Leased Line

Leased line restoral is either attempted after the expiry of a simple time out function when the back up line is inactive, or the modem incorporates active circuitry that monitors and tests the leased line continuously and restores at the first available opportunity.

From: Telecommunications Engineer's Reference Book , 1993

Modems

T J Egginton BEng (Sheffield University) , in Telecommunications Engineer's Reference Book, 1993

39.5.2.9 Auto call back and line restoral

For the leased line application, transferring large amounts of data between two points, a PSTN link can be used as a back up in the event of the leased line failure. Dial up to one or more destinations can be initiated automatically by the modem keeping the network on line. Leased line restoral is either attempted after the expiry of a simple time out function when the back up line is inactive, or the modem incorporates active circuitry that monitors and tests the leased line continuously and restores at the first available opportunity.

The second method is far superior, because it eliminates switching back to a faulty line and avoids using the back up circuit any longer than necessary. Ideally a visual indication of this back up mode is desirable to avoid excessive call charges generated by a consistently poor leased line service.

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Data Communications

HARVEY M. DEITEL , BARBARA DEITEL , in An Introduction to Information Processing, 1986

Polling with Multidrop Lines

Dedicated leased lines are generally used in multidrop configurations. One technique for determining which node (that is, terminal or computer) will transmit next is called polling (Figure 7-23).

Figure 7-23. Polling.

In polling, the communications controller successively tests each terminal on a multidrop line to see if that terminal wishes to transmit data to the central computer. If a terminal is not ready to transmit, then the next terminal is tried. If a terminal does indicate that it wants to transmit, transmission to the central computer is initiated when the terminal is polled.

The most active terminals may be polled more frequently than others to ensure they receive good service. Sometimes, however, a polled terminal that indicates it wants to send data may not be given the uninterrupted attention of the central computer. Its request may, for example, cause it to tie up a line indefinitely. In multiuser systems where all users must receive reasonable response times, such requests cannot be satisfied. Many systems therefore use a timer device to limit transmission time; when time runs out the line is automatically freed for use by other terminals.

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Time division multiplexing

Michael J Simmonds , Luc Ceuppens , in Telecommunications Engineer's Reference Book, 1993

21.8.2 BT's KiloStream

KiloStream is a point to point leased line digital service that was first offered commercially in January 1983. Like DDS, KiloStream is an all synchronous facility. BT provides a Network Terminating Unit (NTU) which is similar to a DSU/CSU to terminate the subscriber's line. The NTU encodes customer data for transmission via a digital local line to the local exchange where it is fed into a CEPT 2.048 Mbit/s multiplexer, as illustrated in Figure 21.14.

Figure 21.14. Basic KiloStream structure

The customer data rate can be 2400, 4800, 9600, 48000 or 64000bit/s. The DTE/NTU interface depends on the data rate. Speeds of 2400, 4800, and 9600bit/s can be serviced through a CCITT X.21 or CCITT X.21 bis/V.24 interface. 48kbit/s can be provided with a CCITT X.21 or CCITT X.21 bis/V.35 interface. A 64kbit/s channel is only available with an X.21 interface. At all data rates, except at 64kbit/s, the NTU encodes the user data into a 6+2 envelope structure as described in CCITT Recommendation X.50. Hence, the line data rate is higher than the customer data rate. For DTE data rates of 2400, 4800, and 9600bit/s the line data rate equals 12.8kbit/s, while a 48kbit/s user channel is transported at a 64kbit/s line rate.

Customer data is framed into a 6+2 format to provide the signalling and control information required by the network for maintenance assistance. This is known as envelope encoding and is illustrated in Figure 21.15. The rate of 8000 octets/s, imposed by the 8kHz sampling frequency of PCM systems, allows a total net rate of 48kbit/s per digital channel. One such channel could, for example, transport 5 sub-channels at 9600 bit/s, 10 sub-channels at 4800bit/s, or 20 sub-channels at 2400bit/s. CCITT Recommendation X.58 optimises this structure by minimising the signalling overhead and is able to transport up to 6 sub-channels at 9600bit/s.

