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156 Satellite Networking: Principles and Protocols signalling is attractive, but one drawback is that when channel patching is required, signalling leads have to be patched as well. 4.4.5 Associated and disassociated channel signalling Traditionally, signalling goes along with the traffic on the same channel it is associated with on the same media. This signalling may or may not go on the same media or path. Most often, this type of signalling is transported on a separate channel in order to control a group of channels. A typical example is the European PCM E1 where one separate digital channel supports all supervisory signalling for 30 traffic channels. It is still associated channel signalling if it travels on the same media and path as its associated traffic channels. If the separated signalling channel follows a different path using perhaps different media, it is called disassociated signalling. See Figure 4.6. ITU-T Signalling System No. 7 (ITU-T SS7) always uses separated channels, but can be associated and disassociated. Disassociated channel signalling is also called non-associated channel signalling. 4.4.6 ITU-T signalling system No. 7 (ITU-T SS7) ITU-T SS7 was developed to meet the advanced signalling requirements of the all-digital network based on the 64 kbit/s channels. It operates in a quite different manner than Switch Network Exchange A SF SF SF SF sender Signal Processor Exchange B Signal Switch Network SF SF SF SF receiver Processor Trunks (a) Conventional associated channel signalling. Switch Network Exchange A Exchange B Switch Network Processor Trunks (b) Separate channel signalling with common channel signalling (CCS). Processor CCS Terminal CCS Terminal Signalling Figure 4.6 Associated and separate signalling Satellite Internetworking with Terrestrial Networks 157 conventional signalling systems. Nevertheless, it must provide supervision of circuits, address signalling, call progressing and alert notification. It is a data network entirely dedicated to interswitching signalling, and can be summarised as the following: • it is optimised for operation with digital networks where switches use stored-program control (SPC); • it meets the requirements of information transfer for inter-processor transactions with digital communication networks for call control, remote control, network database access and management, and maintenance signalling; and • it provides a reliable means of information transfer in the correct sequence without loss or duplication. Since 1980, it has become known as the signalling system for ISDN. The SS No.7 network model consists of network nodes, termed signalling points (SP), which are interconnected by point-to-point signalling links, with all the links between two SPs called a link set. Messages between two SPs may be routed over a link set directly connecting the two points. This is referred to as the associated mode of signalling. Messages may also be routed via one or more intermediate points relaying the messages at the network layer. This is called the non-associated mode of signalling. It supports a special case of static routing, called quasi-associated mode, in which routing only changes in response to events such as link failures or addition of new SPs. The function of relaying messages at the network layer is called the signalling transfer points (STP). There are certain relationships between the SS No.7 and the OSI/ISO reference model as illustrated as Figure 4.7. It can be seen that SS No.7 has three layers corresponding to layers 1–3 of the OSI/ISO reference model within the communication networks. The application processes within a communication network invoke protocol functionality to communicate with each other in much the same way as ‘end users’. The signalling system also encompasses operation, administration and maintenance (OAM) activities related to communications. Sublayer 4 of SS No.7 corresponds to OSI layer 4 upward, and consists of user parts and the signalling connection control part (SCCP). There are three user parts: telephone user part (TUP), data user part (DUP) and ISDN user part (ISDN). Layers 1–3 together make up the message transfer part (MTP). The SCCP User parts Signalling Connection Control Part (SCCP) Signalling network functions Signalling link control Signalling data link OSI/ISOSS7 No. 7 Application Presentation Session Transport Network Data Link Physical 4 3 2 1 7 6 5 4 3 2 1 Figure 4.7 Relationship between the SS No.7 and OSI/ISO reference model 158 Satellite Networking: Principles and Protocols provides additional functions to the MTP for both connection-oriented and connectionless services to transfer circuit-related and non-circuit-related signalling information between switches and specialised centres in telecommunication networks via SS No.7 networks. It is situated above the MTP in level 4 with the user parts. 