Coaxial cable

Coaxial cable:-With ``coax'', the medium consists of a copper core surrounded by insulating material and a braided outer conductor.Coaxial cable carries signals of higher frequency ranges than those in twisted-pair cable.
      Coax has a central core conductor of solid or stranded wire (usually copper) enclosed in an insulating sheath, which is, in turn, encased in an outer conductor of metal foil, braid, or a combination of the two.The outer metallic wrapping serves both as a shield against noise and as the second conductor, which completes the circuit. This outer conductor is also enclosed in an insulating sheath, and the whole cable is
protected by a plastic cover.

Characteristics:-
*)Co-axial cable has superior frequency characteristics compared to twisted-pair and can be used for both analog and digital signaling.
*)Coaxial cables are used both for base band and broadband communication.
*)This cable offers bandwidths of 300 to 400 MHz facilitating high-speed data communication with low        bit-error rate.
*) In broadband signaling, signal propagates only in one direction, in contrast to propagation in both directions in base band signaling.

Applications:-
*)Coaxial cable was widely used in analog telephone networks where a single coaxial  network  could carry 10,000 voice signals.
*)the most popular use of co-axial cable is in cable TV (CATV) for the distribution of TV signals.
*)coaxial cable is used in traditional Ethernet LAN s.

                       

Data flow

Data flow--
The communication between two devices is:-
1)Simplex:-In simplex mode, the communication is unidirectional, as on a one-way street. Only one of the two devices on a link can transmit; the other can only receive .
Keyboards and traditional monitors are examples of simplex devices. The keyboard can only introduce input; the monitor can only accept output.
2.)Half-Duplex:-In half-duplex mode, each station can both transmit and receive, but not at the same time.:
When one device is sending, the other can only receive, and vice-versa.
The half-duplex mode is like a one-lane road with traffic allowed in both direct
ions. When cars are traveling in one direction, cars going the other way must wait.
3)Full-Duplex:-In full-duplex mode (duplex), both stations can transmit and receive simultaneously .
The full-duplex mode is like a two-way street with traffic flowing in both direct
ions at the same time. In full-duplex mode, signals going in one direction share the
capacity of the link with signals going in the other direction. This sharing can occur in
two ways: Either the link must contain two physically separate transmission paths, one
for sending and the other for receiving; or the capacity of the channel is divided.
between signals traveling in both directions.
One common example of full-duplex communication is the telephone network.

                              

Fiber-Optic Cable

Fiber-Optic Cable:-A fiber-optic cable is made of glass or plastic and transmits signals in the form of light.Light travels in a straight line as long as it is moving through a single uniform substance.
In fiber optic technology, the medium consists of a hair-width strand of silicon or glass, and the signal consists of pulses of light.It has a cylindrical shape and consists of three concentric sections: the core, the cladding, and the jacket.
   The core, innermost section consists of a single solid dielectric cylinder of diameter d1 and of refractive index n1. The core is surrounded by a solid dielectric cladding of refractive index n2 that is less than n1.The cladding is surrounded by a jacket made of plastic. The jacket is used to protect against moisture, abrasion, crushing and other environmental hazards.
   Optical fibers are available in two varieties; Multi-Mode Fiber (MMF) and Single-Mode Fiber (SMF).For multi-mode fiber the core and cladding diameter lies in the range 50-200µm and 125-400µm, respectively. Whereas in single-mode fiber, the core and cladding diameters lie in the range 8-12µm and 125µm, respectively.

Advantages:-
1.Higher bandwidth.:-Very high data rate, low error rate. 1000 Mbps (1 Gbps) over distances of kilometers common. Error rates are so low ,negligible.
2.)Less signal attenuation.
3.)Difficult to tap,  higher reliability of this medium.
4.light weight:-Much thinner (per logical phone line) than existing copper circuits.
5.Not susceptible to electrical interference (lightning) or corrosion (rust).
6.Greater repeater distance than coax.
7.)Resistance to corrosive materials. Glass is more resistant to corrosive materials than copper.
Disadvantages:-
*)installation and maintenance.
*)Difficult to tap. It really is point-to-point technology. In contrast, tapping into coax is trivial. No special training or expensive tools or parts are required.
*)One-way channel. Two fibers needed to get full duplex (both ways) communication.
*)Cost. The cable and the interfaces are relatively more expensive than those of other
guided media.

