What is network? Connected Devices? what is network criteria in data communication?

 

Introduction to network ....

Computer network ?

What is network : 

A network is the interconnection of a set of devices capable of communication. 

• A device can be a host such as a 
• large computer, desktop, laptop, workstation, cellular phone, or security system.
• A device in this definition can also be a connecting device.

What are Connected Devices ?

1_Router which connects the network to other networks .

2_Switch: which connects devices together, 

3_modem (modulator-demodulator): which changes the form of data. 

These devices in a network are connected using 

• wired or wireless transmission media 
• such as cable or air.

what is network criteria in data communication?

What are the data communication criteria :

What is Network Criteria :

1__Performance: 

Performance can be measured in many ways, including transit time and response time.

What are the criteria to evaluate the performance of a network ?

1_Transit time :

 is the amount of time required for a message to travel from one device to another.

 2_Response time :

 is the elapsed time between an inquiry and a response. 

The performance of a network depends on a number of factors :

• number of users. 
• type of transmission medium .
• capabilities of the connected hardware .
• and the efficiency of the software.

Performance is often evaluated by two networking metrics:

 throughput and delay. 

We often need more throughput and less delay.

 If we try to send more data to the network, we may increase throughput but we increase the delay because of traffic congestion in the network. 

2__Reliability:

In addition to accuracy of delivery network reliability is measured by :

What are the criteria to evaluate the Reliability of a network ?

• the frequency of failure.
•  the time it takes a link to recover from a failure. 3_and the network’s robustness in a catastrophe. 

3__Security :

Network security issues include protecting data from

• unauthorized access.
• protecting data from damage and development.
• and implementing policies and procedures for recovery from breaches and data losses.

physical structure of computer network :

 physical structures in data communications :

What is the physical structure of computer network ?
 What are  network attributes ?

 What are the types of connection in the network ?
1___Type of Connection :
A network is two or more devices connected through links.
What is A link ?
 is a communications pathway that transfers data from one device to another.
-  For communication to occur :

• two devices must be connected in some way to the same link at the same time.
• There are two possible types of connections:

1_ point-to-point Connection 

2_ and multipoint Connection 

 what is difference between point to point and multipoint connection ?

What is a Point to Point Connection?

• A point-to-point connection provides a dedicated link between two devices.
• The entire capacity of the link is reserved for transmission between those two devices.
•  Most point-to-point connections use an actual length of wire or cable to connect the two ends, but other options, such as microwave or satellite links, are also possible .
• When we change television channels by infrared remote control,
•  we are establishing a point-to-point connection between the remote control and the television’s control system.

What is a Multipoint Connection ?

• A multipoint (also called multidrop) connection is one in which more than two specific devices share a single link .
• In a multipoint environment, the capacity of the channel is shared .
• either spatially or temporally.
• If several devices can use the link simultaneously,
• it is a spatially shared connection.
•  If users must take turns, it is a timeshared connection.

2___Physical Topology
types of physical topology : 

The term physical topology :
What meant by topology :

refers to the way in which a network is laid out physically.

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 (usually called nodes) to one another.

What are Types of network topology :There are four basic topologies possible:
1_mesh 

2_star

3_bus

4_and ring.

1_mesh :
What is Mesh Topology ? 

In a mesh topology :

 every device has a dedicated point-to-point link to every other device.
The term dedicated means that the link carries traffic only between the two devices it connects.
What is the example of mesh topology?

What is a full mesh topology?
Full mesh topology occurs when every node has a circuit connecting it to every other node in a network.
A fully connected mesh topology.

What are the advantages of a mesh network ( topology) ?
Advantages of mesh topology :
A mesh offers several advantages over other network topologies.

• First, the use of dedicated links guarantees that each connection can carry its own data load, thus eliminating the traffic problems that can occur when links must be shared by multiple devices.
• Second, a mesh topology is robust.
• If one link becomes unusable, it does not incapacitate the entire system.

• Third, there is the advantage of privacy or security.
• When every message travels along a dedicated line, only the intended recipient sees it.
• Physical boundaries prevent other users from gaining access to messages.

• 4__Finally, point-to-point links make fault identification and fault isolation easy.
•  Traffic can be routed to avoid links with suspected problems.
• This facility enables the network manager to discover the precise location of the fault and aids in finding its cause and solution.

What are the disadvantages of a mesh network ( topology) ?
disadvantages of mesh topology :
A mesh offers several disadvantages over other network topologies .
The main disadvantages of a mesh are :

• related to the amount of cabling and the number of I/O ports required.
•  First, because every device must be connected to every other device, installation and reconnection are difficult.
• Second, the sheer bulk of the wiring can be greater than the available space (in walls, ceilings, or floors) can accommodate.
•  Finally, the hardware required to connect each link (I/O ports and cable) can be prohibitively expensive.

