Satellite Networking

Transcription

1 as a component of Satellite Communications B (EEM.scmB) Dr Zhili SUN Centre for Communication Systems Research School of Electronic Engineering, Information Technology & Mathematics University of Surrey Guildford Surrey GU2 7XH Tel: Fax: Z.Sun@surrey.ac.uk 1 as a component of Satellite Communications B (EEM.scmB) 1

2 Contents Network Protocols basics and reference models Satellite networks and network services PDH and SDH transmissions technology SDH - Intelsat scenarios ISDN and B-ISDN over satellite TCP/IP over satellite IP QoS over satellite 2 2

3 Review Network protocol basics and reference models Telecommunication Services Network Description and Architecture Basic Technical Issues Digital Transmission (PDH & SDH) SDH over Satellite - Intelsat scenarios Satellite system performance related service requirement Issues on ISDN and B-ISDN and broadband networks over Satellite Internet over Satellite and QoS Standards - ITU-T and ITU-R 3 Satellite communication is another method of extending the communication networks. The links may be used for telephony, data, facsimile, and video, and also for broadband services and Internet services. Historically, communication was closely associated with telecommunication networks or telephony networks. All the services and applications were based on the telephony channel of about 4 KHz bandwidth. Due to the need of data communication, a term, the protocol came into popular use in 1960s. According to the IEEE, the protocol can be defined as: a formal set of rules and convention governing the format and relative timing of message exchange among two or more communication terminals. The protocol provides transport service to the applications of real time and nonreal time. The protocol also shields the transmission technologies such copper cable, optical fibre and radio and satellite links from the applications. Different protocol architectures have been developed based on different the transmission technologies to support different types of applications. When satellite links become a integrated part of the infrastructure, the protocol has significant impact on the performance of the network, quality of service (QoS) provided to the application, and efficiency of making used of the satellite resources. This lecture discuss all the relevant issues listed in the above slide related to satellite networking and protocols. 3

4 Protocol basics 4 4

5 Protocol architecture Layers, protocols and interfaces Connectionoriented and connectionless services 5 There are two types of transmission technologies: broadcast and point-to-point transmissions. The scale of physical size of networks can be LAN, MAN and WAN. Satellite networks are inherently and most useful where broadcast property is important. They can also be set up to support point-to-point transmissions in a very wide area. What is a protocol? It is the rules and convention used in conversation by agreement between communication parties. Why networking need protocols? It is to reduce design complexity that each layer is designed to offer certain services to high layers, shielding those layer from the details of how the services are actually implemented. Each layer has an interface with primitive operations which can be used to access to the offered services. A network architecture is a set of layers and protocols. A protocol stack is a list of protocols (one protocol per layer) 5

6 The ISO reference model 6 An entity is the active element in each layer. Peer entities is the entities in the same layer on different machines. Virtual and actual communication are very useful concepts in protocol design. For example, the HTTP is the protocol used by the WWW. The virtual communication is at the application layer between the local client machine and remote server using the HTTP protocol. The actual communication is in the physical layer with a bit stream. A network layer such as the provides a connection oriented service where a connection needs to be set up before data exchange and is released after being used. Another such as the IP protocol provides connectionless service where connections are not needed for data exchanges. Basic protocol functions include segmentation and reassembly, encapsulation, connection control, ordered delivery, flow control, error control, routeing and multiplexing. 6

7 Data Transmission in the OSI model 7 The physical layer (bit stream): it specifies mechanical, electrical and procedure interfaces and the physical transmission medium. In satellite network, radio links are the physical transmission. Data link layer: it provides a line that appears free of undetected transmission errors to the network layer. Broadcast networks have an additional issues in data link layer, i.e., how to control access to the shared medium. A special sublayer called medium access control (such as Polling, Aloha, FDMA, TDMA, DAMA) deals with this problem. The network routes packets from source to destination. The functions include network addressing, congestion control, accounting, disassembling and reassembling, coping with heterogeneous network protocols and technologies. In broadcast network, the routing problem is simple. The routing protocol is often thin or even non-existent. The transport layer provides reliable data delivery service for high layer users. It is the highest layer of the services associated with the provider of communication services. The higher layers are user data services. It has functions of ordered delivery, error control, flow control and congestion control. The session layer provides the means of for cooperating presentation entities to organise and synchronise their dialogue and to manage the data exchange. The presentation layers are concerned with data transformation, data formatting and data syntax. The application layer is the highest layer of the ISO architecture. It provides services to application processes. 7

8 Internet - the TCP/IP reference model 8 The slide shows the initial Internet protocol architecture. It is based on datagram and best effort approach without guarantee quality of service (QoS). The idea is that the design of a network should only be concerned with transport packets from their source to their destination address. The network is inherently unreliable no matter how you design it. It leaves to users to make it to be reliable if required. It is claimed to be a successful approach based on 40 years experience of Internet supporting data networks. 8

9 The B-ISDN reference model 9 The slide shows the protocol architecture. It based on connection oriented approach with guarantee quality of service (QoS). The idea is that the network should be designed very reliable so that users don t have to worry about if there may be some problems in their data transfer. It is claimed to be a successful approach based on 100 years experience of telephone networks. 9