Controlled Access

Controlled Access--In controlled access, the stations consult one another to find which station has the right to send. A station cannot send unless it has been authorized by other stations.
1.Reservation--In the reservation method, a station needs to make a reservation before sending data.
Time is divided into intervals. In each interval, a reservation frame precedes the data frames sent in that interval.If there are N stations in the system, there are exactly N reservation mini slots in the
reservation frame.When a station needs to send a data frame, it makes a reservation in its own mini slot. The stations that have made reservations
can send their data frames after the reservation frame.
2.Polling--It is similar to the roll-call performed in a classroom.In Polling one device is designated as a primary station and the other devices are secondary stations.All data exchanges must be made through the primary device even when the ultimate destination is a secondary device.The primary device controls the link; the secondary devices follow its instructions.If the primary wants to receive data, it asks the secondaries if they have anything to send; this is called poll function..If there is no data, usually a “poll reject” message is sent back. If the primary wants to send data, it tells the secondary
to get ready to receive; this is called select function.

3.Token Passing--In token passing scheme, all stations are logically connected in the form of a ring and control of the access to the medium is performed using a token.A token is a special bit pattern or a small packet, usually several bits in length, which circulate from node to node. Token passing can be used with both broadcast (token bus) and sequentially connected (token ring) type of networks.
token is passed from a node to the physically adjacent node.

Bridges

Bridges--A bridge operates in both the physical and the data link layer. As a physical layer
device, it regenerates the signal it receives. As a data link layer device, the bridge can
check the physical (MAC) addresses (source and destination) contained in the frame.A bridge has filtering capability. It can check the destination address of a frame and
decide if the frame should be forwarded or dropped. If the frame is to be forwarded, the
decision must specify the port. A bridge has a table that maps addresses to ports.
A bridge does not change the physical (MAC) addresses in a frame.
  A two-layer switch is a bridge, a bridge with many ports and a design that allows better (faster) performance.A bridge has filtering capability.

Random access

Random access:-It is also called contention method.In this method, stations compete for the medium. If two stations transmit data at the same time, then a collision
results—both stations then must determine when to retransmit data.

In this no station is superior to another station and
none is assigned the control over another.No station permits, or does not permit,another station to send. a station that has data to send uses a procedure
defined by the protocol to make a decision on whether or not to send.

If two station transmit data at the some time ,then a collision results,both station then must somehow determine when to transmit data,this decision depends on the state of the medium(idle or busy).

In this scheme, each stations must:-
•- Determine when to access the medium.
• -Determine what to do if the medium is busy.
• -Have some way of determining whether transmission succeeded or not.
•- Have some way of dealing with collisions (access conflicts with other stations).

Use of Data Link Layer

Use of Data Link Layer:-
i)Framing:-Group the physical layer bit stream into units called frames. so that each frame is distinguishable from another.
ii)Flow control:-It Ensuring the sending entity does not overwhelm
the receiving entity.Flow control. Prevent a fast sender from overwhelming a slower receiver.
iii)Error control :-It allows the receiver to inform the sender of any frames lost or damaged in transmission and coordinates the retransmission of those frames by the sender.
iv)Sender checksums the frame and sends checksum together with data. The checksum allows the receiver to determine when a frame has been damaged in transit.
vI)Receiver recomputes the checksum and compares it with the received value. If they differ, an error has occurred and the frame is discarded.
v)Medium Access Control (MAC):-It provide a link with reliable communication or there is a available dedicated link(channel) between the sender and receiver .

Physical Layer

Physical Layer:-The physical layer coordinates the functions required to carry a bit stream over a physical medium. It deals with the mechanical and electrical specifications of the interface and
transmission medium. It also defines the procedures and functions that physical devices and intertäces have to perturb tor transmission to Occur.
The physical layer is responsible for movements of
individual bits from one hop (node) to the next.
It's properties are:-
1.)Physical characteristics of interfaces and medium.:-The physical layer defines
the characteristics of the interface between the devices and the transmission
medium and type of transmission medium.
2.)Representation of bits.:-For transmission , bits must be encoded into signals—electrical or optical. The physical layer defines the type of encoding (how Os and 1s are changed to signals).
3.)Data rate. :-The transmission rate—the number of bits sent each second—is also
defined by the physical layer.
4.)Synchronization of bits:-the sender and
the receiver clocks must be synchronized.
5.)Line configuration.
6.)Physical topology:-how network are connected.
7.)Transmission mode:-It also define the direction of transmission.