4.4.7 Network management In the OSI reference model, there are five categories for network management functions defined as the following: • configuration and name management; • performance management; • maintenance management; • accounting management; and • security management. Configuration and name management comprise a set of functions and tools to identify and manage network objects. The functions include the ability to change the configuration of objects, assign names to objects, collect state information from objects (regularly and in emergencies) and control states of objects. Performance management comprises a set of functions and tools to support planning and improve system performance, including mechanisms to monitor and analyse network performance and QoS parameters, and control and tune the network. Maintenance management comprises a set of functions and tools to locate and deal with abnormal operation of the network, including functions and mechanisms to collect fault reports, run diagnostics, locate the sources of faults, and take corrective actions. Accounting management comprises a set of functions and tools to support billing for the use of network resources, including functions and mechanisms to inform users of costs incurred, limit use of resources by setting a cost limit, combine costs when several network resources are used, and calculate the bills for customers. Security management comprises a set of functions and tools to support management functions and to protect managed objects, including authentication, authorisation, access control and encipherment and decipherment and security logging. Please note that security management is more to provide security for the network than user information. 4.4.8 Network operation systems and mediation functions Network management is implemented in network operation systems including user specific functions and common functions; the later are further subdivided into infrastructure functions and user generic functions. Infrastructure functions provide underlying computer-related capabilities which support a wide range of processes. These include such services as physical communications and message passing, data storage and retrieval and human–machine interface (such as in a workstation computer with windows). Satellite Internetworking with Terrestrial Networks 159 User-generic functions are general utilities in the network operation systems (NOS). They can support a number of user-specific functions. Some of the generic functions are listed in the following as examples: • Monitoring: to observe the system and basic system parameters at a remote site. • Statistics, data distribution and data collection: to generate and update statistics, to collect system data and to provide other functions with system data. • Test execution and test control: independent of the purpose of test, whether it is done to detect a fault or to prove the correct operation of unit or an element, a test is performed in the same way. Tests are used by maintenance installation of equipment or new features, performance management and normal operations. Configuration control and protection actions might be involved if the test uses additional network resources to minimise the resources used for tests and maximise system availability during the test. • Configuration management: to keep track of the actual configuration of the network and to know about valid network or network element configurations. To reconfigure the network or a network element or to support reconfiguration if it is necessary. Network operation systems (NOS) involve four layers of management functions: business management, service management, network management and element management with business at the top of the layers and element at the bottom as shown in Figure 4.8. • Business management includes functions necessary to implement policies and strategies with the organisation owning and operating the services and possibly also the network. These functions are influenced by still higher levels of control such as legislation or macro- economic factors and might include tariff policies and quality management strategies, which give guidance on service operation when equipment or network performance is degraded. Many of these functions may not initially be automated. • Service management looks after particular services such telephone, data, Internet or broad- band services. The service may be implemented across several networks. The functions Business Management Service Management Network Management Network Element Management Network Element Each la y er mana g es multiple occurrences of the la y er below Figure 4.8 Layers of management functions in network operation systems (NOS) 160 Satellite Networking: Principles and Protocols may include customer-related functions (e.g. subscription record, access rights, usage records and accounts) and establishment and maintenance of the facilities provided by the service itself additional to the network facilities. • Network management