Figure 21.15. KiloStream 8-bit envelope encoding

The NTU provides a CCITT interface for customer data at 2.4, 4.8, 9.6 or 48kbit/s to include performing data control and supervision, which is known as structured data. At 64kbit/s, the NTU provides a CCITT interface for customer data without performing data control and supervision, which is known as unstructured data.

The NTU controls the interface via CCITT Recommendation X.21, which is the standard interface for synchronous operation on public data networks. An optional V.24 interface is available at 2400, 4800 and 9600bit/s, while an optional CCITT V.35 interface can be obtained at 48kbit/s. With the X.21 interface, the control circuit (C) indicates the status of the transmitted information, data or signalling, while the indication circuit (I) signals the status of information received from the line. The control and indication circuits control or check the status bit of the 8-bit envelope used to frame six information bits.

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Surface Texture Knowledge Support – ISM

Robert Ohlsson , Bengt Goran Rosén , in Advanced Techniques for Assessment Surface Topography, 2003

12.7.1 Internet/World Wide Web

The internet is a global network where companies, universities, individual users etc are connected via modems or leased lines. The internet has been fully operational for the last twelve years and it is not "owned" by any specific organisation However the World Wide Web is a recent phenomenon that started seeing widespread use from the mid 1990s. Soon after the invention of advanced graphical web browsers like Mosaic and Netscape, companies and their customers gave the web their attention. The new tools made it easy to graphically transfer rich information with just a click with the mouse, instead of having to know a numerous cryptic commands that have to be typed in by hand.

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SCADA and smart energy grid control automation

K. Sayed , H.A. Gabbar , in Smart Energy Grid Engineering, 2017

18.9.4 Host to Outstation Hardware Communications

Serial communications to outstation devices can be implemented through several media: copper wire (RS485/RS232), fiber, radio, leased line, and even satellite. Leased telephone circuits, fiber, and satellites, however, have a comparatively high cost, and new radio technologies offer an attractive communications solution. One of such technologies is the multiple address radio system (MAS). The MAS usually operates in the range of 900  MHz and is omnidirectional, enabling radio coverage in an area with radius up to 25 miles, depending on terrain. A single MAS master radio can collect data from many remote sites to a concentrator. Communication protocol and bandwidth can limit the number of RTU that can be connected by a master radio. The protocol limit is simply the address range supported by the protocol. Bandwidth limitations can be substituted by the use of effective protocols, or by slowing down the scan rate to include more remote units. Combining spread-spectrum and point-to-point radio with MAS offers a chance to address-specific communication issues. MAS radio currently is preferred to packet radio; MAS radio communications tend to be more suitable for smaller timeout values on communication responses, scan time, and controls.

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ISDN

KOICHI ASATANI , TOSHINORI TSUBOI , in Multimedia Communications, 2001

10.2.3.1 Channel Types

A channel is a portion that carries information. The B channel, H channel, and D channel are specified, as shown in Table 10.3. The B and H channels are used for carrying user information, while the D channel is primarily used for carrying signaling information for circuit switching and may be used for carrying packet types of user information when it is not used for signaling.

Table 10.3. Channel Types

Channel Type Channel Rate Use
β 64 Kbit/s
•

User information stream

•

Circuit switching/packet switching/semi-permanent connection
D 16 Kbit/s 64 Kbit/s
•

Signaling information for circuit switching

•

Packet switched data
H0 384 Kbit/s
H H1 H11 1536 Kbit/s
•

User information stream

•

Circuit switching/packet switching/semi permanent connection
H12 1920 Kbit/s

10.2.3.1.1 B Channel

The B channel has 64 Kbit/s capacity. It carries a wide variety of user information streams for circuit switching, packet switching, and leased line. The B channel capacity of 64 Kbit/s is specified in harmonization with the Pulse Code Modulation (PCM) voice bit rate.

10.2.3.1.2 H Channels

H channels are intended to carry high bit-rate signals, such as video and data signals. An H0 channel and an H1 channel are specified. The capacity of H0 is 384 Kbit/s (= 6 × 64 Kbit/s). The capacity of the H1 channel is 1536 Kbit/s (= 24 × 64 Kbit/s) for 1.5 Mbit/s hierarchy countries or 1920 Kbit/s (= 30 × 64 Kbit/s) for 2 Mbit/s countries. The H1 channel with 1536 Kbit/s is designated as H11, and with 1920 Kbit/s as H12.

