by Dinesh Thakur Category: Network Technologies

Integrated Services Digital Network is a telephone system network. It is a wide area network becoming widely available. Prior to the ISDN, the phone system was viewed as a way to transport voice, with some special services available for data. The key feature of the ISDN is that it integrates speech and data on the same lines, adding features that were not available in the classic telephone system.

 
by Dinesh Thakur Category: Network Technologies
1. TDM is the digital multiplexing technique.
2. In TDM, the channel/link is not divided on the basis of frequency but on the basis of time.
3. Total time available in the channel is divided between several users.
4. Each user is allotted a particular a time interval called time slot or time slice during which the data is transmitted by that user.
5. Thus each sending device takes control of entire bandwidth of the channel for fixed amount of time.
 
by Dinesh Thakur Category: Network Technologies

Client/Server Architecture The client/server architecture significantly decreased network traffic by providing a query response rather than total file transfer. It allows multi-user updating through a GUI front end to a shared database. Remote Procedure Calls (RPCs) or standard query language (SQL) statements are typically used to communicate between the client and server.



 
by Dinesh Thakur Category: Network Technologies

Frequency-Division Multiplexing (FDM) is a scheme in which numerous signals are combined for transmission on a single communications line or channel. It is analog multiplexing technique. Each signal is assigned a different frequency (sub channel) within the main channel. its requires channel synchronization. FDM multiplexing technique is based on orthogonality of sinusoids.

 
by Dinesh Thakur Category: Network Technologies

A communications device that multiplexes (combines) several signals for transmission over a single medium. A demultiplexer completes the process by separating multiplexed signals from a transmission line. Frequently a multiplexer and demultiplexer are combined into a single device capable of processing both outgoing and incoming signals. A multiplexer is sometimes called a mux.

 
by Dinesh Thakur Category: Network Technologies

Frame relay has evolved from X.25 packet switching and objective is to reduce network delays, protocol overheads and equipment cost. Error correction is done on an end-to-end basis rather than a link -to-link basis as in X.25 switching. Frame relay can support multiple users over the same line and can establish a permanent virtual circuit or a switched virtual circuit.



 
by Dinesh Thakur Category: Network Technologies

• X.25is a standard used by many older public networks specially outside the U.S.
• This was developed in 1970s by CCITT for providing an interface between public packet-switched network and their customers.
• The packet switching networks use X.25 protocol. The X.25 recommendations were first prepared in1976 and then revised in1978,1980 and 1984.
• X.25 was developed for computer connections, used for terminal/timesharing connection.
• This protocol is based on the protocols used in early packet switching networks such as ARPANET, DATAPAC, and TRANSPAC etc.

• X.25 Packet Switched networks allows remote devices to communicate with each other across high speed digital links without the expense of individual leased lines.
• A protocol X.21 which is a physical layer protocol is used to specify the physical electrical and procedural interface between the host and network.
• The problem with this standard is that it needs digital signal rather than analog signals on telephone lines.
• So not many networks support this standard. Instead RS 232 standard is defined.
• The data link layer standard has a number of variations. It is designed for error detection and corrections.
• The network layer protocol performs the addressing, flow control, delivery confirmation etc.
• It allows the user to establish virtual circuits and send packets on them. These packets are delivered to the destination reliably and in order.

• X.25 is a connection oriented service. It supports switched virtual circuits as well as the permanent circuits.

• Packet Switching is a technique whereby the network routes individual packets of HDLC data between different destinations based on addressing within each packet.

• A switched virtual circuit is established between a computer and network when the computer sends a packet to the network requesting to make a call to other computer.

• Packets can then be sent over this connection from sender to receiver.

• X.25 provides the flow control, to avoid a fast sender overriding a slow or busy receiver.

• A permanent virtual circuit is analogous to-a leased line. It is set up in advance with a mutual agreement between the users.

• Since it is always present, no call set up is required for its use.

• In order to allow the computers which do not use the X.25 to communicate with the X.25 network a packet assembler disassembler (PAD) is used.

• PAD is required to be installed along with each computer which does not use X.25.

• X.25 defines the interface for exchange of packets between a DTE and switch data subnetwork node.

