Some of the terms about Computer Networking with easy to understand definition.

1. Q.1 Write case study on wide area network?
“A wide area network (WAN) is a telecommunication network that covers a broad
area (i.e., any network that links across metropolitan, regional, or national boundaries).”
Business and government entities utilize WANs to relay data among employees, clients,
buyers, and suppliers from various geographical locations. In essence this mode of
telecommunication allows a business to effectively carry out its daily function regardless
of location.
This is in contrast with personal area networks (PANs), local area networks (LANs),
campus area networks (CANs), or metropolitan area networks (MANs) which are usually
limited to a room, building, campus or specific metropolitan area.
Design options
The textbook definition of a WAN is a computer network spanning regions,
countries, or even the world. However, in terms of the application of computer
networking protocols and concepts, it may be best to view WANs as computer
networking technologies used to transmit data over long distances, and between
different LANs, WANs and other localized computer networking architectures. This
distinction stems from the fact that common LAN technologies operating at Layer 1/2
(such as the forms of Ethernet or Wifi) are often geared towards physically localized
networks, and thus cannot transmit data over tens, hundreds or even thousands of miles
or kilometers.
WANs necessarily do not just connect physically disparate LANs. A CAN, for
example, may have a localised backbone of a WAN technology, which connects different

2. LANs within a campus. This could be to facilitate higher bandwidth applications, or
provide better functionality for users in the CAN.
WANs are used to connect LANs and other types of networks together, so that users
and computers in one location can communicate with users and computers in other
locations. Many WANs are built for one particular organization and are private. Others,
built by Internet service providers, provide connections from an organization's LAN to
the Internet. WANs are often built using leased lines. At each end of the leased line, a
router connects the LAN on one side with a second router within the LAN on the other.
Leased lines can be very expensive. Instead of using leased lines, WANs can also be built
using less costly circuit switching or packet switching methods. Network protocols
including TCP/IP deliver transport and addressing functions. Protocols including Packet
over SONET/SDH, MPLS, ATM and Frame relay are often used by service providers to
deliver the links that are used in WANs. X.25 was an important early WAN protocol, and
is often considered to be the "grandfather" of Frame Relay as many of the underlying
protocols and functions of X.25 are still in use today (with upgrades) by Frame Relay.
Connection technology options
Several options are available for WAN connectivity:
Sample
Bandwidth
Option: Description Advantages Disadvantages protocols
range
used
Point-to-Point connection between PPP, HDLC,
Leased
two computers or Local Area Most secure Expensive SDLC,
line
Networks (LANs) HNAS
Circuit A dedicated circuit path is created 28 - 144
Less Expensive Call Setup PPP, ISDN
switching between end points. kbit/s
Devices transport packets via a shared
single point-to-point or point-to-
multipoint link across a carrier X.25
Packet Shared media
internework. Variable length packets Frame-
switching across link
are transmitted over Permanent Relay
Virtual Circuits (PVC) or Switched
Virtual Circuits (SVC)
Similar to packet switching, but uses Best for
fixed length cells instead of variable simultaneous Overhead can
Cell relay ATM
length packets. Data is divided into use of voice and be considerable
fixed-length cells. data
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Q.2. Write a case study on STOP AND WAIT protocol?

3. Stop-and-wait is a method used in telecommunications to send information
between two connected devices. It ensures that information is not lost due to dropped
packets and that packets are received in the correct order.
“ In the stop-and-wait method of flow control, the sender sends one frame and waits
for an acknowledgement before sending the next frame.”
It is the simplest kind of automatic repeat-request (ARQ) method. A stop-and-wait
ARQ sender sends one frame at a time; it is a special case of the general sliding window
protocol with both transmit and receive window sizes equal to 1. After sending each
frame, the sender doesn't send any further frames until it receives an acknowledgement
(ACK) signal. After receiving a good frame, the receiver sends an ACK. If the ACK does
not reach the sender before a certain time, known as the timeout, the sender sends the
same frame again.
Typically the transmitter adds a redundancy check number to the end of each
frame. The receiver uses the redundancy check number to check for possible damage. If

