9 Data Networks
Manoj Kumar
I. Objectives
Objective of this module is to introduce how normal telephone lines are used for data communication. Telephone lines can be used with various equipment to provide data services in addition to voice. The digital technologies used for such integrated services will be discussed in details which include ISDN, ATM, DSL etc. At the end of the module, a student will be able to understand the basic knowledge about the technologies behind various data network solutions and its public use.
II. Learning Outcomes
After going through this lesson, learners would develop understanding about functioning of data networks and tools and techniques involved in using normal telephone lines for data communication. They will learn about digital technologies that are used for developing integrated services and their applications such as ISDN, ATM, DSL, etc. Learners would be equipped with basic knowledge on technologies behind various data network solutions and its public use.
III. Module Structure
1. Introduction
2. Integrated Services Digital Network (ISDN)
2.1 SDN Principle
2.2 SDN Services
2.2.1 Bearer Services
2.2.2 Teleservices
2.2.3 Supplementary Service
2.3 SDN Interfaces
2.3.1 sic Rate Interface (BRI)
2.3.2 Primary Rate Interface (PRI)
2.3.3 Broadband-ISDN (B-ISDN) and ATM
2.4 SDN Architecture
2.5 ATM (Asynchronous Transfer Mode)
2.5.1 ATM Architecture
3. Digital Subscribers Line (DSL)
3.1 Asymmetric DSL (ADSL)
3.2 Symmetric DSL (SDSL)
3.3 High-bit-rate DSL (HDSL)
3.4 Very high bit-rate DSL (VDSL)
4. Summary
5. References
1. Introduction
Alexander Graham Bell invented telephone in 1876, which is used for long distance telephone calls later over a pair of copper wires. The plain old telephone service (POTS) is based on analogue signal transmission. Based on this technology, the advanced forms of telephony services such as Integrated Services Digital Network (ISDN), cellular telephone systems, and voice over Internet Protocol (VoIP) etc. were introduced. The advantage of POTS is that a simple connection in its basic form is used in residence as well as small business. The technology of POTS is characterized by bi-directional communications, frequency range of the human voice, signalling using call-progress tones, such as dial tone and ringing signal, subscriber dialling etc. The pair of wires from the central switch office to a subscriber’s home is used for service which is called a subscriber loop. The communication circuits of the public switched telephone network could be modernized by advances in digital communications. The basic telephone network is working based on the principle of circuit switching, which is called circuit switched networks. There could be dedicated circuit between two points. Instead of copper wires, same circuit can be established with the optical fibre cable also, based on the distance and architecture required. Advanced technology can be introduced in the same line to make it as packet switched networks. These technological solutions can enable the ordinary telephone links for data communication also. Such advanced technological solutions like ISDN, ATM, DSL for data networks are discussed in this module.
2. Integrated Services Digital Network (ISDN)
Public telephone networks (PSTN) and carriers can incorporate circuit switched and packet switched networks. Telephone and other service equipment can be connected to the cloud network through the public carriers using respective equipments. Analogue phones, facsimile, modem and other devices can be connected to the nearest LEC (Local Exchange Carrier) which is connected to the public carriers. Depending upon the technologies used, the transmission will use the carrier which is specified with each transmission. Analogue phone calls can be made available through circuit switched networks and data transmission can be done through packet switched networks which uses same carrier channels.
2.1 ISDN Principle
ISDN is a WAN (Wide Area Network) technology combining best features of circuit- switching and packet-switching technologies. Voice, data and video services can be integrated on this technology by using the same public carriers. ISDN is such a solution for integrating all digital services on the normal telephone carriers. It uses PSTN with a cloud architecture, meaning that users connect to a network and what happens inside of the network “cloud” is hidden from the user. A user using a computer and a modem dials the number of another computer and creates a temporary circuit between the two. When the communications session is completed, the circuit is disconnected.
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. It offers circuit-switched connections (for either voice or data), and packet-switched connections (for data), in increments of 64 Kbit/s. Development of ISDN is governed by a set of recommendations issued by Integrated Telecommunication Union ITU-T, called I-series recommendations, which are based on services offered to users, user- network interfaces (UNI), ISDN capabilities etc.
