Scrambling in digital communication pdf

All optical plant was installed over a period of two - three weeks. The link length was 1. The data stream is continuous with any unused frames packed with PRBS. Conventional AC coupled laser transmitter and optical receivers were used.

The laser launched Receiver sensitivity was measured at dbm. In the upstream direction transmission is by TDMA with each outstation sending packets of data in assigned time slots. In this case DC coupled optical transmitters and receivers were used. Each customer transmitter was turned fully off when no data was being sent to avoid inter-channel interference on the shared fibre.

This was achieved by biasing the laser off, turning it fully on for a logic 'one' and turning it fully off again for a logic 'zero'. This differs from conventional point to point fibre systems in which the transmitter is biased above turn-on and modulated about that point. The optical receiver is also designed to operate in the presence of a burst mode signal. A DC coupled receiver is required to avoid baseline drift in the absence of received data during the quiet period between packets.

The receiver used was based on a long wavelength InGaAs PIN photodiode operating into a high input impedance FET op-amp, with bootstrap feedback to reduce input capacitance.

A ranging function is required at the subscriber's terminal to ensure that packets are transmitted at the correct instant to avoid time overlap at the Head End. The preferred arrangement for a full network is to have 15 exchange lines at the DP, with 1 to 15 exchange lines interfaces per customer optical termination, a two level optical split hierarchy nominally at cabinet and DP sites with a distance of 1.

If a copper wire is made to some customers from the network a single level optical split hierarchy is preferred, nominally sited at the cabinet. Although a conventional exchange to cabinet distance of 1. This can provide a basis for rationalising the number of local exchanges in a given network. The efficient multiplexing structure of such a network arising from the combination of optical splitting and the sharing of the customer's optical connection cost over multiple lines should mean that the enhanced upper network costs associated with the longer links are kept within bounds.

This should allow any significant cost savings identified for exchange rationalisation to be enjoyed to the full. The passive network architecture offered by the present invention presents an opportunity for evolution towards a broadband multiservice network. When considering the evolution to broadband service capability two important principles need to be adhered to as far as possible.

They are: a the need to minimise the cost of any additional features that are required on the initial network in order to allow graceful evolution to a multiservice broadband network and b to be able to add broadband services to an existing system without disturbing the basic telephony customers already connected.

An important consideration for the broadband network is the amount of extra field plant and installation work that will be required to add the new services. The aim here must be to minimise such costs by utilising as much as possible of the installed system base. Expansion of the system to carry higher bitrate services such as cable television requires the use of wavelength division multiplexing WDM techniques unless the bitrate is sufficiently large at the outset to allow for future broadband service.

The latter would load the costs of the initial basic services to an unacceptable degree and the introduction of broadband service must, at minimum, depend on the addition of at least one wavelength, allowing the existing narrowband customers to continue undisturbed in low bitrate mode.

Because broadband services require higher bit rates than the low speed data and speech services the optical receiver sensitivities will be considerably reduced. This implies that the optical splitting ratio used will be too large for the optical power budget available for the broadband services. It follows therefore that different access points will need to be available for the feeder fibres, carrying the broadband services from the Head End, into the optical splitter array.

A bi-directional optical branching network with two stages of splitting can have a service upgrade by providing additional fibre from the exchange to the first splitting point and connecting in at different levels within this splitter.

Although the bi-directional network has received the greatest attention so far, other structures are possible within the passive optical network concept of the applicant's invention and some of these may have advantages either in an initial telephony realisation or in the evolution of broadband services.

The evolution of the optical technology and the service package carried by an enhanced network are obviously closely coupled. For example the number of wavelengths available for broadband upgrade will depend crucially on the optical technology invoked. Also the technologies used for exchange to customer transmission could be economically viable well in advance of customer to exchange transmission because of resource sharing at the exchange end.

The technology available for optical wavelength multiplexing can be crudely divided into three categories of sophistication with many permutations in between a more detailed breakdown of possible optical technology evolution and service packages is illustrated in Figure The production tolerances of the fixed wavelength filters and the center wavelengths and line widths of the F-P laser sources would mean that technology category a would limit the number of wavelengths available to between 6 and 12 wavelengths over both windows of the fibre.

In the customer to exchange direction where temperature control of the laser sources might be prohibitively expensive the number of wavelengths available could be limited to between 2 and 4 over both windows. With the technology b scenario the numbers of potential wavelengths could be considerably greater with maybe as many as one to two hundred being possible in the exchange to customer direction in the longer term.

However it may well be that practical considerations such as the size of split or safety issues would limit the size of the wavelength multiplex before the optical technology did so. Even in the upstream direction, without any means of wavelength drift correction, 10 - 50 channels could be available. Where the coherent technology of scenario c is invoked then many hundreds of wavelengths are possible in principle, the limitations being imposed by non-linear phenomena in the fibres.

With the large number of wavelength channels and the potentially large optical power budgets available, this technology would offer a further major reappraisal of the operating topologies for optical networks. The three technology scenarios are also indicative of relative timescale availability.

With scenario a effectively being "today's" technology, b being possible in the two to five year time scale and c maybe being available within the decade at commercially acceptable prices. However any time scale predictions concerning advanced optical technology must be made with extreme caution and may even, given the pace of earlier optical development, prove to be pessimistic.

Given that wavelength multiplexing will be the method for introducing broadband services into the network and that studies into the optimum topology are still required, the following are some examples of how the bidirectional branching network with two stages of splitting might evolve described with reference to Figures 12 to The narrow pass optical filter at the customer's equipment allows the passage of only the initial wavelength for narrow band services, thus blocking interfering channels from and unauthorised access to broadband services added at a later stage.