provides functions to manage the network in question, including network configuration, performance analysis and statistical monitoring. • Element management provides functions to manage a number of network elements in a region. These functions are most likely to focus on maintenance but could also include configuration capability and some statistical monitoring of the network elements. It does not cater for network wide aspects. The mediation function (MF) acts on information passing between network element functions and the operation systems functions (OSF) to achieve smooth and efficient com- munication. It has functions including communication control, protocol conversion and data handling, and communication primitive functions. It also includes data storage and processing involving decision making. 4.5 Access and transit transmission networks According to ITU-T recommendation Y.101, access network is defined as an implementation comprising those entities (such as cable plant, transmission facilities, etc.) which provide the required transport bearer capabilities for the provision of telecommunications services between the network and user equipment. Transit network can be considered as a set of nodes and links that provide connections between two or more defined points to facilitate telecommunication between them. The interface has to be well defined in terms of capacity and functionality to allow independent evolutions of user equipment and the network, and new interfaces have to be developed to accommodate new user equipment with large capacity and new functionality. The evolution of access and transit networks can be seen from analogue transmission from telephone networks, to digital transmission telephony networks, synchronous transfer mode in transit network, integration of telephony networks and data ISDN, Internet networks, broadband networks in B-ISDN, etc. 4.5.1 Analogue telephony networks Although almost all of today’s networks are digital, the connections from many residential homes to the local exchanges are still in analogue transmission. They are gradually fading away with the installation of broadband access networks such as asymmetric digital sub- scriber line (ADSL). ADSL is a modem technology that converts twisted-pair telephone lines into access paths for multimedia and high-speed data communications. The bit rates transmitted in both directions are different with a typical ratio of 1 to 8 between user terminal and local switch. We discuss analogue telephony networks not because the technology itself is important for the future, but because the principles of design, implementation, control, management and operation developed with the network have been used for many years, are still very important to us today, and will continue to be important in the future. Of course these principles have to be used and developed in the new network context. Satellite Internetworking with Terrestrial Networks 161 The telephony networks were well designed, well engineered and optimised for telephony services. In the context of available technologies and knowledge, the user service was telephony, the network resource was channel, and bandwidth of 4 kHz was allocated to each channel to support good acceptable quality of service. 4.5.2 Telephony network traffic engineering concept The networks were dimensioned to provide the service to a large number of people (almost all the homes and offices today) with 4 kHz channels, taking into account factors of economics such as user demands and costs of the network to meet the demands. There were well- developed theories to model user traffic, network resource and performance of the network and grade of service. • Traffic is described by patterns of arrivals and holding times. Traffic is measured in Erlang, named after the Danish mathematician for his contribution to telephony network traffic engineering. The Erlang is a dimensionless unit. Erlang is defined as a product of number of calls A and average holding time in hours H of these calls: A ×H Erlang. One Erlang represents one call lasting for one hour or one circuit is occupied for one hour. The patterns of call arrivals and holding times are stochastic in nature, hence described by statistical methods in terms of probability distributions, means, variance, etc. Traffic varies in time in different time scales: instantaneously, hourly, daily, seasonal, trend with a gradual increase. • The network can provide full availability of resources to meet all the traffic requirements but is expensive or has limited availability to meet most requirements economically. The network can also allow traffic to queue to wait for network resources to be available or give priority or some kinds of treatments to a portion of the traffic. • Performance criteria allow quantitative measurement of network performance with param- eters including: probability of delay, average delay, probability of delay exceeding a range of time values, number