Physical Topology

Physical Topology:--
The term physical topology refers to the way in which a network is laid out physically..
Two or more device connect (0 a link, two or more links form a topology. The topology--

1.)Mesh:- 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.
for n nodes-we need
n(n—l)/2
duplex-mode links.

*advantages over other network topologies.:---
1.)It uses dedicated links so guarantees that each connection can carry its own data load,so eliminating the traffic problems.
2.)a mesh topology is robust. If one link becomes unusable, it does not incapacitate the entire system.
3.)privacy or security. When every
message travels along a dedicated line, only the intended recipient sees it.
4.)fault identification and fault isolation is easy.

*disadvantage:-
1)the amount of cabling and the
number of i/o ports required.
2.) the hardware required to connect each link (I/O ports and cable) can be
prohibitively expensive. For these reasons a mesh topology is usually implemented in a
limited fashion.

2.)Star Topology :-In a star topology, each device has a dedicated point-to-point link
only to a central controller, usually called a huh. Thc devices arc 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 .

advantage:-1.)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.
2)it easy to install and reconfigure.
3.)robustness
4.)If one link fails, only that link is affected. All
other links remain active.so easy fault identification and fault isolation.

disadvantage:-
1.)dependency of the whole topology
on one single point, the huh. If the huh goes down, the whole system is dead.
2.)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.

3.) Bus Topology :-The preceding examples all describe point-to--point connections. A bus
topology, on the other hand, is multi-point. 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.
Advantages:-
1.) easy of installation.
2.)Due to Backbone , 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 .

Disadvantages :-1.) difficult reconnection and fault isolation.
2.)difficult to add new
devices.
3.)Signal reflection at the taps can cause degradation . 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.
4.)fault or break in the bus cable stops all Transmission.The damaged area reflects signals back in the
direction of origin, creating noise in both directions.

4.)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. 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.

adv-A ring is relatively easy to install and reconfigure.To add or delete a device requires
changing only two connections.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.

disad-unidirectional traffic can be a disadvantage.a break in
the ring (such as a disabled station) can disable the entire network. This

Network Criteria

* NETWORKS:-A network is a combination of hardware and software that sends data from one location
to another. The hardware consists of the physical equipment that carries signals from
one point of the network to another. The software consists of instruction sets that make
possible the services that we expect from a network.
A network is a set of devices ( nodes) connected by communication
links. A node can be a computer, printer, or any other device capable of sending and/or
receiving data generated by other nodes on the network.

*Network Criteria:-A network must he able to meet a certain number of criteria. The most important of
these are performance, reliability, and security.---
1)Performance:-Performance can be measured in many ways, including transit time and response time.
Transit time is the amount of time required for a message to travel from one device to another.
2.)Reliability:-network reliability is measured by the frequency of
failure, the time it takes a link to recover fron a failure, and the networks robustness in
a catastrophe.
3.)Security:-Network security issues include protecting data from unauthorized access. protecting
dat,a from damage.

Type of connection

Type of connection :--
1.)Point-to-Point 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.
2.)Multipoint:-A multipoint (also called multidrop) connection is one in which more
than two speciIic devices share a single link (see Figure 1.3h).
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.

Data communications

Data communications:-Data communications are the exchange of data between two devices via some
form of transmission medium such as a wire cable. For data communications to occur,
the communicating device; must be part of a communication system made up of a combination of hardware (physical equipment) and( software (programs). The effectiveness
of a data communications system depends on four fundamental characteristics: delivery. accuracy, nmeimess, and jitter.
i)Delivery. -The system must deliver data to the correct destination. Data must be
received by the intended device or ricer and only by that derive or user
2. Accuracy. -The system must deliver the data accurately. Data that have been
altered in transmission and left uncorrected are unusable.
3. Timeliness- The system must deliver data in a timely manner. Data delivered late are
useless.
4)Jitter. Jitter refers to the variation in the packet arrival time. It is the uneven delay in
the delivery of audio a video packets.