Networks

• To enable data communications to take place, you need to connect to a network that can carry your signal.
• Commonly telephone networks are Analogue (sound based), though increasingly digital links using fibre optics are being implemented
• The following diagram illustrates analog and digital links in a communications network.

A Standard voice-grade telephone circuit. Dial-up features A Standard voice-grade telephone circuit is used for a Computer and Modem to communicate to another Node. This line may be used for Voice, Fax, Data, and allows transfer speeds up to 33.6 Kbps (with modifying techniques such as using the digital part of the line it is possible to get to 56Kbps on download). A Dial-Up line is used for Data Communications where ; Data communications requirement is only occasional (Eg ; Coolstores transmitting Tray, and Grower Data to the Kiwifruit Marketing Board) The quantity of Data transmitted is low (< 1 Mb / session ) Non-fixed receiver location, or remote Node. Note: An intelligent modem does the dialling (NOT the computer), most often HAYES software compatible.

A Data-grade line that allows data communication between 2 Fixed locations. Leased line features A Data-grade line that allows data communication between 2 Fixed locations at speeds up to 64Kbps. Is hooked up to an Organisations internal system for easy access from within the building, but this circuit is not used by any other Telecom Subscriber. It is dedicated to servicing the 2 fixed locations. A Leased line is used for Data Communications where ; There are 2 fixed locations that wish to communicate to each other There is constant Data Traffic between the 2 locations There is a high volume of data that is required to be transmitted

• Cellular mobile data service based on analogue AMPS technology, claimed to be as good as digital services.
• A Datalink accreditation program for modem suppliers to ensure compatibility.
• Telecom approved modems include data compression to the V42.bis standard, produces actual transmission speeds of more than three times the rated speed of the modem.
• Fleetlink trunked mobile radio services off the option of data over a dedicated radio link, which is good for truck drivers, who no longer have to wait their turn on a single voice channel, and know their business may be overheard.

• [Ref 98]Computerworld NZ, Apr 95

A Packet switched network is a special kind of data network containing Intelligent Switching Nodes. In essence, it is a Group of Computers throughout NZ, and the rest of the world that are connected together by Standard Telecom links. Normally leased lines (but can dialup via PSTN). A User of the Packet Network would simply dial-up the local Packet switching Computer (Data Switching Exchange - DSE), using a Computer, and Modem (if using an analog (standard telephone) connection), or use a permanent digital connection directly into the Packet Network, then Log on to the Packet Network Identify the Node that is to receive the data Begin transmitting the data.

• Prior to transmission, each data message is segmented into short blocks of a specified maximum length, and each block is provided with a header containing addressing and sequencing information.
• The Packets are passed very quickly from node to node, arriving in a fraction of a second at their final destination.
• The Node computers may not store the data.
• "Messages" are forgotten by the sending node as soon as the next node checks for errors and acknowledges receipt of the message.

• Uses X recommendations (CCITT)
• Is synchronous (HDLC) but can be used with asyncronous through an X3 pad ,( X3 Pad - assembles and disassembles async to HDLC sync )
• X25 PSN (packet switch Network) corresponds to the three lower layers of the OSI (see network file)

• A DSE is a network node joining 2 or more links.
• The DSE is the exchange that Users access to get onto the Packet Network.

• For Data applications in which the amount of traffic between terminals cannot justify a dedicated circuit (Leased Line), packet switching may be more economical.
  • Example: Public Libraries (Periods of intermittent high use).
• For applications in which data communication sessions are shorter than the minimum chargeable time unit for a telephone call, packet switching may be more economical than dialled data.
  • Example: Email systems (short Email messages are sent frequently).

• Each packet of data is sent over the network in the most efficient possible path to the destination which can be used to maximise efficiency, and minimise congestion.
• Further, this allows for automatic re-routing in case of a failure in a node on the network.
• Telecom Pacnet

• Provide a higher transmission rate
• Maintain a higher level of data integrity

• Data moves at higher speeds, allowing transmission of higher volumes
• Data Error rates are greatly reduced

• An NTU (Network Terminating Unit) device replaces the modem in the network. Most NTU's support either sync or async data formats and can handle speeds up to 56 Kbps in point-to-point or multipoint configurations.

• Telecom Digital Data Services

• Dial-up or Leased digital phone network
• Uses the same copper cable as an ordinary phone line.
• Three channels
• This all digital network, provides user access via a single port or wall plug.
• Slowest 64k Kbps (Kilo bits per sec) - v's 36.6Kbps for the fastest modem, and doubles as the bandwidth increases
• From Telecom (1996) Basic rate 64/128 Kbps, to Primary Rate (2Mbps and up) - used by large organisations to carry a swag of Basic Rate connections simultaneously.
• Plug in a small box as opposed to the ordinary wall socket.
• More stable and reliable connection than standard analog. Due to the digital signal more resistant to line noise.

• Audio and video
• Computer data
• Videotext
• telex and facsimile
• telemetry service and high speed data transmission
• Telecom ISDN

• Global System for Mobile communications (GSM) fully digital technology, with asynchronous circuit-switched data channel at 300bit/s to 9600bit/s (9.6Kb/s), fully error corrected and encrypted against tapping

• Connecting to office email and Internet
• Real estate agents connecting to a central property listings database.
• Online library book enquiries to the central library from mobile library service vans.
• [Ref 98]Computerworld NZ, Apr 95

Code Division Multiple Access (CDMA)

Telecom (NZ), AAPT (Australia) - due 2001

Works by assigning a unique digital code to each userAccording to Telecom will allow for wireless data speeds of 144kbit/s, eventually to 2Mbit/sec. Broatch, M (2000, May 29) 

References
Broatch, M (2000, May 29), CDMA rings in changes, Computerworld NZ, pp 23-25. 

• Work over regular phone lines (copper wires) using digital modems.
• Allow data speeds currently (Feb 1998) of 6-8Mbit/s using a special modem and switching technology .
• Theoretically able to download data at up to 9Mbit/s, though expected to reach about 1.5Mbit/sec.
• Uploads can be 10 times slower than downloads.
• Maximum length 2km between local modem and exchange.
• Simultaneously supports telephone and data.
• Made up of three components
  • A customer premises digital modem
  • A teleco digital modem and
  • A concentrator that aggregates traffic from multiple modems for delivery to a data network while routing voice traffic to a voice switch)
• Comes in different "flavours"
  • HDSL (High-bit-rate DSL) uses two or three pairs of copper wire to form a T1 (1.5Mb/s) or E1 (2Mb/s) connection.
  • Single pair HDSL - single twisted pair to transmit at 768kb/s in both directions.
  • ISDN DSL - 128kb/s using an ISDN terminal adaptor. ISDL is packet switched while ISDN is circuit switched.
  • VDSL - Very-high speed DSL) 52Mb/s in development
  • CDSL - Consumer DSL - a single phone line allowing 128kb/s upstream and 1Mb/s downstream to the user.

• Malcolm, A (1998, Feb 2)(1997, Aug 4)
• From www.adsl.com Aug 1998

[Rev: 05/06/00 ] 4/8/97 © 1997-2000 V/2-Com (Verhaart), P O Box 8415, Havelock North, New Zealand.