Network Engineer

Forward error correction is the process in which the receiver tries to guess the message by using redundant bits.

The correction of errors is more difficult than the detection. In error detection, checks only any error has occurred. In error correction, the exact number of bits that are corrupted and location in the message are known. The number of the errors and the size of the message are important factors.

Data link protocols are sets of specifications used to implement the data link layer. The categories of Data Link protocols are:

1. Asynchronous Protocols

2. Synchronous Protocols

☛ Character Oriented Protocols

☛ Bit Oriented protocols

☛ Divide the data into sections

☛ Add the sections together using 1's complement arithmetic

☛ Take the complement of the final sum, this is the checksum.

The common Error Detection methods are:

☛ Vertical Redundancy Check (VRC)

☛ Longitudinal Redundancy Check (VRC)

☛ Cyclic Redundancy Check (VRC)

☛ Checksum

Data can be corrupted during transmission. For reliable communication errors must be deducted and Corrected. Error Detection uses the concept of redundancy, which means adding extra bits for detecting errors at the destination.

VRC is the most common and least expensive mechanism for Error Detection. In VRC, a parity bit is added to every data unit so that the total number of 1s becomes even for even parity. It can detect all single-bit errors. It can detect burst errors only if the total number of errors in each data unit is odd.

The concept of including extra information in the transmission solely for the purpose of comparison. This technique is called redundancy.

The most powerful of the redundancy checking techniques, is based on binary division.

A block of bits is divided into rows and a redundant row of bits is added to the whole block. It can detect burst errors. If two bits in one data unit are damaged and bits in exactly the same positions in another data unit are also damaged, the LRC checker will not detect an error. In LRC a redundant data unit follows n data units.

Used by the higher layer protocols (TCP/IP) for error detection.

☛ Single-Bit error:

In a single-bit error, only one bit in the data unit has changed

☛ Burst Error:

A Burst error means that two or more bits in the data have changed.

☛ Cables

☛ Leased Lines

☛ Last-Mile Links

☛ Wireless Links

Nodes and Links.

☛ Guided Media:

1: Twisted - Pair cable

a: Shielded TP

b: Unshielded TP

2: Coaxial Cable

3: Fiber-optic cable

☛ Unguided Media:

1: Terrestrial microwave

2: Satellite Communication

The Session layer is the network dialog Controller. It establishes, maintains and synchronizes the interaction between the communicating systems.

☛ Dialog control

☛ Synchronization

The Application Layer enables the user, whether human or software, to access the network. It provides user interfaces and support for services such as e-mail, shared database management and other types of distributed information services.

☛ Network virtual Terminal

☛ File transfer, access and Management (FTAM)

☛ Mail services

☛ Directory Services

The Transport Layer is responsible for source-to-destination delivery of the entire message.

☛ Service-point Addressing

☛ Segmentation and reassembly

☛ Connection Control

☛ Flow Control

☛ Error Control

The Data Link Layer transforms the physical layer, a raw transmission facility, to a reliable link and is responsible for node-node delivery.

☛ Framing

☛ Physical Addressing

☛ Flow Control

☛ Error Control

☛ Access Control

The Transport layer links the network support layers and user support layers.