4. the receiver sees that the frame is good, it sends an ACK. If the receiver sees that the
frame is damaged, the receiver discards it and does not send an ACK -- pretending that
the frame was completely lost, not merely damaged.
One problem is where the ACK sent by the receiver is damaged or lost. In this case,
the sender doesn't receive the ACK, times out, and sends the frame again. Now the
receiver has two copies of the same frame, and doesn't know if the second one is a
duplicate frame or the next frame of the sequence carrying identical data.
Another problem is when the transmission medium has such a long latency that
the sender's timeout runs out before the frame reaches the receiver. In this case the
sender resends the same packet. Eventually the receiver gets two copies of the same
frame, and sends an ACK for each one. The sender, waiting for a single ACK, receives two
ACKs, which may cause problems if it assumes that the second ACK is for the next frame
in the sequence.
To avoid these problems, the most common solution is to define a 1 bit sequence
number in the header of the frame. This sequence number alternates (from 0 to 1) in
subsequent frames. When the receiver sends an ACK, it includes the sequence number of
the next packet it expects. This way, the receiver can detect duplicated frames by
checking if the frame sequence numbers alternate. If two subsequent frames have the
same sequence number, they are duplicates, and the second frame is discarded.
Similarly, if two subsequent ACKs reference the same sequence number, they are
acknowledging the same frame.
Stop-and-wait ARQ is inefficient compared to other ARQs, because the time
between packets, if the ACK and the data are received successfully, is twice the transit
time (assuming the turnaround time can be zero). The throughput on the channel is a
fraction of what it could be. To solve this problem, one can send more than one packet
at a time with a larger sequence number and use one ACK for a set. This is what is done
in Go-Back-N ARQ and the Selective Repeat ARQ.
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5. Q.3. Write a case study on sliding window protocol?
In the sliding window method of flow control, the sender can transmit several
frames before needing an acknowledgement. Fames can be sent one after another,
meaning that the link can carry several frames at once and its capacity can be used
efficiently. The receiver acknowledges only some of the frames, using a single ack to
confirm the receipt of multiple data frames.
“ In the Sliding window method of flow control, several frames can be transit at a
time.”
A sliding window protocol is a feature of packet-based data transmission protocols.
Sliding window protocols are used where reliable in-order delivery of packets is required,
such as in the Data Link Layer (OSI model) as well as in the Transmission Control Protocol
(TCP).
Conceptually, each portion of the transmission (packets in most data link layers,
but bytes in TCP) is assigned a unique consecutive sequence number, and the receiver
uses the numbers to place received packets in the correct order, discarding duplicate
packets and identifying missing ones. The problem with this is that there is no limit of
the size of the sequence numbers that can be required.
By placing limits on the number of packets that can be transmitted or received at
any given time, a sliding window protocol allows an unlimited number of packets to be
communicated using fixed-size sequence numbers.

6. For the highest possible throughput, it is important that the transmitter is not
forced to stop sending by the sliding window protocol earlier than one round-trip delay
time (RTT). The limit on the amount of data that it can send before stopping to wait for
an acknowledgment should be larger than the bandwidth-delay product of
communications link. If it is not, the protocol will limit the effective bandwidth of the
link.
Conceptually, the sliding window of the sender shrinks from the left when frames
of data are sent. The sliding window of the sender expands to the right when
acknowledgements are received.
Go-Back-N
Go-Back-N ARQ is the sliding window protocol with wt>1, but a fixed wr=1. The
receiver refuses to accept any packet but the next one in sequence. If a packet is lost in
transit, following packets are ignored until the missing packet is retransmitted, a
minimum loss of one round trip time. For this reason, it is inefficient on links that suffer
frequent packet loss.
Selective repeat
The most general case of the sliding window protocol is Selective Repeat ARQ. This
requires a much more capable receiver, which can accept packets with sequence
numbers higher than the current nr and store them until the gap is filled in.
The advantage, however, is that it is not necessary to discard following correct data
for one round-trip time before the transmitter can be informed that a retransmission is
required. This is therefore preferred for links with low reliability and/or a high
bandwidth-delay product.
The window size wr need only be larger than the number of consecutive lost
packets that can be tolerated. Thus, small values are popular; wr=2 is common.
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7. Q.4. Write a brief note on IEEE 802.3, 802.4, 802.5?
Ethernet: IEEE 802.3
IEEE 802.3 supports a LAN standard originally developed by Xerox and later
extended by a joint venture between Digital Equipment Corporation and Xerox. This was
called “Ethernet.”
Ethernet protocols refer to the family of local-area network (LAN) covered by the
IEEE 802.3. In the Ethernet standard, there are two modes of operation: half-duplex and
full-duplex modes. In the half duplex mode, data are transmitted using the popular
Carrier-Sense Multiple Access/Collision Detection (CSMA/CD) protocol on a shared
medium. The main disadvantages of the half-duplex are the efficiency and distance
limitation, in which the link distance is limited by the minimum MAC frame size.
Therefore, the carrier extension technique is used to ensure the minimum frame size of
512 bytes in Gigabit Ethernet to achieve a reasonable link distance.
Four data rates are currently defined for operation over optical fiber and twisted-
pair cables:
• 10 Mbps - 10Base-T Ethernet (IEEE 802.3)
• 100 Mbps - Fast Ethernet (IEEE 802.3u)
• 1000 Mbps - Gigabit Ethernet (IEEE 802.3z)
• 10-Gigabit - 10 Gbps Ethernet (IEEE 802.3ae).
The Ethernet system consists of three basic elements: 1. the physical medium used
to carry Ethernet signals between computers, 2. a set of medium access control rules
embedded in each Ethernet interface, and 3. an Ethernet frame that consists of a
standardized set of bits used to carry data over the system.

