station A wants to send four packets
to station X. First, A requests the establishment of
a connection to X. Once the connection is in
place, the packets are sent one after another and in
sequential order.
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8. Fig. 7 Switched Virtual Circuit
When the last packet has been received and, if necessary, acknowledged, the connection is
released and that virtual circuit ceases to exit. Only one single route exists for the duration of
transmission, although the network could pick an alternate route in response to failure or
congestion. Each time that A wishes to communicate with X, a new route is established. The
route may be the same each time, or it may differ I response to varying network conditions.
2.b.2PERMANENT VIRTUAL CIRCUITS
Permanent, virtual circuits ​(PVC) are comparable to leased lines in circuit switching.
In this method, the same virtual circuit is provided between two users on a continuous basis. The
circuit is dedicated to the specific users. It can be used without connection establishment and
connection termination, whereas two SVC users may get a different route every time they
request a connection, two PVC users always get the same route.​Message switching ​is best
known by the descriptive term ​store and forward, ​a. In this mechanism, a node receives a
message, stores it until the appropriate route is free, then sends it along. Store and forward is
considered a switching technique because there is no direct link between the sender and receiver
of a transmission. A message is delivered to the node along one path then rerouted along another
to its destination.In message switching, the message are stored and relayed from secondary
storage (disk), while in packet switching the packets are stored and forwarded from primary
storage (RAM).
OPEN SYSTEM INTERCONNECTION MODEL OF OSI
The two important network architectures, the OSI reference model and the TCP/IP
reference model.
THE OSI REFERENCE MODEL
Established in 1947, The OSI model is based on a proposal developed by the
international standards organization as a first step towards international standardization of the
protocols used in the various layers, a. The model is called the OSI ​(open systems
interconnection) ​Reference Model because it deals with connecting open systems that are open
for communication with other systems. It was first introduced in the late 1970s. The OSI model
is not a protocol; it is a model for underlying and designing a network architecture that is
flexible, robust, and interoperable.
The OSI model has seven layers. The principles that were applied to arrive all the seven
layers are as follows:
A layer should be created where a different level of abstraction is needed.
Each layer should perform a well defined function.The function of each layer should be chosen
with an eye toward refining internationally standardized protocols.
The layer boundaries should be chosen to minimize the information flow across the interfaces.
The number of layers should be large enough that distinct functions need not be thrown together
in the same layer.
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9. LAYERED ARCHITECTURE​:
THE OSI model is built of seven ordered layers: physical (layer 1), data link
2). network (layer 3), transport (layer 4), session (layer 5), presentation
6). and application (layer 7).
ORGANIZATION OF THE LAYERS
The five layers can be thought of as belonging to three subgroups. Layers 1, 2, 3-
physical, data link, and network - are the network support layers; they deal with the physical
aspects of moving data from one device to another. Layer 5- application-can be thought of as the
user support layer; it allows interoperability among unrelated software systems.Layer 4, the
transport layer, links the two subgroups and ensures that what the lower layers have transmitted
is in a form that the upper layer can use.The process starts at layer 5 then moves from layer to
layer in descending, sequential order. At each layer, a header can be added to the data unit.
At layer 2, a trailer is added. When the formatted data unit passes through the physical layer, it
is changed into an electromagnetic signal and transported along a physical link.
Physical Layer:
Functions:
This Layer is responsible to carry a ​bit stream of data over the physical medium
Physical characteristics of interface and media​: It defines the interfaces and the type of
transmission media.
Representation of Bits​:It represents the type of encoding.ie) to be transmitted the bit stream is
converted in to electrical signals or optical signal.This is known as encoding
Data Rate​: It determines the number of bits transmitted per second
Synchronization of bits​: The sender and receiver clocks must be synchronized
Line configuration​: It determines the type of link. The types of link are point to point aqnd
multipoint link. Point to point is ther is dedicated link between two stations. In multipoint link
many devices share a particular link.
It also determines the ​physical topology and the type of Transmission media​.
Fig.8 physical topology
Data Link Layer:
Functions
Framing:​The data link divides the stream of bits in to smaller units called frames
Physical addressing:​The data Link Layer adds Header and trailer to the frame.The Header
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10. contains the source and Destination address.
Flow control​:It imposes flow control mechanish to mavoid the overwhelming at the receiver
side.