10 Satellite networks and network services 10 10

11 Custom Designed Networks Telecommunication networks Custom Designed Networks Broadcast TV TV distribution Small Dish / VSAT type date network 11 Broadcast TV Satellites play a major role throughout the world in providing TV services directly to home. They are often designed for this purpose. TV distribution Satellites are extensively used to transfer TV material around the world between studios. These satellite are often the same one as the one used to provide the main network services, but they are totally separated in networking terms and subject to different constraints. Small Dish / VSAT type date network These networks use dedicated earth stations to provide a range of services to business customers. Many of the services are identical to those carried via leased circuits on the main networks, particularly when the network penetration to a particular geographical location is limited and the only access possible is via a dedicated satellite network of this kind. The network consists of a large hub earth station to transmit to a large number of small earth stations at customers premises. 11

12 Basic Technical Problems Propagation Delay Limited bandwidth Transmission Errors Transmission Power GEO MEO LEO terrestrial 12 Naturally, there are far greater losses. For LOS microwave we encounter free-space losses possibly as high as 145 db. In the case of a satellite with a range of 22,300 miles operating on 4.2 GHz, the free-space loss is 196 db and at 6 GHz, 199 db. At 14 GHz the loss is about 207 db. This presents no insurmountable problem from earth to satellite, where comparatively high power transmitters and very high gain antennas may be used. On the contrary, from satellite to earth the link is power-limited for two reasons: (1) in bands shared with terrestrial services such as the popular 4-GHz band to ensure noninterference with those services and (2) in the satellite itself, which can derive power only from solar cells. It takes a great number of solar cells to produce the RF power necessary; thus the down-link, from satellite to earth, is critical, and received signal levels will be much lower than on comparative radio-links, as low as -150 dbw. A third problem is crowding. The equatorial orbit is filling with geostationary satellites. Radiofrequency interference from one satellite system to another is increasing. This is particularly true for systems employing smaller antennas at earth stations with their inherent wider beamwidths. It all boils down to a frequency congestion of emitters. It should be noted that low earth-orbit satellites typically orbit some 500 km above the earth. 12

13 Error Control Mechanisms Re-transmission for non-real time applications Forward Error Control (FEC) such as Adaptive Reed-Solomon Coding Interleaving Techniques to randomise burst errors as it is easier to correct random errors than burst errors such as cell based interleaving technique used in COMSAT equipment 13 As we have mentioned previously, satellite communications is down-link limited because down-link EIRP level is strictly restricted. Still we want to receive sufficient power to meet the error performance objectives. One way to achieve such a goal is to FEC-code the links where lower Eb/N0 ratios will still meet error objectives. Thus INTELSAT requires coding on their digital accesses. Typical INTELSAT digital link for the Intermediate Data Rate (IDR) Digital Carrier System are required to use R = 3/4 FEC convolutional coding. INTELSAT recommends using standard information rates specified by CCITT. The occupied satellite bandwidth unit for IDR carriers is approximately equal to 0.6 times the transmission rate. The transmission rate is defined as the coded symbol rate. To provide guard-bands between adjacent carriers on the same transponder, the nominal satellite bandwidth unit is 0.7 times the transmission rate. IDR carriers are designed to provide a service in accordance with CCIR Recs. 522, 614, and 579. To achieve these requirements, the system is designed to provide a nominal BER of 1 x 10E-7 under clear sky conditions. Under degraded sky conditions (typically with rainfall), a worst-case BER of 1 x 10E-3 for all but 0.04% of the year is provided. 13

14 Main network services Voice (bandwidth khz) Voice band data (Facsimile, etc.) via 3.1 khz channel up to 9.6 kbit/s 64 kbit/s digital data (ISDN and leased network) Broadband (64 kbit/s -> 2 Mbit/s -> 155 Mbit/s ->...) Satellite usage must take into account the end-to-end customer requirements as well as signalling/routeing constrains of particular network configuration The requirements of these services may also differ depending on whether they are carried on a dedicated (leased) circuit within the main network or a switched connection. 14 Satellite links may prove optimum for a variety of applications over the telecommunication networks, including the following: On international high-usage trunks country to country. On national trunks, between switching nodes that are fairly well separated in distance [i.e..., >200 miles (320 km)] in highly developed countries. Again, the tendency is to use satellite links for direct high usage connectivity. It may serve as an adjunct to LOS microwave and fibre optics. In areas under development where satellite links replace HF radio and a high growth is expected to be eventually supplemented by radiolink and fiber optic cable. In sparsely populated, highly rural, and the areas where it may be the only form of communication. Northern Canada and Alaska are good examples. On final routes for overflow on a demand-assignment basis. Route length again is a major consideration. In many cases, on international connections reducing such connections to one link. On private and industrial networks including VSAT networks. On specialised common carriers. On thin-line communications and tracking systems. 14