8. As with all IEEE 802 protocols, the ISO data link layer is divided into two IEEE 802
sublayers, the Media Access Control (MAC) sub layer and the MAC-client sublayer. The
IEEE 802.3 physical layer corresponds to the ISO physical layer.
The MAC sub-layer has two primary responsibilities:
• Data encapsulation, including frame assembly before transmission, and frame
parsing/error detection during and after reception
• Media access control, including initiation of frame transmission and recovery
from transmission failure
The MAC-client sub-layer may be one of the following:
• Logical Link Control (LLC), which provides the interface between the Ethernet
MAC and the upper layers in the protocol stack of the end station. The LLC
sublayer is defined by IEEE 802.2 standards.
Access to the shared channel is determined by the medium access control (MAC)
mechanism embedded in the Ethernet interface located in each station. The medium
access control mechanism is based on a system called Carrier Sense Multiple Access with
Collision Detection (CSMA/CD).
Protocol Structure - Ethernet: IEEE 802.3 Local Area Network protocolsThe
basic IEEE 802.3 Ethernet MAC Data Frame for 10/100Mbps Ethernet:
7 1 6 6 2 46-1500bytes 4
Pre SFD DA SA Length Type Data unit + pad FCS
• Preamble (PRE) - 7 bytes. The PRE is an alternating pattern of ones and zeros
that tells receiving stations that a frame is coming, and that provides a means to
synchronize the frame-reception portions of receiving physical layers with the
incoming bit stream.
• Start-of-frame delimiter (SFD) - 1 byte. The SOF is an alternating pattern of ones
and zeros, ending with two consecutive 1-bits indicating that the next bit is the
left-most bit in the left-most byte of the destination address.
• Destination address (DA) - 6 bytes. The DA field identifies which station(s) should
receive the frame..
• Source addresses (SA) - 6 bytes. The SA field identifies the sending station.
• Length/Type- 2 bytes. This field indicates either the number of MAC-client data
bytes that are contained in the data field of the frame, or the frame type ID if the
frame is assembled using an optional format.

9. • Data- Is a sequence of n bytes (46=< n =<1500) of any value. (The total frame
minimum is 64bytes.)
• Frame check sequence (FCS) - 4 bytes. This sequence contains a 32-bit cyclic
redundancy check (CRC) value, which is created by the sending MAC and is
recalculated by the receiving MAC to check for damaged frames.
Token bus: IEEE 802.4
Local area network have a direct application in factory automation and process
control, where the nods are computers controlling the manufacturing process .in this
type of application, real time processing with minimum delay is needed. Processing must
be occur at the same speed as the objects moving along the assembly line. Ethernet
(IEEE 802.3) 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 resembles a bus topology and not a ring
Token bus combine feature of Ethernet and token ring. It combines the physical
configuration of Ethernet and the collision free feature of the token ring. Token bus is a
physical bus that operates as a logical ring using tokens. Token bus is limited to factory
automation and process control and has no commercial application in data
communication.
Token bus was standardized by IEEE standard 802.4. It is mainly used for industrial
applications. Token bus was used by GM (General Motors) for their Manufacturing
Automation Protocol (MAP) standardization effort. This is an application of the concepts
used in token ring networks. The main difference is that the endpoints of the bus do not
meet to form a physical ring. The IEEE 802.4 Working Group is disbanded (FMS).