Errorcontrol​:Trailer added to the frame is meant for error control. This mechanism will allow
to detect and retransmit the damaged or lost frames
Access Control​:When two or more devices are connected to a same Link the protocols in the
data Link layer determine which device has control over the link
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11. Network Layer:
Network Layer is responsible for source to destination delivery of a packet
Functions:
Logical addressing​:If the packet passes to a network there should be source and destination
network address.The network layer adds header thethe packet coming from the upper layer.This
network addressing is known as Logical addressing.
Routing:
Routing is the process of connecting networks.The connection devices used are Routers.The
Routers possess Routing Table which contains the address of the destination and the Router
through which the packet can be sent
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12. Fig.9Data link layer
Transport Layer:
The Transport layer is responsible for process-to process delivery of the packet.
Functions:
Port addressing:
The computer actually runs several process at a time. To deliver the data to the correct
process a header is added to the packet from the above layer.This is known as port addressing.
Segmentation and reassembly:
The message is divided in to transmittable segments ans given a sequence number.According to
the sequence number the message is reassembled at the destination
Connection control​:
The transport layer can be either connectionless or connection oriented.In connection
oriented the transport layer makes a connection with the transport layer at the destination and
then send the packets.In connectionless control the packet is transmitted without establishing
connectionFlow and error control mechanisms are provided for reliable transmission.
Fig.10 network layer
Session Layer:
It establishes,maintains and synchronizes the interaction among communicating systems
Functions:
Dialog control​:It allows either half duplex or full duplex way of tansmission node.
Synchronization​:
The session layer adds check points in the data coming from the upper layer.The check
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13. points are provided at frequent intervals. This allows the retransmission of data from the
checkpoints when the data get damaged or lost
Presentation Layer:
The presentation layer deals with the syntax and semantics of the of the information
exchanged between the systems.
Function
Translation:
The information is converted into stream of bits before transmission in the presentation layer.
Encryption:
The sender transforms the original signal into another form for security purposes
Compression:
Data Compression is reducing the number of bits transmitted.
Application Layer:
The application Layer enables the user to access the internet
DATA LINK CONTROL PROTOCOL​:
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14. HDLC (HIGH-LEVEL DATA LINK CONTROL)
HDLC is a bit-oriented data link protocol designed to support both half-duplex and full-duplex
communication over point -to-point and multipoint links. Systems using HDLC can be
characterized by their station types, their configurations, and their response modes.
Station types of HDLC
HDLC differentiates between three types of stations: primary, secondary, and combined. A
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15. primary station ​is the device in either a point- to -point or multipoint line configuration that has
complete control of the link. The primary sends commands to the ​secondary ​stations. A primary
issues commands; a secondary issues responses.A​combined ​station can both command and
respond. A combined station is one of a set of connected peer devices programmed to behave
either as a primary or as a secondary.
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16. CONFIGURATIONSof data link layer.
Configuration is the relationship of hardware devices on a link. Primary, secondary, and
combined stations can be configured in three ways: unbalanced, symmetrical, and balanced.
An unbalanced configuration ​(master/slave configuration) is one in which one device
is primary and the others are secondary. Unbalanced configuration can be point-to-point if only
two devices are involved; more often they are multipoint, with one primary controlling several
secondary. ​A symmetrical configuration ​is one in which each physical station on ​a ​link
consists of two logical stations, one a primary and the other a secondary. It behaves like an
unbalanced configuration except that control of the link can shift betweenthe two stations.
A balanced configuration ​is one in which both stations in a point-to-point topology are
of the combined type. The stations are linked by a single line that can be controlled by either
station.
MODES OF COMMUNICATION in HDLC
A mode in HDLC is the relationship between two devices involved in an exchange the
mode describes who controls the link. Exchanges over unbalanced configurations are always
conducted in normal response mode. Exchanges over symmetrical or balanced configuration
can be set to a specific mode using a frame designed to deliver the command. HDLC supports
three modes of communication between stations: normal response mode (NRM),
asynchronous response mode (ARM), and asynchronous balanced mode (ABM).
NRM Normal response mode (NRM) ​refers to the standard primarysecondary
relationship. In this mode, a secondary device must have permission from the primary device
before transmitting. Once permission has been granted, the secondary may initiate a response
transmission of one or more frames containing data.
