Next generation networks




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4.3Packet Switched Data networks


A packet switched data network identifies as a digital data communications network that groups all transmitted data, irrespective of content, type, or structure into suitably sized blocks, called packets. The network over which packets are transmitted is a shared network which routes each packet independently from all others and allocates transmission resources as needed.[2]

General meaning of the “packet” means a small container or a pouch, thus “Packet” in networks would be interpreted as a formatted block of data. If this understanding is reasonable enough, by the definition PSDN covers wide range of existing networks (X.25 based, Frame Relay and even IP) as well as future networks. However, following the scope of this Technical Paper and the concept of legacy networks, this Technical Paper focuses on the public telecommunications networks aspect which is a packet switched network using X.25 protocol.

X.25 is an ITU-T standard protocol suite for packet switched public data network. A Packet Switched Data Network (PSDN) based on X.25 consists of packet-switching exchange (PSE) nodes, leased lines and PSTN connections or ISDN connections as physical links. X.25 is a family of protocols that was popular during the 1980s with telecommunications companies and in financial transaction systems such as automated teller machines.

X.25 defined three basic protocol layers with followings and Figure 3 shows overall configuration model of X.25 based PSDN:



  • Physical layer: specifies the physical, electrical, functional and procedural characteristics to control the physical link between a DTE (Data Terminal Equipment) and a DCE (Data Circuit-terminating Equipment). Common implementations use X.21, EIA-232, EIA-449 or other serial protocols;

  • Data link layer: consists of the link access procedure for data interchange on the link between a DTE and a DCE. The Link Access Procedure-Balanced (LAPB) is a data link protocol that manages a communication session and controls the packet framing including error correction and orderly delivery; and

  • Packet layer: defines a packet-layer protocol for exchanging control and user data packets to form a packet-switching network based on virtual calls, according to the Packet Layer Protocol.



Figure 3 – Overall configuration model of PSDN

In addition, a Packet Assembler-Disassembler (PAD) is used for providing multiple of asynchronous terminal connectivity to PSDN or a host computer. PAD collects data from a group of terminals and places the data into X.25 packets (assembly). In the reverse, PAD takes data packets from PSDN or host computer and returns them into a character stream that can be sent to the terminals (disassembly).

Key features of PSDN are to optimize utilization of available link capacity, minimize response times and increase the robustness of communication. Packets are buffered and queued when traversing network adapters, switches and other network nodes. Therefore delay and throughput varies depending on the traffic load in the network, resulting variables and differences of QoS.

4.4Integrated Services Digital Networks


Integrated Services Digital Networks (ISDN) is an integrated services network that provides digital connections between user-network interfaces identified with a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the PSTN.

ISDN is designed to allow digital transmission of voice and data together over ordinary telephone copper wires (twisted copper wires). ISDN is a 64 kb/s channel (called B channel) based circuit-switched telephone network system (offering circuit-switched connections for either voice or data) with providing access to packet networks for data such as X.25 based PSDN. The key feature of ISDN is integrating voice and data on the same lines, adding additional features called supplementary services that were not available in the PSTN. There are several kinds of access interfaces to ISDN defined as Basic Rate Interface (BRI: 2B+1D, D is a signalling channel with 16 kb/s) with maximum of 128 kbit/s (in both upstream and downstream directions) and Primary Rate Interface (PRI) with maximum T1 (23B+1D, here D is a signalling channel but with 64kb/s capacity) and E1 (30B+1D, here D is a signalling channel but with 64kb/s capacity) capacity.

The network capabilities of the ISDN (mainly low layer capabilities) are specified in ITU-T Recommendation I.324 as shown in Figure 4.



Figure 4 – Architecture model of ISDN

Low layer capabilities specifying transport capability of the ISDN are identified as following:



  • Circuit switching capability: Provides circuit-switched connections with information transfer rates based on B-channel (64 kb/s) carried by at the ISDN user-network interfaces and switched at 64 kb/s by the circuit-switching functional entities. This also be applied to information transfer rates greater than 64 kb/s and in the case of user bit rates of less than 64 kb/s are rate adapted to 64 kb/s before any switching can take place in the ISDN;

  • Packet switching capabilities: Provide number of packet mode bearer services based on two types of functional groupings;

  • Frame mode capabilities: Provide the order preserving bidirectional transfer of service data units (layer 2 frames) from one S or T reference point to another routed through the network on the basis of an attached label which contains a logical identifier with local significance, and;

  • Signalling capability: Signalling associated with circuit switched connections is carried by the D-channel at the ISDN user-network interface and processed by the local CRF (Connection Related Function).

Overall configuration model of ISDN is shown in Figure 5 and features of relevant functions are summarized as following:



Figure 5 – Overall configuration model of ISDN

  • Terminal Adapter (TA): A converter device that allows non-ISDN devices to operate on an ISDN network;

  • Terminal Equipment 1 (TE1): A device that supports ISDN standards and connected directly to an ISDN. For example, routers with integrated ISDN interfaces, ISDN telephones, personal computers, or videophones could function as TE1;

  • Terminal Equipment 2 (TE2): A non-ISDN device, such as a router, analog phone or modem, which requires a TA in order to connect to an ISDN network;

  • Network Termination 1 (NT1): A connection box terminates the connection from the Central Office (CO) and converts BRI signals for use by ISDN line signal, and;

  • Network Termination 2 (NT2): A device that provides switching services for the internal network (typically used with PRI), when they need to be divided for several functions. For example, some channels may be used for WAN (Wide Area Networks) data communications and others for the telephone system (such as PBX: Private Exchange) and/or video tele-conferencing.

The connection between two function groups (including cabling) is called a reference point and identified as followings:

  • U (U-interface): is the actual two-wire cable, also called the local loop, that connects the Customer Premise Equipment to the telecommunications provider;

  • R (R-interface): is the wire or circuit that connects the TE2 to the TA;

  • S (S-interface): is a four-wire cable from TE1 or TA to the NT1 or NT2;

  • T (T-interface): The point between the NT1 and NT2 used to divide the normal telephone company's two-wire cable into four-wires, which then allows the connection of up to eight ISDN devices and;

  • S/T: When NT2 is not used on a connection that uses NT1, the connection from the TE1 or TA to the NT1 connection is typically called S/T.

Comparing the previous networks such as PSTN and PSDN, ISDN has been designed for integrating various services not only voice but also multimedia. Benefits of ISDN are summarized following:

  • use the existing telephone wiring system (twisted pair-lines) without incurring additional costs and enhance wide area networks usage;

  • provide integrated access to telephone services, circuit-switched data and digital video by using the telephone network;

  • offer much faster call setup than modem connections by out-of-band (D channel) signalling;

  • provide a faster data transfer rate than modems by using the B channel including multiple B channels are bonded;

  • provide a clear data path over which to negotiate PPP (Point-to-Point Protocol) link;

  • offer “Pay-as you-Go” which means you only pay for the time that you use the link, and;

  • Telephone Company builds a totally digital network for increased bandwidth speeds.
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