The principle of ISDN according to ITU –T is as stated below:
• The ISDN is supported by a wide range of voice and non-voice applications of the same network. It provides a range of services· using a limited set of connections and multipurpose user-network interface arrangements.
• ISDN supports a variety of applications that include both switched and non- switched connections. The switched connections include both circuit and packet switched connections.
• As far as possible, new services introduced into an ISDN should be arranged to be compatible with the 64 Kbps switched digital connections.
• A layered protocol structure should be used for the specification of access to an ISDN.
This is the same as the OSI reference model. The standards which have already been developed for OSI applications such as X.25 can be used for ISDN. (Please refer Module M-11 ISO-OSI and TCP/IP module). ISDNs may be implemented in a variety of configurations.
2.2 ISDN Services
The purpose of the ISDN is to provide fully integrated digital services to users. These services fall into categories- bearer services, teleservices and supplementary services.
2.2.1 Bearer Services: Bearer services provide the means to transfer information (voice, data and video) between users without the network manipulating the content of that information. The network does not need to process the information and therefore does not change the content.
Fig.1: Bearer Services
Bearer services belong to the first three layers of the OSI model and are well defined in the ISDN standard. They can be provided using circuit-switched, packet-switched, frame-switched, or cell-switched networks (ATM).
2.2.2 Teleservices: In teleservices, the network may change the contents of the data. These services correspond to layers 4-7 of the OSI model. Teleservices relay on the facilities of the bearer services and are designed to accommodate complex user needs, without the user having to be aware of the details of the process. Teleservices include telephony, teletex, telefax, videotex, telex and teleconferencing.
2.2.3 Supplementary Service: Supplementary services are those services that provide additional functionality to the bearer services and teleservices. Examples of these services are reverse charging, call waiting, and message handling, all familiar from today’s telephone company services.
2.3 ISDN Interfaces
There are several kinds of access interfaces to the ISDN Basic Rate Interface (BRl)
Primary Rate Interface (PRl) Broadband-ISDN (B-ISDN)
2.3.1 Basic Rate Interface (BRI): Basic Rate Interface service consists of two data- bearing channels (‘B’ channels) and one signalling channel (‘D’ channel) to initiate connections. The B channels operate at 64 Kbps maximum.
The D channel operates at a maximum of 16 Kbps. The two channels can operate independently. For example, one channel can be used to send a fax to a remote location, while the other channel is used as a TCP/IP connection to a different location.
The basic rate interface (BRl) specifies a digital pipe consisting of two B channels and 16 Kbps D channel. Two B channels of 64 Kbps each, plus one D channel of 16 Kbps, equal 144 Kbps. In addition, the BRl service itself requires 48 Kbps of operating overhead. BRl therefore requires a digital pipe of 192 Kbps. Conceptually, the BRl service is like a large pipe that contains three smaller pipes, two for the B channels and one for the D channel.
2.3.2 Primary Rate Interface (PRI): Primary Rate Interface service consists of a D channel and either 23 B channels (depending on the country). The usual Primary Rate Interface (PRI) specifies a digital pipe with 23 B channels and one 64 Kbps D channel. Twenty-three B channels of 64 Kbps each, plus one D channel of 64 Kbps equals 1.536 Mbps. In addition, the PRI service itself uses 8 Kbps of overhead. PRI therefore requires a digital pipe of 1.544 Mbps. Conceptually, the PRI service is like a large pipe containing 24 smaller pipes, 23 for the B channels and 1 for the D channel. The rest of the pipe carries the overhead bits required for its operation.
2.3.3 Broadband-ISDN (B-ISDN) and ATM: Narrowband ISDN has been designed to operate over the current communications infrastructure, which is heavily dependent on the copper cable. B-ISDN however, relies mainly on the evolution of fiber optics. B-ISDN would be able to provide end users with increased transmission rate, up to 155.54 Mbits/s on a switching basis.
(Reference: http://ecomputernotes.com/computernetworkingnotes)
One of the fundamental principles of the B-ISDN is to offer subscribers a large variety of services such as video telephony, video surveillance, high volume file transfer, High Definition Television (HDTV) and many more services not offered by N-ISDN.