ARM ​In ​asynchronous response mode (ARM), ​a secondary may initiate a transmission
without permission from the primary whenever the channel is idle. ARM does not alter the
primary-secondary relationship in any other way. All transmission from a secondary must still
be made to the primary for relay to a final destination.
ABM ​In​asynchronous balanced mode (ABM), ​all stations are equal and therefore only
combined stations connected in point-to-point are used. Either combined station may initiate
transmission with the other combined station without permission.
FRAMES​of HDLC
HDLC defines three types of frames: information frames (I-frames), supervisory frames
(S-frames), and unnumbered frames (U - frames. Each type of frame works as an envelope for
the transmission of a different type of message. ​frames ​are used to transport user data and
control information relating to user data.​S-frames ​are used only to transport control
information, primarily data link layer flow and error controls.
MEDIA ACCESS PROTOCOL
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17. 17

18. The media access control (MAC) sublayer, resolves the contention for the shared media.
It contains the synchronization, flag, flow, and error control specifications necessary to move
information from one place to another, as well as the physical address of eth next station to
receive and route a packet. MAC protocols are specific to the LAN using them (Ethernet,
token bus, token ring).
ETHERNET
IEEE ​802.3 supports a LAN standard originally developed by Xerox and later extended
by a joint venture between Digital equipment corporation, intel corporation, and xerox. This was
called ​Ethernet.​IEEE802.3 defines two categories: baseband and broadband. The word base
specifies a digital signal. The word broad specifies an analog signal.IEEE divides the baseband
category into five different standards: 10Base5, 10Base2, lOBase-T, lBase5, and 100Base-T.
The first number (10, 1, or 100) indicates the data rate in Mbps. The last number or letter (5, 2,
1, or T) indicates the maximum cable length or the type of cable.
IEEE defines only one specification for the broadband category: 10 Broad36. Again, the
first number (10) indicates the data rate. The last number defines the maximum cable length.
The maximum cable length restriction can be changed using networking devices such as
repeaters or bridges.
TOKEN BUS in ​MAC
Local area networks have a direct application in factory automation and process control,
where the nodes are computers controlling the manufacturing process. In this type of
application, real-time processing with minimum delay is needed. Processing must occur at the
same speed as the objects moving along the assembly line.Ethernet is not a suitable protocol for
this purpose because the number of collisions is not predictable and the delay in sending data
from the control center to the computers along the assembly line is not a fixed value. Token ring
is also not a suitable protocol because an assembly line resembles a bus topology and not a
ring.Token bus combines features of ethernet and token ring. It combines the physical
configuration of ethernet and the collision-free feature of token ring. Token bus is a physical bus
that operates as a logical ring using tokens.
TOKEN RING in MAC layer
The network access mechanism used by ethemet (CSMA/CD) is not infallible and may
result in collision stations may attempt to send data multiple times before a transmission makes
it onto the link.This redundancy may create delay of indeterminable length if the traffic is
heavy. There is no way to predict either the occurrence of collisions or the delays produced by
multiple stations attempting to capture the link at the same time.
Token ring resolves this uncertainty by requiring that station take turns sending data. Each
station may transmit only during its turn and may send only one frame during each turn. The
mechanism that coordinates this rotation is called token passing. A token is a simple placeholder
frame that is passed from station to station around the ring .a station may send data only when it
has possession of the token.
COMMAND/RESPONSE
A primary station sends commands. A secondary station sends responses. A combined
station sends commands and responses. The configuration refers to the relationship of hardware
devices on a link. Primary, secondary, and combined stations can be configured in three ways:
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19. unbalanced, symmetrical, and balanced.
Fig.12 Functional diagram of Command/Response
Any of these configurations can support both half-duplex and full-duplex transmission.
An ​unbalanced configuration ​is one in which one device is primary and the others are
secondary. Unbalanced configurations can be point-to-point if only two devices are involved;
more often they are multipoint, with one primary controlling several secondary.
A ​symmetrical configuration ​is one in which each physical station on a link consists of
two logical stations, one a primary and the other a secondary. Separate lines link the primary
aspect of one physical aspect of one physical station to the secondary aspect of another physical
station. A symmetrical configuration behaves like an unbalanced configuration except that
control of the link can shift between the two stations.