10. Token Ring: IEEE 802.5
Token Ring as defined in IEEE 802.5 is originated from the IBM Token Ring LAN
technologies. Both are based on the Token Passing technologies. While them differ in
minor ways but generally compatible with each other.
“ Token ring allow each station to send one frame per turn.”
Token-passing networks move a small frame, called a token, around the
network. Possession of the token grants the right to transmit. If a node receiving the
token has no information to send, it seizes the token, alters 1 bit of the token (which
turns the token into a start-of-frame sequence), appends the information that it wants
to transmit, and sends this information to the next station on the ring. While the
information frame is circling the ring, no token is on the network, which means that
other stations wanting to transmit must wait. Therefore, collisions cannot occur in Token
Ring networks.

11. The information frame circulates the ring until it reaches the intended destination
station, which copies the information for further processing. The information frame
continues to circle the ring and is finally removed when it reaches the sending station.
The sending station can check the returning frame to see whether the frame was seen
and subsequently copied by the destination.
Unlike Ethernet CSMA/CD networks, token-passing networks are deterministic,
which means that it is possible to calculate the maximum time that will pass before any
end station will be capable of transmitting. This feature and several reliability features
make Token Ring networks ideal for applications in which delay must be predictable and
robust network operation is important.
Protocol Structure - Token Ring: IEEE 802.5 LAN Protocol
1 2 3 9 15bytes
SDEL AC FC Destination address Source address
Route information 0-30 bytes
Information (LLC or MAC) variable
FCS (4 bytes) EDEL (1) FS(1)
• SDEL / EDEL - Starting Delimiter / Ending Delimiter. Both the SDEL and EDEL have
intentional Manchester code violations in certain bit positions so that the start
and end of a frame can never be accidentally recognized in the middle of other
data.
• AC - Access control field contains the Priority fields.

12. • FC - Frame control field indicates whether the frame contains data or control
information
• Destination address - Destination station address.
• Source address - Source station address.
• Route information - The field with routing control, route descriptor and routing
type information.
• Information - The Information field may be LLC or MAC.
• FCS - Frame check sequence.
• Frame status - Contains bits that may be set on by the recipient of the frame to
signal recognition of the address and whether the frame was successfully copied.
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Q.5. Explain FDDI in detail?
Fiber Distributed Data Interface (FDDI)
FDDI is a set of ANSI protocols for sending digital data over fiber optic cable. FDDI
networks are token-passing (similar to IEEE 802.5 Token Ring protocol) and dual-ring
networks, and support data rates of up to 100 Mbps. FDDI networks are typically used as
backbones technology because of its support for high bandwidth and great distance. A
related copper specification similar to FDDI protocols, called Copper Distributed
Data Interface (CDDI), has also been defined to provide 100-Mbps service over
twisted-pair copper.
An extension to FDDI, called FDDI-2, supports the transmission of voice and video
information as well as data. Another variation of FDDI called FDDI Full Duplex
Technology (FFDT) uses the same network infrastructure but can potentially support
data rates up to 200 Mbps.
FDDI uses dual-ring architecture with traffic on each ring flowing in opposite
directions (called counter-rotating). The dual rings consist of a primary and a secondary
ring. During normal operation, the primary ring is used for data transmission, and the
secondary ring remains idle, the primary purpose of the dual rings is to provide superior
reliability and robustness.

13. FDDI's four specifications are the Media Access Control (MAC), Physical Layer
Protocol (PHY), Physical-Medium Dependent (PMD), and Station Management (SMT)
specifications. The MAC specification defines how the medium is accessed, including
frame format, token handling, addressing, algorithms for calculating cyclic redundancy
check (CRC) value, and error-recovery mechanisms. The PHY specification defines data
encoding/decoding procedures, clocking requirements, and framing, among other
functions. The PMD specification defines the characteristics of the transmission medium,
including fiber-optic links, power levels, bit-error rates, optical components, and
connectors. The SMT specification defines FDDI station configuration, ring configuration,
and ring control features, including station insertion and removal, initialization, fault
isolation and recovery, scheduling, and statistics collection.