15 Network architecture International Node International Node Main Network Node Main Network Node Switching function Main Network Node Main Network Node Local Network Node Local Network Node Switching function Local Network Node Local Network Node Customer s terminal 15 Customer s terminal INTERNATIONAL NETWORK Before 1980 the ITU-T routing plan was based on a network with a hierarchical structure with descending levels called CT1, CT2, CT3, and CTX (central transits), Since 1980 ITU-T has made a radical change in its international routing plan. The new plan might be called a "free routing structure." It assumes that national administrations (telephone companies) will maintain national hierarchical networks. Obviously the change was brought about by the long reach of satellite communications with which international high-usage (HU) trunks can terminate practically anywhere in the territory of a national administration. The CCITT International Telephone Routing Plan is contained in CCITT Rec. E.171 and is reviewed below. In practice, the large majority of international telephone traffic is routed on direct circuits (i.e.., no intermediate switching point) between international switching centres (ISCs). It should be noted that it is the rules governing routing of connections consisting of a number of circuits in tandem that this recommendation primarily addresses. These connections have an importance in the network because: they are used as alternate routes to carry overflow traffic in busy periods to increase network efficiency they can provide a degree of service protection in the event of failure of other routes they can facilitate network management when associated with ISCs having temporary alternative routing capabilities. 15

16 Circuit switched main network PBX Mobile International Transmission Network (Cable, satellite and radio) Analogue International exchange Digital International exchange PBX ISDN Mobile Voiceband data Cordless phone Fax Phone Modem Analogue Main Network Exchange Analogue local exchange Digital Main Network Exchange Digital local exchange Modem Cordless phone Phone Voiceband data Notes: Satellites can in principle be used on any section (or combination of sections) of the network. In Europe they are mainly used in connections of international gateways worldwide. Need to carefully control circuit routeing to void picking up 2 satellite hops on a particular call. 16 Fax This plan replaces the previous one established in Rec. E. 171 continues under "Principles": The Plan preserves the freedom of administrations: (a) to route their originating traffic directly or via any transit administration they choose; (b) to offer transit capabilities to as wide a range of destinations as possible in accordance with the guidelines it provides. The governing features of this plan are: (a) it is not hierarchical, (b) administrations are free to offer whatever transit capabilities they wish, providing they conform to the Recommendation, (c) direct traffic should be routed over final or high usage circuit groups, (d) no more than 4 international circuits in tandem between the originating and terminating ISCs, (e) advantage should be taken of the non-coincidence of international traffic by the use of alternative routings and provide route diversity (Rec. E.523), 16

17 Main network transmission Local Access Analogue: standard 2-wire, 3.1 khz local line 64 kbit/s: leased line for access using ITU-T X-series interfaces 144 kbit/s: ISDN two 64 kbit/s information channels plus a 16 kbit/s signalling link to control these channels 2 Mbit/s: used for wideband leased circuit access or to connect a PBX with 30x64 kbit/s information channels plus a 64 kbit/s signalling channel. Main Network Analogue transmission (this is being replaced by digital transmission Digital transmission (120 Mbit/s TDMA, IDR 2 Mbit/s) 17 (f) the routing of transit switched traffic should be planned to avoid possibility of circular routings, (g) when a circuit group has both terrestrial and satellite circuits, the choice of routing should be governed by: the guidance given in (CCITT) Rec. G. 114 (e.g., no more than 400 ms one-way propagation time), the number of satellite circuits likely to be utilised in the overall connection, the circuit which provides the better transmission quality and overall service quality, (h) the inclusion of two or more satellite circuits in the same connection should be avoided in all but exceptional cases. Regarding (h), reference should be made to Annex A of Recs. E. 171 and Q

18 Analogue transmission hierarchy Single Channel (3100 Hz) Group (12 or 16 channels) Super-Group (60 channels) nlower order systems from a single channel up to 60 channels. nhigher order systems from 300 up to channels Master-Group (300 Channels) Super-Master-Group (900 Channel) Hyper-Group (900 Channels) 16 Super-Group (960 channels) 12 MHz (2700 Channels) 12 MHz (2700 Channels) 60 MHz (10800 Channels) 18 Notes: Analogue satellite channels using a variety of access/modulation techniques are still used internationally to support this hierarchy. This is rapidly being replaced in the network by the digital hierarchies. This uses mainly FDMA. 18

19 PDH and SDH transmissions technology 19 19

20 History of Digital Transmission Systems Until 1970 achievement in long-haul routes: Frequency Division Multiplexing (FDM) Early 1970s begin to appear: Digital Transmission Systems Pulse Code Modulation (PCM) technique Represent standard 4 khz analogue telephone signal as a 64 Kbit/s digital bit stream 20 A Brief History of Transmission Systems In the early 1970s, digital transmission systems began to appear, utilizing a method known as Pulse Code Modulation (PCM), first proposed in PCM allowed analogue waveforms, such as the human voice, to be represented in binary form, and using this method it was possible to represent a standard 4 khz analogue telephone signal as a 64 kbit/s digital bit stream. Engineers saw the potential to produce 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. 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 channels carrying control information, to produce a channel with a bit rate of 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 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 Mbit/s links, it was decided to create a further level of multiplexing. The standard adopted in Europe involved the combination of four 2 Mbit/s channels to produce a single 8 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 Mbit/s, 140 Mbit/s, and 565 Mbit/s to produce a full hierarchy of bit rates. 20

