Chapter 1. Data Communications, Data Networks, and the Internet

Data Communications, Data Networks, and the Internet

Networking Technology

• Local Area Networks (LANs)
• Wide Area Networks (WANs)

Local Area Networks (LANs)

A group of computers and associated devices that share a common communications line or wireless link to a server.

Several key features of LAN

1. The scope of LAN is small, typically a single building or a cluster of buildings
2. LANs are usually owned by the same organization that owns attached devices
3. Internal data rates are greater than WANs
4. Most common configurations are switched LANs and wireless LANs

High-speed LANs are emerging

Examples of higher-speed LANs:

• Centralized server farms (e.g., Facebook)
• A server farm is a set of many servers interconnected together within the same physical facility
• E.g., Data-center, supercomputer
• Power workgroups
• These groups typically consist of a small number of cooperating users who need to manage massive data files across the network
• E.g., a computer-aided design (CAD) company (regularly runs simulations of new designs)
• High-speed local backbone
• As processing demand grows and high-speed interconnection is necessary.

Wide Area Networks (WANs)

A computer network that extends over a large geographical distance and place

• A WAN connects many LANs and is used for larger areas
• A WAN is similar to a banking system, where hundreds of branches in different cities are connected with each other.
• A WAN works in a similar fashion to a LAN, just on a larger scale.

High-speed Wide Area Networking Needs

Changes in data traffic patterns are driving the creation of high-speed WANs.

1. High performance computing
2. Average traffic load has risen
3. Most organizations require access to the internet
4. Traffic patterns have become more unpredictable
5. More reliance on personal computers, workstations, and server

Simplified Communications Model

Source -> Transmitter -> Transmission system -> Receiver -> Destination

* Source system consists of a source and a transmitter.

* Destination system consists of a receiver, a destination.

Example

Workstation -> Modem -> Public Network -> Modem -> Server

Key elements of the simplified communications model

Purpose : The exchange of data between two parties

• Source : Generates data to be transmitted (e.g., telephones & PCs)
• Transmitter : Converts data into transmittable electromagnetic signals
• Transmission system : Carries data (single transmission line or network)
• Receiver : Converts received signal into data
• Destination : Takes incoming data

Communications Tasks

Transmission System Utilization	Make efficient use of transmission facilities
Interfacing	A device must interface to communicate
Signal generation	Capable of being propagated through the transmission system
Synchronization	The receiver must be able to determine when a signal begins to arrive and when it ends
Exchange management	The two parties must cooperator (transmit simultaneously or take turn, data format)
Error detection and correction	In all communications systems, there is a potential for error
Flow control	Sending data faster than destination can be processed
Addressing	Only intended destination system must receive the data
Routing	Specific route through the network
Recovery	Resume activity at the point of interruption or restore the state
Message formatting	Form of the data to be exchanged
Security	Receiver may wish the data have not been altered
Network Management	Configuring the system, monitoring its status

Building Networks

Networks have been implemented using one of two technologies

• Circuit switching
• Packet switching

Circuit Switching

A method of implementing a telecommunications network in which two network nodes establish a dedicated communications channel (circuit) through the network

• Uses a dedicated communications path
• Connected sequence of physical links between nodes
• Logical channel dedicated on each link
• The most common example of circuit switching is the telephone network

Packet Switching

• Data are sent out in a sequence of small chunks called packets
• Packets are passed from node to node along a path leading from source to destination
• Packet-switching networks are commonly used for terminal-to-terminal computer and computer-to-computer communications

Packet Switching	Circuit Switching
Store and forward	No store and forward
Shared	Dedicated
Not reserved	Reserved
Higher delay	Less delay
Internet line	Telephone line
Less reliable	Highly reliable
No call setup	Call setup