of delayed calls and number of blocked calls. • Grade of service is one of the parameters used to measure probability of loss of calls to be achieved by the network and expected by users as acceptable quality of service. There are well-established mathematical theories to deal with these factors in classical scenarios in terms of call arrivals and holding-time distribution, number of traffic sources, availability of circuits and handling of lost calls. Some of the mathematical formulas are simple and useful and can be summarised as the following: • Erlang B formula to calculate the grade of service E B  is: E B = A n /n n  x=0 A x /x! where n is number of circuits available and A is the mean of the traffic offered in Erlang. The formula assumes an infinite number of sources, equal traffic density per source and traffic lost call cleared. 162 Satellite Networking: Principles and Protocols • Poisson formula to calculate the probability of lost calls or delayed calls P because of insufficient number of channels n with the traffic offered A is: P = e −A   x=n A x x! The formula assumes an infinite number of sources, equal traffic density per source and lost calls held. • Erlang C formula is: P = A n n! n n−A n−1  x=0 A x x! + A n n! n n−A The formula assumes an infinite number of sources, lost call delayed, exponential holding times and calls served in order of arrival. • Binomial formula is: P =  s −A s  s−1 s−1  x=n  s −1 x  A s −A  x The formula assumes a finite number of sources s, equal traffic density per source and lost calls held. 4.5.3 Access to satellite networks in the frequency domain In the frequency domain, we can see each signal telephony channel is allocated a bandwidth of 4 kHz to access the local exchange, or many of the single channels are multiplexed together to form the transmission hierarchy. To transmit the telephony channel over satellite, a carrier has to be generated which is suitable for satellite radio transmission on the allocated frequency band and channel signal modulating the carrier can be transmitted over satellite. At the receiving side, the demodulating process can separate the channel signal from the carrier; hence the receiver can get back the original telephony signal to be sent to a user terminal or to a network which can route the signal to the user terminal. If a single channel modulates the carrier, we call it single carrier per channel (SCPC), i.e., each carrier carries only a single channel. This is used normally for user terminals to be connected to the network or other terminals as an access network. It is also possible to use this as a thin route to connect a local exchange to the network where the traffic density is low. If a group of channels modulate the carrier, we call it multi channel per carrier (MCPC). This is normally used for interconnect between networks as a transit network or local exchange to the access network. 4.5.4 On-board circuit switching If all connections between earth stations used single global beam coverage, there would be no need to have any switching functions on-board satellite. If multiple spot beams are used, Satellite Internetworking with Terrestrial Networks 163 Spot beam Spot beam Global beam coverage Figure 4.9 Illustration of on-board circuit switching there are great advantages to using on-board switching, since it allows the earth stations to transmit multiple channels to several spot beams at the same time without separating these channels on the transmitting earth stations. Therefore, on-board switching will give satellite networks great flexibility and potentially save bandwidth resources. Figure 4.9 illustrates the concept of on-board switching with two spot beams. If there is no on-board switching function, the two transmissions have to be separated at the transmission earth station by using two different bent-pipes, one of which is for connection within the spot beam and the other is for connection between the spot beams. If the same signal is to be transmitted to both spot beams, it will require two separate transmissions of the same signal; hence it will need twice the bandwidth at the uplink transmissions. It is also possible to reuse the same bandwidth in different spot beams. By using on-board switching, all the channels can be transmitted together and will be switched on-board satellite to their destination earth stations in the different spot beams. Potentially, if the same signal is to be sent to different spot beams, the on-board switch may be able to duplicate the same signal to be sent to the spot beams without multiple transmissions at the transmitting earth station. The same frequency band can be used in the two spot beams by taking appropriate measures to avoid possible interferences. 