2.4 ISDN Architecture
Fig. 6: ISDN Architecture
2.5 ATM (Asynchronous Transfer Mode)
ATM stands for Asynchronous Transfer Mode which is a high speed networking ITU standard, similar to technologies discussed above, which support voice, video and data communications. ATM networks are connection-oriented. ATM technology design to improve utilization and Quality of Service (QoS) on high traffic network. ATM operates at a data link layer (layer 2 in the OSI model ) . Ethernet and other technologies, uses variable-length packets and use routing techniques in network for data transmission but ATM uses fixed -sized cells (53 bytes) for data transfer and no routing techniques. 53 bytes ATM cells includes 48 bytes of data and 5 bytes of header information as shown in figure below. Performance of ATM is expressed in the form of OC (Optical Carrier) levels, written as ‘OC-xx’ and ‘DS-xx’ for normal links. Common levels for ATM are 155 Mbps (OC-3) and 622 Mbps (OC-12), but 10 Gbps performance level is also feasible. The scalable speeds of 1.544 Mbps (DS-1) and 6.312 Mbps (DS-3) are also common. ATM provides scalable bandwidth from Mbps to Gbps due to its asynchronous nature and time slots are available on demand.
The technology integrated data, voice and video with fixed packets length (called cells). This technology is combined with circuit and packet switching networks. Best features like fixed path, known delay and guaranteed quality of service (QoS) is borrowed from circuit switching. The concept of using bandwidth when needed is borrowed from packet switching. The benefits of ATM are the following.
• High performance wire hardware switching
• Dynamic bandwidth for burst traffic
• Class-of-service support for multimedia
• Scalable speed and network size
• Common LAN/WAN architecture
• Opportunities for simplification via VC architecture
• International standards compliance
2.5.1 ATM Architecture
ATM architecture consist of three layers such as Physical layer, ATM layer and Adaptation Layer. ATM layer is analogous to the data link layer of the OSI model. ATM layer is responsible for the simultaneous sharing of virtual circuit (VC) over physical link and passing cell through ATM network to do this, it uses Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI) information in the header of each ATM cell. ATM define a path before transmission starts and path is called virtual circuits (VC). The technology supports and combines voice with real time and low delay, video with high bandwidth, low delay and jitter and data transfer for internet services. ATM allows multiple traffic stream to share the same physical path.
Adaptation layer is only at the edge of ATM network for data segmentation and reassembly of data. ATM Adaptation Layer (AAL) form upper layers to ATM layer below. This is not available in ATM switch, but only in end systems. Different versions of AAL layers are available depending on ATM service class. AAL-1 uses constant bit rate (CBR) for services for e.g. circuit emulation. AAL-2 uses variable bit rate (VBR) for services like MPEG videos. Low overhead AAL is used to carry IP datagrams, which is called AAL-5. Forming of 48 bytes data packets is the job of AAL-5.
3. Digital Subscribers Line (DSL)
Telephonic services are provided to home users through copper wires. DSL is a medium for transferring data over normal telephone copper lines. This can be used to connect to the Internet. Therefore, DSL is family of technologies that provides Internet access by transmitting digital data over the wires of a local telephonic network. Generally, modems are used for getting Internet connection. Modem is an equipment which convert analog signal to digital signal and vice-versa. DSL circuit is much faster than regular phone lines even though it uses normal copper wire. The same phone jacket can be used for DSL modem with a splitter. To get a connection one need to contact local ISP (Internet Service Provider) who provides Internet services. Phone line splitter can be connected to phone line wall jack. The splitter will have two RJ-11 sockets, For Example, one for DSL modem and another for telephone. A phone line filter is used to connect to telephone from the splitter. One connection will go to the DSL modem and modem will be connected to the computer through an Internet cable. The sample connection is shown in the diagram. Generally, DSL connections are called Broadband connection. There are different technologies of DSL, such as ADSL ( Asymmetric) , SDSL (Symmetric), HDSL (High-bit-rate ), VDSL (Very high bit-rate).
