Class notes BTCOC602(Computer Network)

[ Computer Network ]
[ Unit 2 ]




 [ LAN Technology]



A local Area Network (LAN) is a data communication network connecting
various terminals or computers within a building or limited geographical area. The
connection among the devices could be wired or wireless. Ethernet, Token Ring and
Wireless LAN using IEEE 802.11 are examples of standard LAN technologies.


LAN has the following topologies:

 Star Topology
 Bus Topology
 Ring Topology
 Mesh Topology
 Hybrid Topology
 Tree Topology


Note : above points are described in unit 1

,  [ X.25 ]



X.25 was a standard suite of protocols used for packet-switched
communications over a wide area network — a WAN. A protocol is an agreed-
upon set of procedures and rules. Two devices that follow the
same protocols can understand each other and exchange data.




X.25 is generally a protocol that was developed by International
Telecommunications Union (ITU). It usually allows various logical channels to make
use of same physical line. It basically defines a series of documents particularly
issued by ITU. These documents are also known as X.25 Recommendations. X.25
also supports various conversations by multFiplexing packets and also with the
help of virtual communication channels. X.25 basically encompasses or suits to the
lower three layers of the Open System Interconnection (OSI) reference model
for networking.

,These three protocol layers are :
1. Physical Layer
2. Frame Layer
3. Packet Layer




These are explained as following below.


1. Physical Layer :
This layer is basically concerned with electrical or signaling. The physical
layer interface of X.25 also known as X.21 bis was basically derived from RS-232
interface for serial transmission.
This layer provides various communication lines that transmit or transfer
some electrical signals. X.21 implementer is usually required for linking.

, 2. Data Link Layer :
Data link layer is also known as Frame Layer. This layer is an implementation
or development of ISO High-Level Data Link Layer (HDLC) standard which is
known as LAPB (Link Access Procedure Balanced). It also provides a communication
link and transmission hat is error-free among any two physically connected nodes
or X.25 nodes.

LAPB also allows DTE (Data Terminal Equipment) or DCE (Data Circuit-
Terminating Equipment) simply to start or end a communication session or start
data transmission. This layer is one of the most important and essential parts of
X.25 Protocol. This layer also provides a mechanism for checking in each hop
during the transmission. This service also ensures a bit-oriented, error-free, and
also sequenced and ordered delivery of data frames or packets.


There are many protocols that can be used in frame-level as given below :
 Link Access Procedure Balanced (LAPB) –
It is specified by ITU-T Recommendation X usually derived from
HDLC. It is the most commonly used protocol that allows establishing a
logical connection.

 Link Access Protocol (LAP) –
This protocol is very rarely used. This is usually used for framing
and transferring data packets across point-to-point links.

 Link Access Procedure D-channel (LAPD) –
It is used to convey or transfer data over D-channel. It also enables
and allows transmission of data among DTEs through D channel especially
among a DTE and an ISDN node

 Logical Link Control (LLC) –
It is used to manage and ensure the integrity of transmissions of
data. It also allows transmission of X.25 data packets or frames through a
LAN (Local Area Network) channel.