14. Protocol Structure - FDDI: Fiber Distributed Data Interface
2 6 6 0-30 Variable 4bytes
Frame Destination Source Route
Information FCS
control address address information
Frame control - The frame control structure is as follows:
C L F F Z Z Z Z
• C - Class bit: 0 Asynchronous frame; 1 Synchronous frame/
• L - Address length bit: 0 16 bits (never); 1 48 bits (always).
• FF - Format bits.
• ZZZZ - Control bits.
Destination address - The address structure is as follows:

15. I/G U/L Address bits
Source address - The address structure is as follows:
I/G RII Address bits
• I/G - Individual/group address: 0 Group address; 1 Individual address.
• RII - Routing information indicator: 0 RI absent; 1 RI present.
Route Information - The structure of the route information is as follows:
3 5 1 6 1 16 16 16
RT LTH D LF R RD1 RD2 RDn
• RC - Routing control (16 bits).
• RDn - Route descriptor (16 bits).
• RT - Routing type (3 bits).
• LTH - Length (5 bits).
• D - Direction bit (1 bit).
• LF - Largest frame (6 bits).
• R - reserved (1 bit).
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Q.6. Explain various Network Topologies?
TOPOLOGY

16. The term topology refers a way a network is laid out, either physically or logically.
Two or more devices connect to a link; two or more link forms a topology. There are
basic five topologies are possible
• Mesh topology
• Star Topology
• Tree Topology
• Bus Topology
• Ring Topology
Mesh
“In a Mesh Topology, every device has a dedicated point to point link to every other
device.”
The dedicated means that the link carries traffic only between the two devices it
connects.
A fully connected mesh network therefore has n (n-1)/2 physical channels to link n
devices. To accommodate that many links, every device on the network must have n-1
input/output ports.
Advantages:-
• The use of dedicated links guarantees that each connection can carry its own
data load.
• A mesh topology is robust.
• It also provide the privacy and security of the network.
Disadvantages:-
• The main disadvantages of mesh are related to the amount of cabling and the
number of I/O ports required.

17. Star
“ In a star topology, each device has a dedicated point to point link only to a
central controller, usually called a hub”
HUB
The devices are not directly connected to each other. Unlike a mesh topology, a
star topology does not allow direct traffic between two devices.
The controller acts as an exchange: if one device wants to send data to another, it
sends data to the controller, which then relays the data to the other connected devices.
A star topology is less expansive than a mesh topology. In a star, each device needs
only one link and one I/O post to connect it to any number of others. It includes
robustness.
HUB
Advantages:-
• A star topology is less expansive than a mesh topology. In a star, each device
needs only one link and one I/O post to connect it to any number of others.
• It includes robustness. If one link fails, only that link is affected. All other links
remain active. This factor also lends itself to easy fault identification and fault
isolation. As long as the hub is working. It can be used for monitor link problems
and bypass defective lines.
• It is easy to install and configure.
Disadvantages:-
• If the central controller hub fails. Then the network gets destroyed.
Tree

18. “A tree topology is a variation of star topology. As in a star, nodes in a tree are
linked to a central hub that controls the traffic to the network.”
HUB
However, not every device plugs directly into the central hub.
The central hub in a tree is an active hub. An active hub contains a repeater, which is
a hardware device that regenerates the received bit pattern before sending them out.
Advantages:-
• It allows more devices to be attached to a single central hub and can therefore
increase the distance a signal can travel between those devices.
• It allows the network to isolate and prioritize communications from different
computers.
A good example of tree topology can be seen in cable TV technology where the
main cable from the main office is divided into main branches and each branch is divided
into smaller branches and so on. The hubs are used when the cable are divided.
As in a star, nodes in a tree are linked to a central hub that controls the traffic to the
network. However, not every device plugs directly into the central hub.
The central hub in a tree is an active hub. An active hub contains a repeater, which is
a hardware device that regenerates the received bit pattern before sending them out.
Bus

19. “A bus topology, on the other hand, is multipoint. One long cable act 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 main cable or punctures the sheathing of the cable to create a
contact with the metallic core.
Advantages:-
• A bus topology includes easy of installation. Backbone cable can be laid along the
most efficient path, and then connected to the nodes by drop lines.
• A bus uses less cabling then of other topologies.
Disadvantages:-
• It includes difficult reconfiguration and fault isolation.
• A fault or break in the bus cable stops all the transmission.