3.1 Asymmetric DSL (ADSL)
ADSL type of Broad Band communication technology used for connecting to the Internet as mentioned above. ADSL allows more data to be sent to over existing copper line when compared to traditional modem line. As special filter is to be installed on subscriber telephonic line to allow voice and Internet services at the same time. subscriber must be geographically close to the ISP preferably within a radius of 2 to 2.5 miles. Radius are support data rates from 1.5 to 9 Mbps when receiving data (downstream) and 16 to 640 Kbps when sending data (Upstream). It is called asymmetric because the download and upload speeds are not symmetric. For voice service, the frequency reach 0KHz and 4KHz and 20KHz to 2.2 MHz; for DSL service in some cases data rates can go up to 52Mbps.
3.2 Symmetric DSL (SDSL)
SDSL is technology developed in Europe and functions by transmitting digital pulses in the high frequency area of telephone wire. This cannot provide voice service on same wires. SDSL needs a special modem and allows equal bandwidth downstream from ISP to customer premises and upstream from the subscriber to the ISP. SDSL can transmit up to 1.54 Mbps. Due to the symmetric speed, this can be used as WAN technology for small and medium businesses. Application such as web costing, file transfer and distance learning can be deployed with SDSL.
3.3 High-bit-rate DSL (HDSL)
HDSL is communication protocol standardizing in 1994. This is the first DSL technology to use a high frequency of copper cables. HDSL gives 1.544 Mbps for DS1 services in America and 2.048 Mbps over telephone without a need for repeaters. Unlike ADSL, HDSL operates in the baseband and does not allow both services to coexist on the same wires.
3.4 Very high bit-rate DSL (VDSL)
VDSL line lengths are generally between 150 mts and 2000 mts. VDSL provides high amount of bandwidth with speeds up to 52 Mbps while ADSL can give maximum of 8 to 10 Mbps. In India, MTNL as well as BSNL provides high speed combo broadband plans on VDSL technology. It uses frequency band from 25 KHz to 12 MHz. Second generation VDSL provides data rates exceeding 100 Mbps with frequency up to 30 MHz. VDSL is capable of supporting applications such as High Definition Television, Voice over IP, Internet access etc.
4. Summary
Copper cable, despite its limitations, is a very good medium of communication. The technology has built around this wire by utilizing its full potential by carrying different frequencies of signals for data networks. A normal telephone cable could be used with various equipment to provide data services. A circuit switching technology as well as packet switching technology is used to transfer data over telephone networks with sophisticated digital technologies such as ISDN, ATM, DSL, etc. Introduction of Optical cable has revolutionized the communication speed & bandwidth requirement in the data networks. Various options with DSL have given tailor made choices to users for optimum implementation of data networks. This module discussed basic as well as detailed technologies available for data networks.
5. References
2. DSL Advances by Thomas Starr, Massimo Sorbara, John M. Cioffi, Peter J. Silverman from Prentice Hall Proffessional.
3. http://www.telecom.otago.ac.nz/tele201/handouts/lec20_h.pdf
4. http://ecomputernotes.com/computernetworkingnotes/network-technologies/
5. http://www.doc.ic.ac.uk/~nd/surprise_95/journal/vol2/vm4/article2.html
6. http://compnetworking.about.com/od/networkportocols/
7. http://shareengineer.blogspot.in/2012/10/asynchronous-transfer-mode.html;
Additional References
1. http://www.cis.ohio-state.edu/~jain/cis788-95/isdn/index.html
2. http://www.protocols.com/pbook/isdn.htm
Book
- ISDN and Broadband ISDN with Frame Relay and ATM, 4/E By William Stallings http://www.pearsonhighered.com/educator/product/ISDN-and-Broadband-ISDN- with-Frame-Relay-and-ATM/9780139737442.page
- ISDN: How to Get a High-Speed Connection to the Internet By Charles Summers (Author), Bryant Dunetz (Author) http://books.google.co.in/books/about/ISDN.html?id=phpOAQAAIAAJ&redir_esc =y
- ISDN: concepts, facilities, and services By Gary C. Kessler http://www.garykessler.net/library/isdnbook.html
- ISDN By Raj Jain, Professor of CIS, The Ohio State University, Columbus, OH 43210 http://www.cis.ohio-state.edu/~jain/
- ISDN protocol reference model By International Telecommunication Union (CH) https://www.itu.int/rec/T-REC-I.320-199311-I/en