Another key provision for wideband service is the installation at the outset of a multi-stage cabinet splitter which operates over a broad optical bandwidth in both and windows. This facilitates partial bypass by wideband service feeder fibres between the exchange and cabinet see below. These extra fibres may be installed either within the cable or separately at a later date.

Figure 13 shows how additional wavelengths can be used to add new services eg. The extra wavelengths are carried to the cabinet via additional feeder fibres and are fed into the network at space inputs to the cabinet splitter. The additional wavelengths will in general carry a higher bitrate than the telephony and ISDN channels. Customers destined to receive the additional broadband services would be equipped with a simple wavelength demultiplexer to separate the broadband and narrowband wavelengths.

At this bitrate the optical split could be limited to 32 ways compared with say for the telephony optical split. However the addition of only one or two extra optical wavelengths could provide a CATV service delivering 16 to 32 channels on the basic optical telephony network. This would require very few additional optical components - i.

Additional wavelengths provided in this way give rise to an important choice for the operation of the CATV services: the customers could access any of the broadcast wavelengths via a tunable optical filter incorporated into their terminal equipment. This would allow simultaneous reception of several channels chosen from the electrical multiplex of 8 or 16 channels carried on the selected wavelength. Simultaneous reception of more than one optical wavelength would require additional optical filtering and an optical receiver for each additional wavelength selected.

In this case the network operates as a star with the switch sited centrally at the exchange. This system would use fixed wavelength demultiplexer and one optical receiver in the customer's equipment. Although this might simplify the customer equipment it could mean a compromise between service penetration and number of simultaneous channels received by the customers. A more advanced stage using DFB lasers and illustrated in Figure 14 will allow the allocation of at least one dedicated wavelength per customer.

For example, with say 12 to 32 wavelengths available on a 32 way split it would be possible to allocate each CATV customer with one wavelength to carry all the required broadband services eg. Rather than simply dedicating the wavelengths to individual customers there is also at this stage the opportunity of using tunable optical filters at the customers' premises as a broadband switching stage. This could significantly simplify the exchange switching of disparate broadband services eg mixtures of broadcast and dedicated services from multiple sources could be multiplexed onto different optical wavelengths and be selected by the customer equipment.

For each of the technology stages described the number of wavelengths that are possible depends critically on the tolerancing and stability of the lasers, filters and the useable bandwidth of the fibre and couplers. Low cost narrowband services such as telephony and ISDN may necessarily operate without temperature stabilisation in customers' terminals which could mean significant wavelength drifting of the customers' lasers. Hence if schemes such as those shown in Figure 2 to 7 are used, large channel spacings would be necessary for services in the customer to exchange direction of transmission.

Closer spacing would be possible in the exchange to customer direction by using temperature controlled sources at the exchange and tunable filters within the customers' equipment to eliminate filter centre wavelength tolerances.

A central station for a digital communications network, arranged to transmit data to outstations in the form of a stream of frames, each comprising a synchronisation portion containing a synchronisation signal in the form of a predetermined pattern of bits, the central station comprising a scrambling means for scrambling the frames in accordance with a predetermined binary sequence, means for inverting successive nth bits in at least a part of the scrambled frames adjacent to the scrambled synchronisation portions thereof, but not inverting successive nth bits in the scrambled synchronisation portions themselves, thereby preventing false recognition of synchronisation due to an identical pattern of bits in said part of the frames adjacent to the synchronisation portion.

A central station as claimed in Claim 1 wherein each frame comprises a first portion containing housekeeping data for the outstations and said synchronisation portion, and also comprises a second portion containing message data for the outstations, and the inverting means is arranged to invert successive nth bits of only the housekeeping data in the first portion of each frame.

A central station as claimed in Claim 2 wherein the scrambling means comprises a first scrambler arranged to scramble the contents of the first portions of the frames in accordance with a first predetermined binary sequence, and a second scrambler arranged to scramble the second portions of the frames in accordance with a second predetermined binary sequence.

A central station as claimed in Claim 3 wherein the first and second scramblers of the scrambling means share a single predetermined binary sequence generator and utilize respective taps of a shift register of the generator. A central station as claimed in any one of the preceding Claims wherein n is sixteen. An outstation for a digital communications network, arranged to receive frames from a central station as claimed in Claim 1, the outstation comprising descrambling means for descrambling the frames in accordance with the predetermined binary sequence, means for identifying the synchronisation signal in the output of the descrambling means, and means arranged to correct the inverted nth bits thereby to reconstitute the original form of said at least a part of the scrambled frames.

An outstation as claimed in Claim 6 and arranged to receive frames from a central station as claimed in Claim 3, wherein the descrambling means comprises a self-syndronising descrambler coupled to said identifying means; another descrambler for descrambling the contents of the first portions of the frames in accordance with said first predetermined binary sequence, and a further descrambler for descrambling the contents of the second portions of the frames in accordance with said second predetermined binary sequence.

An outstation as claimed in Claim 7, wherein said another and said further descramblers share a single binary sequence generator and utilize respective taps of a shift register of the generator.

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Line Coding is always needed, whereas Block Coding and Scrambling may or may not be needed depending upon need. Scrambling is a technique that does not increase the number of bits and does provide synchronization. Note —. Skip to content. Change Language. Related Articles. Computer Network Fundamentals. Physical layer.

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