Computer Network

Data Communication

Def: Exchange of data between two devices via some form of transmission medium such as a wire cable.

Effectiveness of Data communication system depends on

1. Delivery: Data must be received by intended device or user and only that device or user
2. Accuracy: system must deliver data accurately
3. Timeliness: Late delivery of data is useless.
4. Jitter: Variation in packet arrival time.

Components

1. Message: data to be communicated. eg text, images etc
2. Sender: device that sends data. Eg computer, telephone
3. Receiver: device that receives data
4. Transmission Medium: physical path by which a message travels. Eg twisted pair wire, coaxial cable, fibre optic
5. Protocol: set of rules that govern data communications. Represent agreement between communicating devices.

Data Representation

1. Text: ASCII, Unicode systems are used to encode text data.
2. numbers: numbers can be directly converted into binary
3. Images: Image is composed of matrix of pixels. Most popular method for colored image is RGB
4. Audio: It is continuous. We can send it as it is or can discretized it.
5. Video: number of pictures.

Data Flow

1. Simplex: Eg. Keyboard & traditional monitors
2. Half Duplex: Eg. Walkie talkies
3. Full duplex: Eg. telephone

Networks

def: It is set of devices connected by communication links.

Network Criteria

1. Performance: can be measured in transit time (time required for message to travel from one device to another) & response time (time between inquiry & response). performace depends on number of users, type of transmission medium,capabilities of connected devices & efficiency of software. performace is evaluated by Throughput & delay
2. Reliability: It is frequency of failure, the time it takes a link to recover from failure & networks robustness in catastophe
3. Security: Includes protecting data from unauthorized access, protecting data from damage & development and implementing policies, procedures for recovery from breaches and data losses.

Physical Structures

Type of Connection

1. Point-to-Point: dedicated link between two devices. Entire capacity reserved.
2. Multipoint: more than two specific devices share a single link. capacity is shared, either spatially or temporally.

Physical topology

How physical network is laid out.

Mesh

every device has a dedicated point-to-point link to every other device. physical links in a fully connected mesh with $n$ nodes, will be $n(n-1)/2$. To accomodate that many links, every device on network must have $n-1$ IO ports.

Mesh topology is usually implemented in limited fashion. eg backbone connecting main computers.

Star

each device has dedicated point-to-point link only to a central controller (hub).

used in LANs

Bus

multipoint. single cable acts as backbone. nodes are connected to bus cable by drop lines and taps.

Ring

each device has a dedicated P2P connection with only two devices on either side of it.