10.2.3.1.3 D Channel

A D channel primarily carries signaling information for circuit switching. The capacities of a D channel are 16 Kbit/s for a basic interface and 64 Kbit/s for a primary rate interface. User information for packet switching is also supported by the D channel.

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Electronics Elements (Detailed Discussion)

Thomas Norman CPP, PSP, CSC , in Integrated Security Systems Design (Second Edition), 2014

Digital

For enterprise systems, the drawbacks of circuit-switched networks present major problems. How do you maintain a circuit from New York to California? The cost of leased lines is too high, so digital intercoms are used. As digital video systems become more prevalent, it is also easier to "piggyback" voice communications on that digital path. Digital systems use packet-switched networks instead of circuit-switched networks for communications. Therefore, their path can be dynamic. If the organization wants to move its console, it only has to connect the new console to the nearest digital switch.

Digital intercoms have certain drawbacks too. For security, audio is more important than any other communication. Dropped audio could mean a lost life. So audio communications must be configured as the priority communication in order to ensure that no communication is lost as, for example, a new screen of video cameras is loading from one guard tour to the next. This is a programming element and cannot be configured in software if it is not written into the code. Audio compression protocols commonly include the G.7xx series, including G.711, G.721, G.722, G.726, G.728, and G.729. Another common protocol is the MPEG protocol, including MP-3. These are all UDP protocols. The software designer must ensure that the audio protocol is given priority of communications over the digital and data protocols. Where this is not done, the security officer can find it difficult to talk while video is loading. This condition is unacceptable, although at the time of this writing several video software manufacturers publish software with this flaw. Their common workaround is to require an additional client workstation just for audio. This is one of those design flaws that no one would buy if he or she knew about it ahead of time.

Digital audio has many advantages. Additional intercom stations can be added on anywhere an extra switch port exists, dramatically reducing wired infrastructure costs. Additional switches can be added at little cost in new infrastructure. For enterprise systems, the Internet or asynchronous transfer mode networks permit communications across state and national boundaries, making possible a monitoring center in one state that monitors sites throughout the world.

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Strategies for Implementing Hybrid E-Learning in Rural Secondary School in Uganda

P.O. Lating , ... L. Trojer , in Proceedings from the International Conference on Advances in Engineering and Technology, 2006

4.1 The SchoolNet Project

SchoolNet connected Internet to some schools at a capital cost of 30,340 USD. Generally VSAT connectivity methods are used with some schools connected using the Broad Spectrum technology. Dial-up and other wired Internet connectivity methods like ISDN, DSL, Leased Lines and Fiber Optic are not suitable for rural areas. They are narrow band and teledensity is low in rural schools. The project is mainly funded by donors especially the World Bank. However, such schools have problems in sustaining the project and cannot meet the recurrent monthly expenditure of 1,680 USD. And Internet in those schools is not being used for e-learning. Furthermore, the schools that SchoolNet chose are the best urban schools in the country with relatively good science laboratories, libraries, infrastructures and qualified teachers. SchoolNet intends to introduce a commercial, proprietary e-learning platform, the Blackboard. This is a very expensive platform to acquire (the cheapest version id at 12,000USD) and maintain. No rural secondary schools can afford this. Makerere University has also thrown it out.

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Computers and their application

Ian Robertson , in Mechanical Engineer's Reference Book (Twelfth Edition), 1994

4.15.3 Network types

There are a number of network types.

4.15.3.1 Point to point

This is the simplest form of network and involves the connection of two devices - two computers or a computer and a terminal. If the communication line goes down for any reason then the link is broken, and so it is usual to back up leased lines with dial-up facilities ( Figure 4.18).

Figure 4.18. A point-to-point link

4.15.3.2 Multi-point

As the name implies, Multi-point describes the connection of several tributary stations to one host. It is usual for the host to 'poll' the tributary stations in sequence, requesting messages, and for the network to be based on leased lines. In the case of one 'spur' being disconnected, the tributary station will dial into the host using a port reserved for that purpose ( Figure 4.19).