What are mesh networks used for?
Why would you choose a mesh topology?
For these reasons a mesh topology is usually implemented in a limited fashion,
for example, as a backbone connecting the main computers of a hybrid network that can include several other topologies.
One practical example of a mesh topology is the connection of telephone regional offices in which each regional office needs to be connected to every other regional office.

2__Star Topology :
What is 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 one another. Unlike a mesh topology.
• a star topology does not allow direct traffic between devices.

How does star topology work?

• The controller acts as an exchange:
•  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 .

What is the difference between star topology and mesh topology ?
In the 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 one another. Unlike a mesh topology.
• a star topology does not allow direct traffic between devices.
• The controller acts as an exchange:
•  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 .
• A star topology is less expensive than a mesh topology.
•  In a star, each device needs only one link
• and one I/O port to connect it to any number of others.

In a mesh topology :

• every device has a dedicated point-to-point link to every other device.
• The term dedicated means that the link carries traffic only between the two devices it connects.

What is star topology advantages and disadvantages?
What are the advantages of a Star network ( topology) ?
Advantages of Star topology :

• easy to install and reconfigure.
• Far less cabling needs to be housed, and additions, moves, and deletions involve only one connection:
• between that device and the hub.
• Other advantages include 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 to monitor link problems and bypass defective links.

What are the disadvantages of a Star network ( topology) ?

disadvantages of Star topology :

• One big disadvantage of a star topology is the dependency of the whole topology on one single point, the hub.
• If the hub goes down, the whole system is dead.

Although a star requires far less cable than a mesh, each node must be linked to a central hub.
For this reason, often more cabling is required in a star than in some other topologies (such as ring or bus). 

The star topology is used in local-area networks (LANs).
High-speed LANs often use a star topology with a central hub.

3_ Bus Topology

What is Bus Topology ?

•  is multipoint. 
• One long cable acts as a backbone to link all the devices in a 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 a cable to create a contact with the metallic core.

 As a signal travels along the backbone, some of its energy is transformed into heat.
Therefore, it becomes weaker and weaker as it travels farther and farther.
For this reason there is a limit on the number of taps a bus can support and on the distance between those taps.

What are Bus topology advantages and disadvantages?
What are the advantages of a Bus network ( topology) ?
Advantages of Bus topology :
Advantages of a bus topology include

• ease of installation.
•  Backbone cable can be laid along the most efficient path, then connected to the nodes by drop lines of various lengths.

What is the difference between star topology , mesh topology and bus topology?

• In this way, a bus uses less cabling than mesh or star topologies.
•  In a star, for example, four network devices in the same room require four lengths of cable reaching all the way to the hub.
• In a bus, this redundancy is eliminated. Only the backbone cable stretches through the entire facility.
• Each drop line has to reach only as far as the nearest point on the backbone.

What are the Disadvantages of a Bus network ( topology) ?
Disadvantages of Bus topology :
Disadvantages include :

• difficult reconnection and fault isolation.
• A bus is usually designed to be optimally efficient at installation.
•  It can therefore be difficult to add new devices.
• Signal reflection at the taps can cause degradation in quality.
• This degradation can be controlled by limiting the number and spacing of devices connected to a given length of cable.
• Adding new devices may therefore require modification or replacement of the backbone.
• In addition, a fault or break in the bus cable stops all transmission, even between devices on the same side of the problem.
• The damaged area reflects signals back in the direction of origin, creating noise in both directions.

Note :
Bus topology was the one of the first topologies used in the design of early local area networks. Traditional Ethernet LANs can use a bus topology.

4_Ring Topology

What is Ring Topology ?
Ring Topology :
In a ring topology, :

each device has a dedicated point-to-point connection with only the two devices on either side of it.

How does a ring topology work?

• A signal is passed along the ring in one direction, from device to device, until it reaches its destination.
• Each device 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 .

What are Ring topology advantages and disadvantages?
What are the advantages of a Ring network ( topology) ?
Why do we use ring topology?
Advantages of Ring topology :

• A ring is relatively easy to install and reconfigure.
• Each device is linked to only its immediate neighbors (either physically or logically).
• To add or delete a device requires changing only two connections.
• fault isolation is simplified. 

in a ring a signal is circulating at all times.

 If one device does not receive a signal within a specified period, it can issue an alarm.
The alarm alerts the network operator to the problem and its location.

What are the Disadvantages of a Ring network ( topology) ?
Disadvantages of Ring topology :
Disadvantages include :

• unidirectional traffic can be a disadvantage.
• In a simple ring, a break in the ring (such as a disabled station) can disable the entire network.
• This weakness can be solved by using a dual ring or a switch capable of closing off the break.

Where do we use ring topology?
Ring topology was prevalent when IBM introduced its local-area network, Token Ring.
Today , the need for higher-speed LANs has made this topology less popular.

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