Three Layers of X.25:

The X.25 interface is defined at three levels:

The three levels are:

(i) Physical layer (level 1)

(ii) Data link layer (level 2)

(iii) Packet layer (level 3).

• These three layers correspond to the three lower most layers of the ISO-OSI reference model. The physical layer takes care of the interface between a computer terminal and the link which attaches it to the packet switching node.

• The X.25 defines the interface for exchange of packets between the user's machine (DTE) and the packet switching node to which this DTE is attached which is called as DCE.

• The three layers of X.25 interface are as shown in Fig.(a).

• At the physical level X.21 physical interface is being used which is defined for circuit switched data network. At the data link level, X.25 specifies the link access procedure-B (LAP-B) protocol which is a subset of HDLC protocol.

                 Different layers of X.25 and interface between DTE and DCE

• At the network level (3rd level), X.25 defines a protocol for an access to packet data subnetwork.

• This protocol defines the format, content and procedures for exchange of control and data transfer packets. The packet layer provides an external virtual circuit service.

• Fig.(b) shows the relationship between the levels of x'25. User data is passed down to X.25 level 3.

• This data then appends the control information as a header to form a packet. This control .information is then used in the operation of the protocol.

• The entire X.25 packet formed at the packet level is then passed down to the second layer i.e. the data link layer.

• The control information is appended at the front and back of the packet forming a LAP-B frame. The control information in LAP-B frame is needed for the operation of the LAP-B protocol.

• This frame is then passed to the physical layer for transmission.

                  Relationship between the levels of X.25

Virtual Circuit Service

• With the X25 packet layer, data are transmitted in packets over external virtual circuits, The virtual circuit service of X25 provides for two types of virtual circuits,

• The virtual circuit service of X25 provides for two types of virtual circuits i.e. "virtual call" and "permanent virtual circuit".

• A virtual call is a dynamically established virtual circuit using a call set up and call clearing procedure.

• A permanent virtual circuit is a fixed, network assigned virtual circuit. Data transfer takes place as with virtual calls, but no call set up or clearing is required.

Characteristics of X.25

In addition to the characteristics of the packet switched network, X.25 has the following characteristics:

  1. Multiple logical channels can be set on a single physical line
  1. Terminals of different communication speeds can communicate
  1. The procedure for transmission controls can be changed.

Multiple Logical Channels can be set on a Single Physical Line

The terminal connected to the packet switched network can communicate with multiple terminals at the same time using a single physical line. This makes it possible to set multiple logical paths called logical channels on a single physical line. Multiple communications thus takes place through these logical channels. Based on the X.25 rules, 4096 logical channel can be set on a single physical line. To enable control of 4096 logical channels there are 16 logical channel groups. Each logical channel group is divided into 256 logical channels. These channel groups are known as LCGN (Logical Channel Group Number) and LCN (Logical Channel Number).

                  X.25 Representing Logical Channel

Terminals of Different Communication Speeds can communicate

As X.25 uses the store and forward method, therefore, the communication is possible. In other words, a terminal of 1.2 Kbits/s can communicate with a host computer at 9600 bits/s through the packet switched network. When the 'telephone network or a leased line is used, this type of communication cannot be established. In other words, in these environments, the transmission speed of the sender should be the same as that of the receiver.

The reason that communication between terminals with different communication speeds is possible is that the senders and the receivers are not physically connected. Data transmission from a 1.2 Kbits/s terminal is temporarily stored in the receiving buffer of the packet switched network and the data is then passed through the network and transmitted to the host computer at 9600 bits/s.

                      Communication Using Different Speeds

By using the above 2 features the network can be established. By applying a higher line speed to the host computer than the terminal and setting multiple logical channels, the number of lines at the computer can be reduced.

                   Packet interleaved Multiplexing

The Procedure for Transmission Controls can be changed

It is possible to change the procedure for transmission control. As we know that X.25 uses the store and forward method, therefore, all data must be once stored in the packet switched unit. By implementing a protocol conversion function to the packet switched unit can connect the devices with different transmission control (basic procedure and X.25 protocol).With the help of this method, any terminal that cannot handle packets cannot be connected to the packet switched network. A terminal that cannot handle packets is called an NPT (Non-packet mode terminal).