Figure 4.19. A multi-point communications network

4.15.3.3 Centralized

Also known as a 'star' network, in this type of network the host exercises control over the tributary stations, all of which are connected to it. The host may also act as a message-switching device between remote sites (Figure 4.20).

Figure 4.20. A typical centralized single-site network

4.15.3.4 Hierarchical

A hierarchical structure implies multiple levels of supervisory control. For example, in an industrial environment special-purpose 'micros' may be linked to the actual process equipment itself. Their function is to monitor and control temperature and pressure. These 'micros' will then be connected to supervisory 'minis' which can store the programs and set points for the process computers and keep statistical and performance records (Figure 4.21). The next link in the chain will be the 'resource management computers', keeping track of the materials used, times taken, comparing these with standards, calculating replenishment orders, adjusting forecasts and so on. Finally, at the top of the network, the financial control system records costs and calculates the financial performance of the process.

Figure 4.21. A hierarchical network

4.15.3.5 Fully distributed

Here a station may be connected to several others in the network. The possibility then exists to share resources such as specialized peripheral devices or large memory capacity and to distribute the database to the systems that access the data most frequently. It also provides alternative routes for messages when communication lines are broken or traffic on one link becomes excessive (Figure 4.22). However, the design of such systems requires sophisticated analysis of traffic and data usage, and even when set up is more difficult to control than less sophisticated networks.

Figure 4.22. A fully distributed wide area network

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Case Studies

XiPeng Xiao , in Technical, Commercial and Regulatory Challenges of QoS, 2008

QBone Premium Service

The QBone Premium Service (QPS) would make inter-domain, peak-limited bandwidth assurances with virtually no loss, and with virtually no delay or jitter due to queuing effects. The intent of the QPS service was to approximate the "virtual leased line" or "Premium" service proposed by Van Jacobson in [2Bit] and initially demonstrated across Abilene, ESNet, and the LBNL QoS testbed to the show floor of SuperComputing 2000 [CBQdemo]. QPS exploited the Expedited Forwarding (EF) per hop forwarding behavior. EF requires that the "departure rate of the aggregate's packets from any Diffserv node must equal or exceed a configurable rate" and that the EF traffic "SHOULD receive this rate independent of the intensity of any other traffic attempting to transit the node." EF may be implemented by any of a variety of queueing disciplines, but is best thought of in terms of forwarding EF packets through a strict priority queue. Services like QPS were built from EF through careful conditioning of EF aggregates so that the arrival rate of EF packets at any node was always less than that node's configured minimum departure rate. A QPS reservation {source, destination, route, startTime, stopTime, peakRate, serviceMTU} was an agreement to provide the transmission assurances of the QBone Premium Service starting at startTime and ending at endTime across the domain-to-domain chain route between source and destination (these are arbitrary network prefixes) for EF traffic entering at source and conforming to the token bucket traffic profile parameterized by:

•

token rate equal to peakRate bytes per second; and

•

bucket depth equal to serviceMTU bytes.

The transmission assurance offered by the QBone Premium Service is as follows:

•

Low loss: this should be very close to zero, but will not be quantified in the service definition.

•

Low latency: queueing delay will be minimized, but no assumption regarding minimal latency was made.

•

Low jitter: delay variation caused by queueing effects should be no greater than the packet transmission time of a service MTU-sized packet at the subscribed rate; no assumption was made about jitter due to other effects (for example, route instability).

Traffic exceeding the profile {peakRate, serviceMTU} would be dropped on ingress and not allowed to progress into downstream DS domains. Bilateral QPS reservations had the same structure as wide-area inter-domain QPS reservations and had comparable implications for the configuration of ingress traffic conditioners. Note that if two domains wanted to use different serviceMTUs, then reshaping must happen at the boundary if going from a larger to a smaller value. Consistent with the Diffserv architectural model, all QPS SLAs were determined bilaterally between adjacent QBone networks (dubbed "DS domains"). Although the agreements that defined SLAs were strictly bilateral, there were technical implications of the QPS described above that impose minimum requirements on QBone SLAs. These requirements, as well as certain recommendations regarding the EF codepoint and the routing of EF traffic, were discussed in the QBone Architecture document [QBarch].

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