A balanced configuration ​is one in which both stations in a point-to-point topology are
of the combined type. The stations are linked by a single line that can be controlled by either
station.
Token Passing method for connecting Links
Token passing method is purely for multipoint Link.
Multipoint Link is one in which ​many devices are connected to a particular Link. If many
Devices are connected to a particular link ​collision ​will occur. ie if many devices sent their
frames at a given time in a same link there is a possibility in which two frames can ​collide and
get ​damaged.
So inorder to avoid this Token Passing method is developed.
In this Token passing method only one stationcan access the medium at a given time. In
order to access the medium the particular station is given a Token.All the stations in a network
is organized as a Logical Ring. TheToken Circulates through the Ring. Those station which
posses the Token is given the right to access the channel.
Logical Ring of the stations connected in a Multipoint Link
Fig.13 Token connecting ling
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20. The token passes from one Logical station to another logical station.When a station
needs to send the data it will wait until it receives the Token from the station just before it.
When it receives the Token it will hold it and sends the data. When the station has no more to
send it will release the Token to another station just after it.
Token Management​:
Token can be possessed by the station only for a limited Time.
Priority can also given to higher priority stations such that Lower priority station can release the
Token to higher priority station
CSMA/CD
Carrier Sense Multiple Access/collision detection [CSMA/CD​] is a ​multiple access
protocol​when multiple devices are connected to a single link.
When multiple devices are connected to a single medium there is possibility of collision of
frames sent by the stations connected to that medium and get damaged.
CSMA/CD has an algorithm to handle Collision.
In CSMA/CD transmission and collision detection is a continuous and simultaneous .If a station
sends a frame it will continuously sense the channel for collision.
It will apply any of the persistant method to sense the channel.
The persistant methods available are
1-persistant method, Non-persistant method, p-persistant method
In this method the station needs to send a frame will continuously sense the channel. If
the channel is found to be idle it will send the frame immediately.
Non-persistent method​:
In this method the station will not continuously sense the channel.If the medium is busy
it will wait for a period of time and sense the channel after that.
p-persistent method​:
In this method the station will continuously sense the channel. If it is idle with a
probability p it will sends its frame.If the probability outcome is less than a particular limit p the
station can transmit. If the probability outcome is greater than a particular limit it will not send
the frame.it will wait for a time slot of time until the probability outcome is less than p and then
transmit.
Fig.14
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21. Continuously sense
Using the persistant method the medium is sensed inorder detect the two conditions
i)Either Transmission is finished or ii) collision is detected
If collision is not detected it means the transmission is complete and the entire frame is
transmitted.Otherwise collision has occurred, If collision has occurred the station will send a
jamming signal and aborts transmission of its frame
Fig .15 Figure Collision detection
In the above figure the station A sends the frame to station C at time t1 by sensing the
channel.After sometime station C sends a frame to station A at time t2.At time t2 the medium
seems to be idle since the frame send by the station A does not reaches the station C due to
propagation delay. Hence the station seems that the channel is idle and send the frame.
Therefore the two frames get collide at some instant.
Since it get collided the frames get damaged and reaches the station C at time t3.the
frame of station A reaches the station A at time t4.At time t3 the station A will detect the
collision and stops the transmission of itsframe.Same way once the damaged frame of A reaches
at time t4 it will detect the collision and stops transmission by sending a jamming signal.After
sending the jamming signal it will wait for a particular period of time T​B and then again sense
the channel by any one of the persistant method
TCP/IP protocol in NETWORK
If two devices need to communicate there should be set of rules to govern the
transmission. These rules are known as protocol.
This protocol is a layered framework that allows the communication between two stations.
The layers in the TCP/IP protocol are
1.Physical Layer
2.Data Link Layer
3.Network Layer
4.Transport Layer
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22. 5.Application Layer
The Application layer do the functions of the session layer and the presentation layer of OSI
layer.