Conversion of Transmission Procedure

 



 
by Dinesh Thakur Category: Network Technologies

Definition: Asynchronous Transfer Mode (ATM) also called cell relay (transferring data in cells of a fixed size) that is operates at the data link layer (Layer 2) of OSI Model over fiber or twisted-pair cable, a high-speed switched network technology based on ITU-T Broadband Integrated Services Digital Network (B-ISDN) standard, developed by the telecommunications industry to implement the next generation network. ATM was designed for use in WANs such as the public telephone system and corporate data networks, though it has also been applied to create super-fast LANs.

ATM can carry all kinds of traffic: voice, video and data simultaneously at speeds up to 155 megabits per second. It Convert voice, video data to packets and passing large packet data through the same medium. ATM is differing from TCP/IP because it use fixed channel routing protocol routes between two end points. A real-time low-latency application such as VoIP and video takes precedence on an ATM network.

What is Asynchronous Transfer Mode (ATM)?

Asynchronous Transfer Mode

ATM is a dedicated connection-oriented switching technology, in which switches create a virtual connection or virtual circuit between the sender and receiver of a call that permanent or switched for the duration of the call. It is a small-packet switched system or similar to circuit-switched network, which breaks down messages into very small, fixed length packets called cells generally organizes digital data into 53 bytes in length (48 bytes of data plus a 5-byte header).   ATM frame structure

An ATM header can have User-Network Interface (UNI) and Network-Node Interface (NNI) two formats.
User-Network Interface (UNI) used for communication between end systems.
Network-Node Interface (NNI) used for communication between switches.

Two type of connections are supported by ATM (Asynchronous Transfer Mode)

Point-to-point connections: It connects either unidirectional or bi-directional two end-systems.
Point-to-multipoint connections: It connects one unidirectional ATM to number of destination ATM.
It is different in packet sizes from Ethernet data or frames. ATM is a core protocol for SONET that is the backbone of ISDN. The advantage conferred by such small cells is that they can be switched entirely in hardware, using custom chips, which makes ATM switches very fast (and potentially very cheap).
The asynchronous part of the name refers to the fact that although ATM transmits a continuous stream of cells over a physical medium using digital signal technology, some cells may be left empty if no data is ready for them so that precise timings are not relevant. Every cell is encoding data with asynchronous time-division multiplexing (TDM) and it queued before being multiplexed over the transmission path.
Every cell are encodes data and processed within their time slot allocated to it. When cell time slot allocated is finished, the next cell starts same procedure. That’s why it's called asynchronous time-division multiplexing (TDM);
This is ATM's greatest strength, as it enables flexible management of the quality of service (QoS) so; an operator can offer different guaranteed service levels (at different prices) to different customers even over the same line. This ability will enable companies to rent virtual private networks based on ATM that behave like private leased lines but in reality share lines with other users.
Available ATM service: Generally four data bit rates are available for ATM services: constant bit rate (CBR), variable bit rate (VBR), available bit rate (ABR) and unspecified bit rate (UBR).



 
by Dinesh Thakur Category: Network Technologies

In Computer network, a proxy server is a server (a computer system or an application program) that acts as an intermediary for requests from clients seeking resources from other servers. A client connects to the proxy server, requesting some service, such as a file, connection, web page, or other resource, available from a different server.



 
by Dinesh Thakur Category: Network Technologies

The Token-Passing Protocol relies on a control signal called the token. A token is a 24-bit packet that circulates throughout the network from NIC to NIC in an orderly fashion. If a workstation wants to transmit a message, first it must seize the token. At that point, the workstation has complete control over the communications channel. The existence of only one token eliminates the possibility of signal collisions. This means that only one station can speak at a time.

 

Page 1 of 5



About Dinesh Thakur

Dinesh ThakurDinesh Thakur holds an B.SC (Computer Science), MCSE, MCDBA, CCNA, CCNP, A+, SCJP certifications. Dinesh authors the hugely popular blog. Where he writes how-to guides around Computer fundamental , computer software, Computer programming, and web apps. For any type of query or something that you think is missing, please feel free to Contact us.



Related Articles