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Friday, October 24, 2008

Baseband Communication

Baseband communication refers to the transmission over a baseband channel or a low pass channel. Low pass channel refers to channels that support only low frequency signals such as twisted pair, coaxial cable etc. The digital signals are sent through the medium without modulation on a high frequency carrier. Two major constraints for baseband communication are channel noise and intersymbol interference(ISI) that will result in bit errors in the received signal. Channel noise is assumed to be additive white Gaussian noise AWGN whose spectral density is independent of the frequency. The effects of channel noise on the information can be minimized by using a matched filter. Intersymbol interference is due to the dispersive nature of the channel and it will degrade the performance considerably. ISI can be controlled by adopting the raised cosine spectrum with the optimum roll-off factor. Eye patterns are used to study the combined effects of channel noise and ISI on the received signal.

Tuesday, August 12, 2008

IP Address

Just like every home has an address every node in the network has a unique address. IP address is the address used for communication between the computers at the network layer level. Every computer linked to the web will be allotted a 32 bit IP address during start up. The IP address can be broadly classified as two types namely classful and classless addressing. Let us take a look at the classful addressing

Every IP address can be dissected into two parts as netid and hostid. The netid identifies the network and the hostid identifies the host on the network.

Class A can be allocated to large size universal networks and they can accommodate a whopping 16,777,216 computers.

Class B addresses can incorporate 65,536 hosts and are most commonly allocated for intermediate size networks like MAN etc.

Class C addresses can support only 256 hosts and they are allocated to many small organizations with lower number of terminals.

Class D address serve as multicast addresses which are used to send data to several nodes at the same time.

Class E addresses are reserved for future use. Recently Classless addresses are being used. IP addresses are often used conservatively with the help of techniques like Network Address Translation(NAT) and Subnetting.

Saturday, August 9, 2008

ALOHA protocol

Aloha means "Hello". Aloha is a multiple access protocol at the datalink layer and proposes how multiple terminals access the medium without interference or collision. The protocol was developed by Norman Abramson and his colleagues at the University of Hawaii. The protocol allows every system to send a frame if it ready to send. But when a collision occurs the node will wait for a random amount time and then send the frame again. The process continues till the node has sent all the frames. Since the nodes send their frames without sensing the medium there is a high probability for collisions to occur. The maximum success rate or throughput that can be achieved with Aloha protocol is only 18%.

In 1972 Roberts developed a protocol that would increase the capacity of aloha two fold. The Slotted Aloha protocol involves dividing the time interval into discrete slots and each slot interval corresponds to the time period of one frame. This method requires synchronization between the sending nodes to prevent collisions.

Thursday, August 7, 2008

Home Area Networks

The Home Area Network (HAN) is a technology that interconnects all the home appliances and also links them to the internet via a central home portal. Devices such as personal computers, laptops, telephones, washing machines, security systems and entertainment systems can be tied together with the internet to facilitate remote controlling and inter-operation. The application diversity and dissimilar bandwidth requirements of each device are the constraints in HAN design. A HAN is meant to be user friendly, low cost, flexible and scalable without significant performance degradation.

There are different technologies adopted to develop the HAN infrastructure. Home Phone Network Access (HPNA) aims at providing internet access to home appliances through the single Telephone wiring. This is possible by using different frequencies for different applications on the same wire employing frequency division multiplexing. Power Line Modems is yet another HAN technology that tries to deliver broadband via power lines supplied to our homes. But the medium is noisy and the cost associated is high. Therefore we need to look out wireless solution alternatives. Home Access Networks are wireless networks that provide wireless internet connection to homes directly and enable communication of devices.

Tuesday, August 5, 2008

Mac frame format

Medium access control is very important to prevent collisions and delays. The data link layer is responsible for medium access control and several protocols are used for this purpose. Carrier sense multiple access is one way of controlling access to the medium. In CSMA/CD the system first senses the carrier before transmitting when the carrier is idle the system sends the data. When the carrier is busy the system backs of for a certain period of time before sensing the carrier again.

let us take a closer look at the Mac frame format

The Preamble is a seven byte field made up of alternating 0's and 1's which is very essential for synchronization between the transmitter and the receiver.

Start frame delimiter has a fixed sequence 10101011 which indicates the start of a frame and identifies the first bit for the receiver.

The Destination address is either 2byte or 6byte which refers to the address of the receiving system.

The Source address is either 2byte or 6byte which refers to the address of the sending system.

The length field provides the length of the data field of the logical link control layer.

The data from the LLC layer is encapsulated in to the Mac frame.

There are some extra bits needed for padding to ensure that the frame is matching the size perfectly.

A 32 bit CRC is provided at the end of the frame for error detection.

Sunday, August 3, 2008

Internet Services

Lets look at the services offered by the internet at the application level. The internet has changed our life and we use internet applications in our daily life directly or indirectly. The power of internet lies in its interoperability wherein multiple systems communicate with each other in solving multiple problems and tasks.

The worldwideweb is a repository of information allowing the users to access text, graphics and multimedia instantly. The users can follow hyperlinks to other websites.

Electronic mail has revolutionised the way people communicate with each other. The user can send messages and files as attachments via email. Every user is presented with an inbox to store incoming mails for free by all leading companies.

File transfer allows users to send/receive a copy of a file online. Small files can be attached to emails and large files can also be shared online.

Remote login allows users to sit at home and access the files in a remote computer by logging on to the system as a guest or registered user.

Friday, August 1, 2008

Transmission Media

In networking there are two types of transmission namely wired and wireless transmission. Technically speaking the medium is of two types namely guided and unguided media. The guided media commonly used for transmission are twisted pair, coaxial cable and optical fibers. Twisted pair is the normal telephone cable that provides broadband connection to our homes. It can support data rates upto 8Mbps. The other option available is coaxial cables like the ones used for cable television which can support data rates upto 500Mbps. Optical fibers can provide ultra speed internet access with a data rate of 2Gbps but they are commonly used in trunking circuits and long haul networks. Skin effect is the limiting factor at higher frequencies.

Wireless transmission enables you to access internet on the move. Mobile access is possible with the help of a device called antenna. The unguided transmission maybe directional or omnidirectional depending on the requirement and the type of antenna used. Wireless technology is much more complex and scientists are working on higher data rates and security aspects.

Tuesday, July 22, 2008

OSI Model

The OSI model is the universally adopted reference model for networking. Networking is essentially the communication between electronic devices governed by certain protocols. All the layers in the model are imaginary or abstract and they depict the functions carried out by the system grouped at a particular level of abstraction. The OSI model is made of 7 layers as follows

The Physical layer is the bottom layer in the protocol stack and it governs the physical and electrical characteristics of the devices and the medium of communication. Modulation, line coding and the medium characteristics are some of the functions performed by this layer.

The Datalink layer is second layer above the Physical layer which is responsible for hop-to-hop or node-to-node delivery. Medium access, flow control and error control are performed by this layer.

The Network layer above the Datalink layer is responsible for addressing and and routing mechanisms. IP is the most popular protocol used in this layer.

The Transport layer provides end-to-end delivery between the source and destination. It also performs error control, flow control and congestion control besides segmentation and reassembly.

The Session layer establishment,management and termination of the dialogue/connection between two parties.

The Presentation layer decides the presentation of information and performs encryption and decryption.

The topmost layer is the Application layer which acts as the user interface to provide services to the user.