20. Ring
“In a ring topology, each device has a dedicated point to point line configuration
only with the two devices on either side it.”
A signal is passed along the ring in one direction from device to device, until it
reaches its reaches its destination..
Each device in a ring incorporates a repeater. When a device receives a signal
intended for another device, its repeater regenerates the bit and passes them along.
A ring is relatively easy to install and reconfiguration.
Advantages:-
• A ring is relatively easy to install and reconfigure. Each device is linked only to its
immediate neighbors.
• Fault isolation is also simplified. Generally in a ring, a signal is circulating at all
times.
Disadvantages:-
• Unidirectional traffic is a disadvantage of ring topology.
• A break in the ring can disable the entire network.
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21. Q.7. Establishing and studying the various parameters of a home
LAN network?
Installing a Wireless Router
One wireless router supports one WLAN. Use a wireless router on your network if:
• you are building your first home network, or
• you want to re-build your home network to be all-wireless, or
• you want to keep your WLAN installation as simple as possible
Try to install your wireless router in a central location within the home. The way Wi-
Fi networking works, computers closer to the router (generally in the same room or in
"line of sight") realize better network speed than computers further away.
Connect the wireless router to a power outlet and optionally to a source of Internet
connectivity. All wireless routers support broadband modems, and some support phone
line connections to dial-up Internet service. If you need dial-up support, be sure to
purchase a router having an RS-232 serial port. Finally, because wireless routers contain
a built-in access point, you're also free to connect a wired router, switch, or hub.
Next, choose your network name. In Wi-Fi networking, the network name is often
called the SSID. Your router and all computers on the WLAN must share the same SSID.
Although your router shipped with a default name set by the manufacturer, it's best to
change it for security reasons. Consult product documentation to find the network name
for your particular wireless router, and follow this general advice for setting your SSID.
Last, follow the router documentation to enable WEP security, turn on firewall
features, and set any other recommended parameters.
Installing a Wireless Access Point
One wireless access point supports one WLAN. Use a wireless access point on your home
network if:
• you don't need the extra features a wireless router provides AND
• you are extending an existing wired Ethernet home network, or
• you have (or plan to have) four or more wireless computers scattered throughout
the home
Install your access point in a central location, if possible. Connect power and a dial-
up Internet connection, if desired. Also cable the access point to your LAN router, switch
or hub. See the diagram in the Page 3 sidebar for details.
We won't have a firewall to configure, of course, but we still must set a network
name and enable WEP on your access point at this stage.

22. Configuring the Wireless Adapters
Configure your adapters after setting up the wireless router or access point (if you
have one). Insert the adapters into your computers as explained in your product
documentation. Wi-Fi adapters require TCP/IP be installed on the host computer.
Manufacturers each provide configuration utilities for their adapters. On the
Windows operating system, for example, adapters generally have their own graphic user
interface (GUI) accessible from the Start Menu or taskbar after the hardware is installed.
Here's where you set the network name (SSID) and turn on WEP. You can also set a few
other parameters as described in the next section. Remember, all of your wireless
adapters must use the same parameter settings for your WLAN to function properly.
Configuring an Ad-Hoc Home WLAN
Every Wi-Fi adapter requires you to choose between infrastructure mode (called
"access point" mode in some configuration tools) and ad-hoc ("peer to peer") mode.
When using a wireless access point or router, set every wireless adapter for
infrastructure mode. In this mode, wireless adapters automatically detect and set their
WLAN channel number to match the access point (router).
Alternatively, set all wireless adapters to use ad hoc mode. When you enable this
mode, you'll see a separate setting for channel number. All adapters on your ad hoc
wireless LAN need matching channel numbers.
Ad-hoc home WLAN configurations work fine in homes with only a few computers
situated fairly close to each other. You can also use this configuration as a fallback
option if your access point or router breaks:
Configuring Software Internet Connection Sharing
As shown in the diagram, you can share an Internet connection across an ad hoc
wireless network. To do this, designate one of your computers as the host (effectively a
substitute for a router). That computer will keep the modem connection and must
obviously be powered on whenever the network is in use. Microsoft Windows offers a
feature called Internet Connection Sharing (ICS) that works with ad hoc WLANs.
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Q.8. Explain Routers, Bridges, and Switches and gateways?