21 Transmission Hierarchies North American 64 64Kbit/s X X3 X X X6 X Europe X X X4 X X Deployment of synchronous transmission systems will be straightforward due to their ability to interwork with existing plesiochronous systems. The SDH defines a structure which enables plesiochronous signals to be combined together and encapsulated within a standard SDH signal. This protects network operators investment in plesiochronous equipment, and enables them to deploy synchronous equipment in a manner suited to the particular needs of their network. As synchronous equipment becomes established within the network, the full benefits it brings will become apparent. The network operator will experience significant cost savings associated with the reduced amount of hardware in the network, and the increased efficiency and reliability of the network will lead to savings resulting from a reduction in maintenance and operations. Another result of increased reliability will be a reduction in the need to hold spare equipment. The sophisticated network management capabilities of a synchronous network will give a vast improvement in the control of transmission networks. Improved network restoration and reconfiguration capabilities will result in better availability, and faster provisioning of services. SDH has been designed to support future services such as Metropolitan Area Networks (MANs), Broadband ISDN, and personal Communications networks. 21

22 Principles of Plesiochronous Operation " Greek meaning of plesiochronous: almost synchronous" "fast" incoming bits at 2 Mbit/s channel "slow" incoming bits at 2 Mbit/s channel Bit rate adaptor Bit rate adaptor Less justification bit added J J 4 Master oscillator J J J More justification bit added Principles of Plesiochronous Operation The multiplexing hierarchy described above 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 recognize as demultiplexing occurs, and discarded, leaving the original signal. This process is know as plesiochronous operation, from Greek, meaning almost synchronous. The same problems with synchronization, 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, or PDH. 22

23 The Synchronous Digital Hierarchy (SDH) 1989 CCITT Blue Book covering SDH: Recommendation G707, G708 & G709 Basic transmission rate STM-1 (Synchronous Transport Module): Mbit/s Higher Transmission rates STM-4 & STM-16: Mbit/s and Gbit/s Suggested higher rate STM-8 & STM-12: & Gbit/s Introducing Operations Administration and Maintenance (OAM) 23 Origins of the SDH As explained in the previous chapter, PDH has reached a point where it is no longer sufficiently flexible or efficient to meet the demands being placed on it. As a result, synchronous transmissions has been developed to overcome the problems associated with plesiochronous transmission, in particular the inability of PDH to extract individual circuits from high capacity systems without having to demultiplex the whole system. Synchronous transmission can be seen as the next stage in the evolution of the transmission hierarchy. A concerted standards effort has been involved in its development. The opportunity of defining this new standard has been used to address a number of other problems. Among these have been a network management capability within the hierarchy, the need to define standard interfaces between equipment, and European transmission hierarchies. This standards work culminated in ITU-T (formerly CCITT) Recommendations G.707, G.708, and G.709 covering the Synchronous Digital Hierarchy (SDH). These were published in the ITU-T Blue Book in In North America ANSI published its SONET standards, which can now be thought of as a subset of the worldwide SDH standards. In addition to the three main ITU-T recommendations, a number of working groups were set up to draft further recommendations covering other aspects of the SDH, such as the requirements for standard optical interfaces and standard OAM functions. 23

24 Digital transmission hierarchy (SDH) The primary rate STM-1 (synchronous transport module - 1) has a bit rate of Mbit/s Each frame consists of payload space of carrying a PDH 140 Mbit/s signal completely, with extra capacity for errorchecking and management channels. The current defined higher SDH levels are STM-4 (4 STM-1s) and STM-16 (16 STM-1s). The proposed STM-R, the reduced bitrate STM-1 is an attempt to design STM with a bit rate of Mbit/s. The satellite community should note that all levels of the SDH contain a considerable percentage of overhead (3.33%) much of which is at present undefinded. 24 The ITU-T recommendations define a number of basic transmission rates within the SDH. The first of these is 155 Mbit/s, normally referred to as STM - 1 (where STM stands for synchronous Transport Module ). Higher transmission rates of STM - 4 and STM - 16 (622 Mbit/s and 2.4 Gbit/s respectively) are also defined, with further levels proposed for study. The recommendations also define a multiplexing structure whereby an STM - 1 signal can carry a number of lower rate signals as payload, thus allowing existing PDH signals to be carried over a synchronous network. This process will be explained in more detail below. 24

25 Mapping PDH to SDH STM-N XN AUG X1 AU-4 s X3 AU-3 s: ANSI SONET specific option e: Europe ETSI specific option multiplexing mapping aligning s VC-4 VC-3 X3 X7 e s TUG-3 X7 e X1 e C Mb/s e TU-3 VC-3 C-3 45/34 Mb/s X1 TUG-2 TU-2 VC-2 C-2 6 Mb/s X3 TU-12 VC-12 C-12 X4 s 2 Mb/s AUG: Administrative Unit Group TUG: Tributary Unit Group VC: Virtual Container TU-11 VC-11 C Mb/s 25 Principles of the Synchronous Digital Hierarchy (SDH) Despite its obvious advantages over PDH, SDH would have been unlikely to have gained acceptance if its adoption had immediately made all existing PDH equipment obsolete. This is why the ITU-T Recommendations made provisions from the outset for any currently used transmission rate to be packaged into an STM-1 frame. All plesiochronous signals between 1.5 Mbit/s and 140Mbit/s can be accommodated, with the ways in which they can be combined to form an STM-1 signal defined in Recommendation G.709. The SDH multiplexing hierarchy is shown in the slide. A brief explanation of how the hierarchy works follows. Mapping PDH to SDH SDH defines a number of Containers, each corresponding to an existing plesiochronous rate. Information from a plesiochronous signal is mapped into the relevant container. The way in which this is done is similar to the bit stuffing procedure carried out in a conventional PDH multiplexer. Each container then has some control information known as the path overhead added to it. The path overhead bytes allow the network operator to achieve endpath monitoring of things such as error rates. Together the container and the path overhead form a Virtual Container. 25