4.6 Digital telephony networks In the early 1970s, digital transmission systems began to appear, utilising the pulse code modulation (PCM) method first proposed in 1937. PCM allowed analogue waveforms, such as the human voice, to be represented in binary form (digital). It was possible to represent a standard 4 kHz analogue telephone signal as a 64 kbit/s digital bit stream. The potential with digital processing allowed more cost-effective transmission systems by combining several PCM channels and transmitting them down the same copper twisted pair as had previously been occupied by a single analogue signal. 4.6.1 Digital multiplexing hierarchy In Europe, and subsequently in many other parts of the world, a standard TDM scheme was adopted whereby thirty 64 kbit/s channels were combined, together with two additional 164 Satellite Networking: Principles and Protocols channels carrying control information including signalling and synchronisation, to produce a channel with a bit rate of 2.048 Mbit/s. As demand for voice telephony increased, and levels of traffic in the network grew ever higher, it became clear that the standard 2.048 Mbit/s signal was not sufficient to cope with the traffic loads occurring in the trunk network. In order to avoid having to use excessively large numbers of 2.048 Mbit/s links, it was decided to create a further level of multiplexing. The standard adopted in Europe involved the combination of four 2.048 Mbit/s channels to produce a single 8.448 Mbit/s channel. This level of multiplexing differed slightly from the previous in that the incoming signals were combined one bit at a time instead of one byte at a time, i.e. bit interleaving was used as opposed to byte interleaving. As the need arose, further levels of multiplexing were added to the standard at 34.368 Mbit/s, 139.246 Mbit/s, and even higher speeds to produce a multiplexing hierarchy, as shown in Figure 4.10. In North America and Japan, a different multiplexing hierarchy is used but with the same principles. 4.6.2 Satellite digital transmission and on-board switching Digital signals can be processed in the time domain. Therefore, in addition to sharing bandwidth resources in the frequency domain, earth stations can also share bandwidth in the time domain. Time division multiplexing can be used for satellite transmission at any level of the transmission hierarchy as shown in Figure 4.10. Concerning on-board switching, a time-switching technique can be used often working together with circuit switching (or space switching). 1 32 MUX 1 4 MUX 1 4 MUX 1 4 E3 rate of 34.368 Mbit/s MUX E1 rate of 2.048 Mbit/s E2 rate of 8.448 Mbit/s E4 rate of 139.246 Mbit/s … Time slot Figure 4.10 Example of traffic multiplexing and capacity requirement for satellite links Satellite Internetworking with Terrestrial Networks 165 4.6.3 Plesiochronous digital hierarchy (PDH) The multiplexing hierarchy appears simple enough in principle but there are complications. When multiplexing a number of 2 Mbit/s channels they are likely to have been created by different pieces of equipment, each generating a slightly different bit rate. Thus, before these 2 Mbit/s channels can be bit interleaved they must all be brought up to the same bit rate adding ‘dummy’ information bits, or ‘justification bits’. The justification bits are recognised as de-multiplexing occurs, and are discarded, leaving the original signal. This process is known as plesiochronous operation, meaning in Greek ‘almost synchronous’ as illustrated in Figure 4.11. The same problems with synchronisation, as described above, occur at every level of the multiplexing hierarchy, so justification bits are added at each stage. The use of plesiochronous operation throughout the hierarchy has led to adoption of the term plesiochronous digital hierarchy (PDH). 4.6.4 Limitations of the PDH It seems simple and straightforward to multiplex and de-multiplex low bit streams to higher bit-rate streams, but in practice it is not so flexible and not so simple. The use of justification bits at each level in the PDH means that identifying the exact location of the low bit-rate stream in a high bit-rate stream is impossible. For example, to access a single E1 2.048 Mbit/s stream in an E4 139.246 Mbit/s stream, the E4 must be completely de-multiplexed via E3 34.368 and E2 8.448 Mbit/s as shown in Figure 4.12. Once the required E1 line has been identified and extracted, the channels must then be multiplexed back up to the E4 line. Obviously this problem with the ‘drop and insert’ of channels does not make for very flexible connection patterns or rapid provisioning of services, while the ‘multiplexer mountains’ required are extremely expensive. Another problem associated with the huge amount of multiplexing equipment in the network is one of control. On its way through the network, an E1 line may have travelled via a number of possible switches. The only way to ensure it follows the correct path is to keep careful records of the interconnection of the equipment. As the amount of reconnection activity in the network increases it becomes more difficult to keep records current and the 01 0 1 0 1 1 ‘Fast’ incoming bits at 2 Mbit/s channels Bit rate adaptor 0 1 0 1 J J Bit rate adaptor 0 1 1 J J J Master oscillator Less justification bit added More justification bit added A high speed multiplexed bit stream ‘Slow’ incoming bits at 2 Mbit/s channels Figure 4.11 Illustration of the concept of plesiochronous digital hierarchy (PDH) [...]