Both sender and receiver has those five layers. These five layers are responsible for efficient
transmission of data to the receiver
TCP/IP contain some independent protocols. These protocols support the data transmission
Physical and Data Link Layer:
This Layer does not define any specific protocol. It supports all standards and any
protocol in the upper layer
Network Layer​:
The protocols in the internetworking layer are IP,ARP,RARP,ICMP,IGMP
Internetworking protocol (IP):
IP transports data in packets called DatagramsDatagrams travel along different paths and
arrive the destination out of sequence.So this is a unreliable Transmission
Address Resolution Protocol(ARP):
Address Resolution Protocol is used to find the a physical address of the station when
the network address is known
Reverse address Resolution Protocol:
This is the reverse process of ARP. This is used to find the network address when the
physical address is known
Internet control message protocol​:
It sends query and error reporting messages
Internet group Message protocol​:
It is used for the simultaneous transmission of data to group of recipients
Transport Layer​:
Transport Layer has two protocols
i)UDP(User datagram protocol):
ii)TCP(Trasmission Control Protocol)
User datagram Protocol (UDP)
It’s a process to process protocol that adds port address,checksum,error control and information
length of data
Transmission Control Protocol (TCP):
It divides the stream of bits into smaller units called segments.These segments are given
a sequence number together with an acknowledgement number.At the receiver side the data is
reordered according to the sequence no
Stream control Transmission Protocol (SCTP):
Itsupports newer applications
Application Layer​:
It is the combined section of session layer,Presentation Layer of OSI model.
Network Interface Layer
The Network Interface layer (also called the Network Access layer) is responsible for
placing TCP/IP packets on the network medium and receiving TCP/IP packets off the network
medium. TCP/IP was designed to be independent of the network access method, frame format,
and medium. In this way, TCP/IP can be used to connect differing network types. These include
22

23. LAN technologies such as Ethernet and Token Ring and WAN technologies such as X.25 and
Frame Relay. Independence from any specific network technology gives TCP/IP the ability to
be adapted to new technologies such as Asynchronous Transfer Mode (ATM).
The Network Interface layer encompasses the Data Link and Physical layers of the OSI
model. Note that the Internet layer does not take advantage of sequencing and acknowledgment
services that might be present in the Data-Link layer. An unreliable Network Interface layer is
assumed, and reliable communications through session establishment and the sequencing and
acknowledgment of packets is the responsibility of the Transport layer.
Fig .16 Layers of OSI
The​ ​Internet layer​ ​is responsible for addressing, packaging, and routing functions. The
core protocols of the Internet layer are IP, ARP, ICMP, and IGMP.
● The ​Internet Protocol​ (IP) is a routable protocol responsible for IP addressing, routing,
and the fragmentation and reassembly of packets.
● The ​Address Resolution Protocol​ (ARP) is responsible for the resolution of the Internet
layer address to the Network Interface layer address such as a hardware address.
● The ​Internet Control Message Protocol​ (ICMP) is responsible for providing diagnostic
functions and reporting errors due to the unsuccessful delivery of IP packets.
● The ​Internet Group Management Protocol​ (IGMP) is responsible for the management of
IP multicast groups.
Transport Layer
The ​Transport layer​ (also known as the Host-to-Host Transport layer) is responsible for
providing the Application layer with session and datagram communication services. The core
protocols of the Transport layer are ​Transmission Control Protocol​ (TCP) and the ​User
Datagram Protocol​ (UDP).
● TCP provides a one-to-one, connection-oriented, reliable communications service. TCP
is responsible for the establishment of a TCP connection, the sequencing and
acknowledgment of packets sent, and the recovery of packets lost during transmission.
● UDP provides a one-to-one or one-to-many, connectionless, unreliable communications
service. UDP is used when the amount of data to be transferred is small (such as the data
that would fit into a single packet), when the overhead of establishing a TCP connection
is not desired or when the applications or upper layer protocols provide reliable delivery.
Application Layer
23

24. The ​Application layer​ provides applications the ability to access the services of the other
layers and defines the protocols that applications use to exchange data. There are many
Application layer protocols and new protocols are always being developed.
The most widely-known Application layer protocols are those used for the exchange of user
information:
● The Hypertext Transfer Protocol (HTTP) is used to transfer files that make up the Web
pages of the World Wide Web.
● The File Transfer Protocol (FTP) is used for interactive file transfer.
● The Simple Mail Transfer Protocol (SMTP) is used for the transfer of mail messages and
attachments.
● Telnet, a terminal emulation protocol, is used for logging on remotely to network hosts
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