26 Simplification of PDH Add-Drop principle 140 Mbit/s line terminator Mbit/s line terminator PDH Customer site SDH Multipleter SDH Multipleter SDH Multipleter SDH 26 Customer site In a synchronous network, all equipment is synchronised to an overall network clock. It is important to note, however, that the delay associated with a transmission link may vary slightly with time. As a result, the location of virtual containers within an STM-1 frame may not be fixed. These variations are accommodated by associating a pointer with each VC. The pointer indicates the position of the beginning of the VC in relation to the STM-1 frame. It can be increased or decreased as necessary to accommodate of the position of the VC. G.709 defines different combinations of virtual containers which can be used to fill up the payload area of an STM - 1 frame. The process of loading containers, and attaching overhead is repeated at several levels in the SDH, resulting in the nesting of smaller VCs within larger ones. This process is repeated until the largest size of VC (a VC - 4 in Europe) is filled, and this is then loaded into the payload of the STM - 1 frame. (This subject will be discussed in more detail in Chapter 4). When the payload area of the STM - 1 frame is full, some more control information bytes are added to the frame to form the Section Overhead. The section overhead bytes are so-called because they remain with the payload for the fiber section between two synchronous multiplexers. Their purpose is to provide communication channels for functions such as OAM; facilities, alignment and a number of other functions. When a higher transmission rate than 155 Mbit/s of STM-1 is required in synchronous network, it is achieved by using a relatively straightforward byte - interleaved multiplexing scheme. In this way, rates of 622 Mbit/s (STM - 4) and 2.4 Gbit/s (STM - 16) can be achieved. 26

27 Synchronous Operation Example: European mapping route for primary rate service STM-1 X1 AUG X1 X3 X7 AU-4 VC-4 TUG-3 TUG-2 s: ANSI SONET specific option e: Europe ETSI specific option multiplexing mapping aligning AUG: Administrative Unit Group TUG: Tributary Unit Group VC: Virtual Container 27 X3 TU-12 VC-12 C-12 2 Mb/s s VC-11 C Mb/s Synchronous Operation The basic element of the STM signal consisting of a group of bytes allocated to carry the transmission rates defined in G.702 (i.e.. 1.5Mbit\s and 2Mbit\s transmission hierarchies). VIRTUAL CONTAINER VC-n : (n = 1-4) Built up from the container plus additional capacity to carry the path overhead (POH). The path overhead provides end-to-end path control and monitoring information. For a VC-3 or VC-4 the payload may be a number of TUs or TUGs as opposed to a simple basic vc-n, where n=1,2. TRIBUTARY UNIT TU-n: (n= 1-3) The tributary unit consists of a Virtual Container plus a Tributary Unit Pointer. The position of the VC within the TU is not fixed, however the position of the TU pointer is fixed with relation to the next step of the multiplex structure, and indicates the start of the VC. TRIBUTARY UNIT GROUP TUG: This is formed by a group of identical TUs. ADMINISTRATION UNIT AU-n: (n=3,4) This consists of a VC plus an AU pointer. The phase alignment of the AU pointers are fixed with relation to the STM-1 frame as a whole and indicate the positions of the VC. 27

28 Transmission rates Levels Referring to PDH: Mbit/s Mbit/s Mbit/s Mbit/s Mbit/s Mbit/s Mbit/s Levels Referring to SDH: STM-1: Mbit/s STM-4: Mbit/s STM-8: Mbit/s Sugested high rates: STM-12: Mbit/s STM-16: Mbit/s 28 SYNCHRONOUS TRANSPORT MODULE: LEVEL 1 (STM-1) This is the basic element of the SDH. It is formed from a payload (made up of the AU) and additional bytes to form a section overhead (SOH). The section overhead allows control information to be passed between adjacent synchronous network elements. SYNCHRONOUS TRANSPORT MODULE: LEVEL N (STM-N) Formed by combining lower level STM signals using byte interleaving. The basic transmission rate defined in the SDH standards is Mbit/s (STM-1). Given that an STM-1 frame consists of bit bytes, this corresponds to a frame duration of 125 microseconds. Two higher bit rates are also defined: Mbit/s (STM-4) and 2, Mbit/s (STM-16). Within an STM-1 frame, information type repeats every 270 bytes. Thus, the STM-1 frame is often considered as a 270 byte x 9 line structure, as shown in the figure bellow. The first 9 columns of this structure constitutes the Section Overhead area, while the remaining 261 columns are the Payload area. The synchronous digital hierarchy does away with a number of the lower multiplexing levels defined in PDH. 2 Mbit/s tributaries are multiplexed to the STM-1 level in a single step. However, in order to achieve compatibility with non-synchronous equipment, the SDH recommendations define methods of subdividing the payload area of an STM-1 frame in various ways so that it can carry different combinations of tributaries, both synchronous and asynchronous. Using this method, synchronous transmission systems can accommodate signals generated by equipment from various levels of the plesiochronous digital hierarchy. 28