23. Routers
Network router is a device or a piece of software in a computer that forwards and
routes data packets along networks. A network router connects at least two networks,
commonly two LANs or WANs or a LAN and its ISP network. A router is often included as
part of a network switch. A router is located at any where one network meets another,
including each point-of-presence on the Internet. A router has two key jobs:
• The router ensures that information doesn't go where it's not needed. This is
crucial for keeping large volumes of data from clogging the network.
• The router makes sure that information does make it to the intended destination.
In performing these two jobs, a router joins the two networks, passing information
from one to the other and, in some cases, performing translations of various protocols
between the two networks. It also protects the networks from one another, preventing
the traffic on one from unnecessarily spilling over to the other. This process is known as
routing.
Routing is a function associated with the Network layer (layer 3) in the Open
Systems Interconnection (OSI) model. Routers use network layer protocol headers, such
as IP header where the source and destination addresses are included and routing tables
to determine the best path to forward the packets. For the communication among
routers and decide the best route between any two hosts, routing protocols such as
ICMP are used.
Actually, routers are specialized computers that send messages speeding to their
destinations along thousands of possible pathways. One of the tools a router uses to
decide which path a packet should go is a routing table. A routing table contains a
collection of information, including:
• Information on which connections lead to particular groups of addresses
• Priorities for connections to be used
• Rules for handling both routine and special cases of traffic
Information in the routing tables can be static (with routes manually entered by
the network administrator) or dynamic (where routers communicate to exchange
connection and route information using various routing protocols).
A routing table can be as simple as a few lines in the smallest routers, but can
grow to massive size and complexity in the very large routers that handle the bulk of
Internet messages.
As the number of networks attached to one another grows, the routing table for
handling traffic among them grows, and the processing power of the router is increased.

24. Packets routed by routers to their destinations
Bridges
Bridges operate in both the physical and data link layer of the OSI modal. Bridges
can divide a large network into smaller segments. They can also relay frames between
two originally separate LAN’s.
Bridges contain logic that allows them to keep the traffic for each segment
separate. In this way, they filter traffic, a fact that makes them useful for controlling
congestion and isolating problem links. Bridges can also provide security through this
partitioning of traffic.
When a frame enters a bridge, the bridge not only regenerates the signal but checks
the address of the destination and forwards the new copy only to the segment to which
the address belongs. As a bridge encounters a packet, it reads the address contained in

25. the frame and compares that address with a table of all the stations on both segments.
When it finds a match, it discovers to which segment the station belongs and relays the
packet only to that segment.
Types of Bridge
To select between segments, a bridge must have a look up table that contains the
physical address of every station connected to it. The table indicates to which the
segment each station belongs.
Simple Bridge
Simple bridges are the most primitive and least expansive type of bridge. A simple
bridge links two segments and contains a table that lists the addresses of all the stations
included in each of them. What makes it primitive is that these addresses must be
entered manually. Before a simple bridge can be used, an operator must sit down and
enter the addresses of every station. Whenever a new station is added, the table must
be modified. If a station is removed, the newly invalid address must be deleted.
Multiport bridge
A multiport bridge can be used to connect more than two LAN’s. The bridge has
three tables, each one holding the physical addresses of stations reachable through the
corresponding port.
Transport Bridge
A transparent, or learning, bridge builds its table of station addresses on its own
as it performs its bridge function. When the transparent bridge is first installed, its table
is empty. As it encounters each packet, it looks a both the destination and the source
addresses. It checks the destination to decide where to send the packet. If it is not
recognize the destination address, it relays the packet to all of the stations on both
segments. It uses the source address to build its table. As it reads the source address, it
notes which side the packet came from and associates that addresses with the segment
to which it belongs.
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Switch

26. “Switches are hardware or software devices which are capable of creating
temporary connection between two or more devices linked to the switch not to the
each other.”
Switch is a network exchange facility operating at the data link layer (layer 2) and
sometimes the network layer (layer 3) of the OSI Reference Model. Classified by working
protocols, there are two-layer switch, three-layer switch, four-layer switch and multiple-
layer switch. Switch also can be classified into managed switch and unmanaged switch.
Generally, three-layer switch and above has management function (managed switch).
Unlike hubs, switches prevent promiscuous sniffing. In a switched network
environment, Javvin Packet Analyzer (or any other packet analyzer) is limited to
capturing broadcast and multicast packets and the traffic sent or received by the PC on
which it is running.
However, most modern switches (management switches) support "port
mirroring", which is a feature that allows you to configure the switch to redirect the
traffic that occurs on some or all ports to a designated monitoring port on the switch.
With this feature, you can monitor the entire LAN segment in switched network
environment. Please refer to the documentation coming with your switch for the
availability information about this feature and configuration instructions.
If your switch dose not support "port mirroring", you can install Javvin Packet
Analyzer on a workstation connected to the same hub as your Internet gateway, or on