... areas where terrestrial Satellite Networking: Principles and Protocols © 2005 John Wiley & Sons, Ltd Zhili Sun 188 Satellite Networking: Principles and Protocols lines are expensive and uneconomical to install and operate Satellite networking was considered as an alternative solution to ‘broadband for all’ to complement terrestrial broadband networks due to its flexibility and immediate global coverage... broadband network interconnection and terminal access • Describe the major roles of satellites in broadband networks with ATM over satellite networking • Understand the basic concept of satellite transparent and on-board switching payload for ATM networks • Understand ATM QoS and performance issues and enhancement techniques for satellite ATM networks 5.1 Background In the early 1990s, research and development... broadband communications based on ATM and fibre optic transmission cable generated a significant demand for cost-effective interconnection of private and public broadband ATM LANs (also called ATM islands), experimental ATM networks and testbeds, and for cost-effective broadband access via satellite to these broadband islands However, there was a shortage of terrestrial networks to provide broadband... between networks operating at bit rates less than 64 kbit/s with 64 kbit/s-based ISDN and B-ISDN, 08/19 96 1 86 Satellite Networking: Principles and Protocols [9] ITU-T Recommendation Y.101 Global information infrastructure terminology: terms and definitions, 03/2000 [10] ITU-T Recommendation E.800, Terms and definitions related to quality of service and network performance including dependability, 08/94... transmission rates of STM-4 270 bytes 270 9 10 1 1 2 3 4 Section overhead AU ptr 5 8 9 POH J1 B3 C2 6 7 STM-1 Payload Section overhead G1 F2 H4 Z3 Z4 125 microseconds Z5 Figure 4.14 STM-1 frame of the SDH network VC-4 9 bytes 168 Satellite Networking: Principles and Protocols and STM- 16 (62 2 Mbit/s and 2.4 Gbit/s respectively) are also defined, with further levels proposed for study 4.7.3 Mapping from... emergency and disaster relief scenarios and remote/rural medical care where the infrastructure was either disrupted or lacking 5.1.1 Networking issues One of the key networking issues was to provide interconnection and also access to geographically dispersed broadband islands in the context of ATM networks with the required QoS and bandwidth Due to their global coverage and broadcasting nature, satellite. .. rate and high speed IDR Therefore, there is a significant difference between the interconnections of different networks Satellite networks can be used as thin routes between pairs of earth stations, as access networks to provide basic rate and primary rate and as transit networks to interconnect main networks with a capacity measured in thousands of circuits 184 Satellite Networking: Principles and Protocols. .. the basic principles and techniques developed for ATM networks to be able to support quality of service (QoS), class of service (CoS), fast packet switching, traffic control and traffic management When you have completed this chapter, you should be able to: • Know the design issues and concepts concerning ATM over satellites • Know the GEO satellite ATM networking and advanced satellite networking. .. terminals, networks and services and applications in the telecoms industry and Internet towards the next generation of Internet by taking advantage of both the IP and ATM networks 5.1.2 Satellite services in the B-ISDN networking environment The principal advantages of satellite systems are their wide coverage and broadcasting capabilities There are enough satellites to provide broadband connections anywhere... networks and mobile networks ATM over Satellite Networks 189 In a broadband networking environment, satellite networking can be used for user access mode and also for network transit mode In the user access mode, the satellite system is positioned at the border of the broadband network It provides access links to a large number of users directly or via local networks The interfaces to the satellite . Payload POH 1 2 3 4 5 6 9 bytes J1 B3 C2 G1 F2 H4 Z3 Z4 Z5 VC-4 Figure 4.14 STM-1 frame of the SDH network 168 Satellite Networking: Principles and Protocols and STM- 16 (62 2 Mbit/s and 2.4 Gbit/s respectively). (NOS) 160 Satellite Networking: Principles and Protocols may include customer-related functions (e.g. subscription record, access rights, usage records and accounts) and establishment and maintenance. hierarchy (PDH) 166 Satellite Networking: Principles and Protocols E4 E3 E2 E3 E2 Customer site PDH E4 line terminator E3 E2 E3 E4 E1 E2 E1 E4 line terminator Figure 4.12 Multiplexing and de-multiplexing