29 The STM-1 Frame 1 Section overhead AU ptr Section overhead 125 microseconds 270 bytes POH J1 B3 C2 G1 F2 H4 Z3 Z4 Z5 STM-1 Payload VC-4 9 bytes 29 The STM-1 Frame As was explained in the last section an STM-1 frame consists of 2430 bytes which can be considered as a structure of 270 columns x 9 lines. The frame is divided into three main sections: Payload Area AU Pointer Area Section Overhead Area PAYLOAD We have seen previously that signals from all levels of the PDH can be accommodated in a synchronous network by packaging them together in the payload area of an STM-1 frame. The plesiochronous tributaries are mapped into the appropriate synchronous container, and a single column of nine bytes, known as the Path Overhead (POH), is added to form the relevant Virtual Container (VC). The path overhead provides information for use in end-toend management of a synchronous path. The slide describes VC-4 packaging with VC-4 Path Overhead B3 BIP-8 (Bit Interleaved Parity): This byte provides bit error monitoring over the path using an even bit parity code, BIP-8. C2 Signal Label: This byte indicates the composition of the VC-n payload. F2 Path User Channel: This byte provides a user communication channel. G1 Path Status: This byte allows the status of the received signal to be returned to the transmitting end of the path from the receiving end. H4 Multiframe indicator: Single byte for multiframe indication. J1 Path Trace: This byte verifies the VC-n path connection. Z3-Z5: Three bytes for National use. After the path overhead is added, a pointer indicates the start of the VC relative to the STM-1 frame. This unit is then known as a Tributary Unit (TU) if it carries lower order tributaries, or an Administrative Unit (AU) for higher order. 29

30 STM-1 Section Overhead A1 A1 A1 A2 A2 B1 E1 A2 C 1 F1 Regenerator section overhead D1 AU pointers D2 D 3 B2 B2 B2 K1 K2 STM-1 Payload D4 D5 D 6 D7 D8 D 9 D10 D11 D12 Multiplex section overhead Z1 Z1 Z1 Z2 Z2 Z2 E2 Bytes reserved for future use. For example, these are proposed by whin ITU-T to be used for media specific applications, e.g. Forward error correction in radio systems. 30 TUs can be bundled together into Tributary Unit Groups (TUGs), which are then mapped into a higher order VC. Once the STM-1 payload area is filled by the largest unit available, a pointer is generated which indicates the position of the unit in relation to the STM-1 frame. This is known as the AU pointer. It forms part of the section overhead area of the frame. The use of pointers in the STM-1 frame structure means that plesiochronous signals can be accommodated within the synchronous network without the use of buffers. This is because the signal can be packaged into a VC and inserted into the frame at any point at time. The pointer then indicates its position. Use of the pointer method was made possible by defining synchronous virtual containers as slightly larger than the payload they carry. This allows the payload to slip in time relative to the STM-1 frame in which it is contained. Adjustment of the pointers is also possible where slight changes of frequency and phase occur as a result of variations in propagation delay and the like. The result of this is that in any data stream, it is possible to identify individual tributary channels, and drop or insert information, thus overcoming one of the main drawbacks of PDH. Section Overhead The Section Overhead (SOH) bytes are used for communication between adjacent pieces of synchronous equipment. As well as being used for frame synchronization, they perform a variety of management and administration facilities. The purpose of individual bytes is detailed below: A1, A2: Framing B1, B2: These bytes are simple parity checks for error detection. C1: Identifies an STM-1 in an STM-N frame. D1-D12: Data communication channel. Used for network management. E1, E2: Orderwire channels. F1: User channel. K1, K2: Automatic Protection Switching (APS) channel Z1, Z2: Reserved bytes for National use. 30

31 SDH over satellite - Intelsat Scenarios (1/2) Full STM-1 transmission (point to point) through a standard 70 MHz transponder. STM-R uplink with STM-1 downlink (point to multipoint) 31 Intelsat, in conjunction with its signatories and ITU-T & ITU-R standards bodies has developed a series of SDH compatible network configurations with satellite forming part of the transmission link. A full description of these network configurations, refer to by Intelsat as scenarios, is out side the scope of this lecture. Recent chairman s report of the ITU-R SG4 contain fuller descriptions of these scenarios. In summary, the options are as follows: (a) Full STM-1 transmission (point to point) through a standard 70 MHz transponder. - This requires the development of an STM-1 modem capable of converting the STM-1 digital signal to an analogue format which can be transmitted through a standard 70 MHz transponder. While this development work is generally supported by the Intelsat signatories, there is limited confidence that this approach will yield reliable long term results. It has been suggested in the Technical Advisory committee of the Intelsat Board of Governors that the carriage of an STM-1 will very closely approach the theoretical limits of a 70 MHz transponder. In addition there is (as yet) no recognised need for this amount of capacity via an SDH satellite links. Current high bit rate PDH IDR links are generally used for submarine cable restoration (although there are some exceptions), but for SDH cables, the capacity of submarine cables is such that a complete current generation Intelsat satellite would have to be held in reserve for SDH restoration. This is clearly not a cost-effected use for telecommunication satellites. (b) STM-R uplink with STM-1 downlink (point to multipoint) - This scenario suggests a multi-destinational system, and requires considerable on-board processing of SDH signals, however, the advantage is flexible transponder usage for the network operator(s) using the system. This approach is not generally favoured by most network operators for reliability and future proofing reasons. This approach may prevent alternative usage of the satellite transponders in the future, and additional complexity is likely to reduce the reliability/lifetime of the satellite, and increase its initial expense. 31