27. your Internet gateway (if acceptable), thus you can monitor all network traffic between
your intranet and the Internet.
Configuring a switch
Javvin Packet Analyzer should be installed on the host/server connected with the
switch’s mirror port (span port).
Mirror port configuration:
• Mirror the way out port to the management port (mirror port), in this way the
entire data transmitted into/out of LAN can be monitored.
• Mirror all way out ports to the management port (mirror port), in this way not
only the entire data transmitted into/out of LAN but also the communication
among hosts in LAN can be monitored. (Recommend)
The following are two examples for CISCO switch using the "monitor" command in
configuration mode:
Format:
#monitor session number source interface mod_number/port_number
#monitor session number destination interface mod_number/port_number
In electronics, a switch is an electrical component that can break an electrical circuit,
interrupting the current or diverting it from one conductor to another.
CROSS-BAR SWITCH

28. A switch may be directly manipulated by a human as a control signal to a system,
such as a computer keyboard button, or to control power flow in a circuit, such as a light
switch. Automatically operated switches can be used to control the motions of
machines, for example, to indicate that a garage door has reached its full open position
or that a machine tool is in a position to accept another work piece. Switches may be
operated by process variables such as pressure, temperature, flow, current, voltage, and
force, acting as sensors in a process and used to automatically control a system. For
example, a thermostat is a temperature-operated switch used to control a heating
process. A switch that is operated by another electrical circuit is called a relay. Large
switches may be remotely operated by a motor drive mechanism. Some switches are
used to isolate electric power from a system.
Gateway
In telecommunications, the term gateway has the following meaning:
• In a communications network, a network node equipped for interfacing with
another network that uses different protocols.
o A gateway may contain devices such as protocol translators, impedance
matching devices, rate converters, fault isolators, or signal translators as
necessary to provide system interoperability. It also requires the
establishment of mutually acceptable administrative procedures between
both networks.
o A protocol translation/mapping gateway interconnects networks with
different network protocol technologies by performing the required
protocol conversions.
• Loosely, a computer or computer program configured to perform the tasks of a
gateway. For a specific case, see default gateway.
“ Gateways, also called protocol converters, can operate at any network layer. The
activities of a gateway are more complex than that of the router or switch as it
communicates using more than one protocol.”

29. A gateway is a network point that acts as an entrance to another network. On the
Internet, a node or stopping point node or a host (end-point) node. Both the computers
of Internet users and the computers that serve pages to users are host nodes, while the
nodes that connect the networks in between are gateways. For example, the computers
that control traffic between company networks or the computers used by internet
service providers (ISPs) to connect users to the internet are gateway nodes.
In the network for an enterprise, a computer server acting as a gateway node is
often also acting as a proxy server and a firewall server. A gateway is often associated
with both a router, which knows where to direct a given packet of data that arrives at
the gateway, and a switch, which furnishes the actual path in and out of the gateway for
a given packet.
Internet-to-Orbit Gateway

30. An Internet to orbit gateway (I2O) is a machine that acts as a connector between
computers or devices connected to the Internet and computer systems orbiting the
earth, like satellites or even manned spacecrafts. Such connection is made when the I2O
establishes a stable link between the spacecraft and a computer or a network of
computers on the Internet, such link can be control signals, audio frequency, or even
visible spectrum signals.
Project HERMES is the first project to have brought this kind of machine into
operation. The HERMES-A/MINOTAUR Space Flight Control Center became operative on
June 6, 2009 and was operated by representatives of 34 countries on the UNOOSA
Symposium of Small Satellites for Sustainable Development in Graz, Austria on
September 10, 2009. Project HERMES is an initiative of the Ecuadorian Civilian Space
Agency and has a maximum coverage of 22,000 km, HERMES-A is supposed to be the
first gateway of a network of five covering all South America. HERMES-A/MINOTAUR is
not only capable of data transmission but voice also.
Project GENSO is an initiative from NASA and ESA, and it is expected to begin
operations on April 2010, it is supposed to have worldwide coverage.
Cloud Gateway
A Cloud storage gateway is a network appliance or server which resides at the
customer premises and translates cloud storage APIs such as SOAP or REST to block-
based storage protocols such as iSCSI or Fiber Channel or file-based interfaces such as
NFS or CIFS. Cloud storage gateways enable companies to integrate cloud storage into
applications without moving the applications into the cloud. In addition they simplify
data protection.
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