32 SDH over satellite - Intelsat Scenarios (2/2) Intermediate data rate (IDR) of 2 Mbit/s PDH IDR link with SDH to PDH conversion at the earth station 32 (c) Extended TU-12 Intermediate data rate (IDR) of 2 Mbit/s - This approach is favoured by a large number of signatories, since it retains the inherent flexibility of the satellite (regarded as a major advantage over cable systems), and would require the minimum of alterations to satellite and earth station design. Additionally, some of the management advantages of SHD are retained, including end-to-end path performance monitoring, signal labelling and other part of the Overhead. Current development work is centred around determining what aspects of the Data Communication Channels could also be carried with the TU-12. Since the bit rate of the TU-12 is not much greater than an existing 2 Mbit/s PDH signal, it is likely that minimal rearrangement of the transponder band-plans would be required, with the possibility of mixing PDH and SDH compatible IDR carriers. Additionally, development work is currently taking place to modify existing IDR modems to carry the TU-12 signal, rather than more expensive options of develop new modems (for example, for the STM-1 and STM-R options. (d) PDH IDR link with SDH to PDH conversion at the earth station - This is the simplest option of all, but provide the operator with any SDH compatibility. All the advantages of SHD are lost, with additional costs incurred in the SDH to PDH conversion equipment. In the early days of SDH implementation, it mat be the only available method, however. 32

33 Satellite system performance related to service requirement Echo: some form of echo control is always advisable on satellite based networks carrying voice traffic, irrespective of the associated delay. Delay: the one way propagation delay between satellite earth station via a geostationary satellite is approximately 260 ms - see ITU-T G.114 Digital transmission error performance objective: G.821 based on 64 Kbit/s circuit switched connection: Bit Error Ratio (BER): is the ratio of the number of bits in error to the total number of bits transmitted during a measurement period. Objective is to get BER < 10 E-6. Errored second (ES) - BER > 10E-6 Severely errored second (SES) - BER > 10E-3 G.826: define ES and SES differently at high bit rates 33 Echo: some form of echo control is always advisable on satellite based networks carrying voice traffic, irrespective of the associated delay. The ITU-T recommends if the mean roundtrip propagation time exceed 50 ms for a particular circuit, an echo suppresser or echo cancellor should be used. Delay: the one way propagation delay between satellite earth station via a geostationary satellite is approximately 260 ms - see ITU-T G.114). Errors: The principle measure of quality of service (QoS) of a data circuit is its error performance. The parameters are defined In G821: the percentage of averaging periods each of time interval T(0) during which the error bit rate (BER) exceeds a threshold value. The percentage is assessed over a much longer time interval T(L). A suggested T(L) is 1 month. ITU-T G.821 based on 64 Kbit/s circuit switched connections: BER: is the ratio of the number of bits in error to the total number of bits transmitted during a measurement period. Objective is to get BER < 10 E-6. Errored second (ES): is any 1-s interval containing at least 1 error. Objective is to get fewer than 8% of 1-second intervals to have any errors worse (equivalent to 92% error free seconds). Severely errored second (SES): is any 1-s interval with BER > 10E-3. Objective is to get Fewer than 0.2% of 1-second intervals to have a bit errors worse that 1x10E-3. ITU-T G.826 makes use of block-based error measurement so that in-service (error) measurements (ISM) are easier to carry out. Error block (EB): a block one or more bits are in error Error Second (ES): A 1-second period with one or more errored blocks. Severely Error Second (SES): A 1-second period that contain more than 30% Ebs. Background block error (BBE): An EB not occurring as part of an SES. 33

34 Path end point (PEP) Error Performance Objectives for G.826 Hypothetical reference path (HRP) Terminating country 17% Intermediate countries (4 assumed) Intercountry path (e.g. cable, sat.) IG IG IG IG IG IG 1% 2% 2% 2% 2% 1% International portion Terminating country 17% Path end point (PEP) Hypothetical reference path: 27,500 km IG: International Gateway 1% per 500 km 34 ES ratio (ESR): The ratio of ESs to total seconds available time during a fixed measurement interval. SES ratio (SESR): The ratio of SESs to total seconds in available time during a fixed measurement interval. BBE ratio: The ratio of Ebs to total blocks during a fixed measurement interval, excluding SESs and unavailable time. Error performance objectives (EPOs) are measured over available time in a fixed measurement interval. All three objectives (i.e.., ESR, SESR and BBER) must hold concurrently to satisfy G.826, they apply end-to-end for a 27,500 km hypothetical reference path (HRP), which is shown in the slide. The following table show the objectives for G826: Rate (Mbit/s) Bits/block ESR SESR BBER x 10 e x 10 e x 10 e x 10 e-4 Under the assumption of 4 intermediate countries and no satellite hop, the following breakdown for the apportionment can be obtained. Terminating Countries: 2 x 17.5% + 2 x 1% = 37% Intermediate Countries: 4 x 2% = 8% Distance allowance: (27500/500) x 1% = 55% Total: 100% If satellites are used, each receive 35% of the apportionment which corresponding to a nominal hop distance of 17,500 km. But the distance of the hop is removed from the distance allowance. 34

35 ISDN over Satellite 35 35

36 Issues on ISDN? The ITU definition of an Integrated Services Digital Network (ISDN) is: A network evolved from the telephony IDN that provides end-to-end digital connectivity to support a wide range of services, including voice and non-voice services, to which users have access by a limited set of standard multipurpose customer interfaces. 36 ISDN is an effort to standardize subscriber services, user/network services and inter-network capabilities. It is supposed to ensure a level of international compatibility. Standardizing the User and Network Interfaces (UNI) stimulates development and marketing not only by large manufacturers of central office equipment but also by third party manufacturers. It achieved the goal of Worldwide Connectivity because ISDN easily provides intercommunication between them. The ISDN UNI includes beyond the physical network a wide range of protocols. The ISDN Standards with the advantages provide the telecommunication world with new capabilities for users and standardizes connection to most equipment/networks. It also gives a good start for new standards like Broadband-ISDN and. 36

37 ISDN Access Two customer access schemes - the basic rate access and the primary rate access. Large business customers will access an ISDN network via a digital PABX at the primary (or possibly higher) PCM multiplex rates of (US) or Mbits/s (European) This corresponds to a TDM group of 30 B-channels in Europe (or 23 in US) plus 1 D-channel operating at 64 kbits. The signalling over this D-channel will be handled using an extension to the No 7 signalling scheme. The small business or domestic customer will access at 2 B- channels of b4 Kbit/s plus a D-channel of 16 Kbit/s 37 BASIC RATE INTERFACE (BRI) The basic rate interface is specified in ITU-T recommendation I.430. The recommendation defines ISDN communication between terminal equipment. The BASIC RATE INTERFACE (BRI) comprises two B-channels and one D-channel (2B+D). Basic rate access may use a point-to-point or point-to-multipoint configuration. In a point-topoint configuration, the network terminals (NT1 or NT2) and terminals equipment (TE1 or TA) can be up to 1 km apart. The physical connection between the TE and NT requires at least two wire pairs, one pair for each direction of transmission, these are the transmit and receive loops. The ISDN TAs and TEs will have some internal memory identifying its address and bearer service attribute profile and supporting the ISDN protocols. Primary rate interface (PRI) The PRI is defined by physical layer protocol and also by higher protocols included LAPD. It has a full duplex point to point serial, synchronous configuration. The CCITT recommendation G703, G704 defines the electrical interfaces and the frame formats. There are two different interfaces: North America T1 (1.544Mbit/s): It multiplexes 24 of 64khz channels. One PRI frame contains 1 framing bit plus a single 8-bit sample from each of 24 channels-193 bits per frame. Europe CEPT E1 (2.048Mbit/s): it multiplexes 32 channels. 37

38 ISDN over satellite Satellite links can easily support ISDN services with Basic rate: 144 Kbit/s (2 x 64 Kbit/s B-channel + 16 Kbit/s D-channel) Primary rate: Mbit/s = 23B + D = 23x (for North America configuration) Mbit/s = 30B + D = 30x64 + 2x64 (for Europe) - where one time slot is used for framing and one general network maintenance Routeing Plan - no hierarchical, no more than 2 hops 38 38

39 and B-ISDN over Satellite 39 39

40 Broadband? B-ISDN or Broadband ISDN: Broadband Integrated Services Digital Network ITU-T definition: A service or system requiring transmission channels capable of supporting rates greater than the primary rate. 40 Fundamental Concept From a technical point of view, the fundamental underpinning of is: to support all existing services as well as emerging services in the future, fixed-size cells with VPI and VCI to minimises the switching complexity, statistical multiplexing to utilises network resources very efficiently, to minimise the processing time at the intermediate nodes and supports very high transmission speeds as well as very low speed by negotiate service contract for a connection with required quality of services, to minimise the number of buffers required at the intermediate nodes to bound the delay and the complexity of buffer management, guarantees performance requirements of existing and emerging applications, layered architecture, and Capable of handling bursty traffic. 40

41 Relationship Between and B-ISDN evolved from the standardization efforts for B-ISDN. is the technology upon which B-ISDN is based. 41 Principle is a fast packet oriented transfer mode based on asynchronous time division multiplexing and it uses fixed length (53 bytes) cells. Each cell consists of a information field (48 bytes) and a header (5 bytes). The header is used to identify cells belonging to the same virtual channel and thus used in appropriate routing. Cell sequence integrity is preserved per virtual channel. Adaptation layers (AAL) are used to support various services and provide service specific functions. This AAL specific information is contained in the information field of the cell. Basic cell structure is used for the following functions. Routing is a connection oriented mode. The header values (i.e.. VCI and VPI etc.) are assigned during the connection set up phase and translated when switched from one section to other. Signalling information is carried on a separate virtual channel than the user information. In routing, there are two types of connections, i.e.., Virtual channel connection(vcc) and Virtual path connection(vpc). A VPC is an aggregate of VCCs. Switching on cells is first done on the VPC and then on the VCC. Resources Management is connection-oriented and the establishment of the connections includes the allocation of a virtual channel identifier (VCI) and/or virtual path identifier (VPI). It also includes the allocation of the required resources on the user access and inside the network. These resources, expressed in terms of throughput and quality of service, can be negotiated between user and network either before the call-set up or during the call. Cell Identifiers cell identifiers including VPI, VCI and Payload Type Identifier (PTI) are used to recognise an cell on a physical transmission medium. VPI and VCI are same for cells belonging to the same virtual connection on a shared transmission medium. 41