AU583428B2

AU583428B2 - Optical distribution system

Info

Publication number: AU583428B2
Application number: AU63790/86A
Authority: AU
Inventors: Ian Murray Dr. McGregor
Original assignee: Australian Telecommunications Corp; Australian Telecommunications Commission
Current assignee: Telstra Corp Ltd
Priority date: 1985-09-25
Filing date: 1986-09-23
Publication date: 1989-04-27
Anticipated expiration: 2006-09-23
Also published as: DK265887A; WO1987002205A1; DK265887D0; EP0239591A1; AU6379086A; EP0239591A4

Description

"OPTICAL DISTRIBUTION SYSTEM"

This invention relates to an optical distribution system.

More specifically, the invention relates to an optical distribution system which can serve a group of users or customers by means of a fibre optic communica¬ tion line and provide for two-way communication between an exchange and the users. The provision of a fibre-optic communication line to a user of course enables a high capacity communications link to be established between the exchange and the users so that signals such as video signals, hi-fi signals or high speed data communications can be handled.

There have been some proposals to provide fibre links from an exchange to users but these proposals have drawbacks. In one proposal the exchange is connected to the respective users by means of separate fibre optic fibres. This is relatively expensive because of the need to provide and install separate fibres from the exchange to the users. Further the exchange needs a separate transmitter/receiver for each fibre. In an alternative arrangement, a single loop has been proposed which extends from the exchange to all of the users and back to the exchange, each user being coupled to the loop by a coupling element. This arrangement, however, would require a considerable upgrade in the installation practices at and near users premises to ensure that an acceptable level of reliability is achieved.

Accordingly, the object of the present invention is to provide a relatively inexpensive optical distribution system for two-way communication from an exchange to users.

According to the present invention there is provided an optical distribution system for two-way communication from an exchange to up to N user stations, said system comprising an exchange transmitter, an exchange receiver, an exchange coupler for coupling said transmitter and receiver to an optical fibre communica¬ tion line, a passive N-port directional coupler having one fibre lead coupled to said fibre communications line and N fibre tails extending to respective user stations which are required in the system, each user station including a user transmitter, a user receiver, and a station coupler for coupling the fibre tail to the user transmitter and user receiver.

The invention will now be further described with reference to the accompanying drawings, in which:

Figure 1 is a schematic view of one form of optical distribution system of the invention;

Figures 2A, 2B and 2C are graphs illustrating the performance of the system; Figure 3 is a schematic representation of one frame for signals transmitted in the system;

Figures 4A and 4B are schematic representations of alternative frames for signals transmitted on the signal;

Figure 5 is a schematic view of an alternative optical distribution system;

Figures 6A to 6D are possible transmitter spectrums and optical filter characteristics;

Figure 7 schematically shows a line code for clock regeneration;

Figure 8 shows schematically a circuit for clock phase recovery at the exchange; and

Figure 9 is a waveform diagram useful in under¬ standing the operation of the circuit of Figure 8.

The system illustrated in Figure 1 comprises an exchange transmitter 2 and an exchange receiver 4 coupled to fibre optic lines 6 and 8, the fibre optic lines preferably being single mode optical fibres. The lines 6 and 8 form two input/output ports of a 3-port directional coupler 10. The coupler 10 has its other input/output port in the form of fibre optic line 12 which is coupled to one end of a relatively long fibre optic communication line 14 via a connector 16. The coupler 10 is a passive element and which may be formed by a technique known in the art of twisting together two optical fibres, fusing them so that there are four fibre portions extending from the fused portion, and thereafter removing one of the fibre portions.

The line 14 extends from the exchange to a conven¬ ient point adjacent to a group of say sixteen users of the system. The system includes a 16-port directional coupler 18 having one input/output fibre 20 coupled to the other end of line 14 by a fused connection on splice 22. The coupler 18 has typically sixteen input/output ports 24 in the form of optical fibres which are connected to optical fibre tails 26 by means of fused connections 28. The coupler 18 is formed in a similar manner to the coupler 10 except that sixteen optical fibres are twisted together, fused and thereafter fifteen of the projecting fibre portions are removed. The fibre optic tails 26 extend from the coupler 18 to user locations 30, each of which might for instance comprise an office or a home.

Each user station 30 includes a user transmitter 32, user receiver 34, 3-port coupler 36 having fibre ports 38 and 40 connected to the receiver 32 and 34 respectively. The transmitter 32 may comprise LED transmitter hich is possibly cheaper than a laser transmitter. The other port 42 of the coupler 36 is connected to the end of the fibre optic tail 26 by means of a connector 44. The use of the fused connections 22 and 28 may be important because they have a relatively low return-loss, typically 26 dB return-loss, whereas the reflection at the connectors 16 and 44 would typically be as high as 17 dB return-loss.

Consider the operation of this system with identical transmitters in the exchange and the users' stations, and with continuous simultaneous transmissions to and from the exchange.

It can be shown that the minimum attenuation of the exchange transmission reflected back to.the exchange by all the reflections at the connectors 44 in the user stations 30 is:-

^A pi " ^RLP ^{+ A}0 ^{+ A}F ^{+ A}E

where RL = the return-loss of a connector 44,

A_Q = the end-to-end attenuation of the system, including all couplers,

A^ = the end-to-end attenuation of the fibre, line 14, excluding all couplers,

A_ = the excess attenuation of the multiport coupler 18.

Typical values are: RL = 17dB, A = (20-35)dB, A^ = (5-20)dB, A_E = 4dB. Hence,

A . = (46 - 76)dB

The attenuation of the exchange transmission reflected back to the exchange by the connector 16 in the exchange is:-

Ap = RLp + 2A.

where RLp = the return-loss of the connector 16, A_ = the attenuation of the 3-port coupler 10.

Typical values are: RLp = 17dB, A., = 4dB. Hence,

The NEXT of the 3-port coupler 10 is specified to be in excess of 40dB. Hence, the combined cross-talk (which is taken to be the power addition of A I. , ApΛ and coupler NEXT) is about 25dB, the dominant term being the reflection from the connector 16 in the exchange.

The desired signal at a receiver 4 must be at least 6dB higher than the combined cross-talk to provide an adequate margin for detection (assuming the system is operating well above the noise limits) . Hence, if all ^** the transmitters are identical (same power and spectral properties) the over-all attenuation of the optical path cannot exceed 19dB for satisfactory signal reception at the exchange receiver 4. If 8dB is allowed for the two 3-port couplers, this leaves a total loss of lldB for the main distribution fibre 14, the multi-port coupler 18 and the fibre tail 26. Typically a multi-port coupler with eight receiver stations 30 connected has about 12dB loss, and as a result, such a system is not particularly satisfactory. With four stations 30, the system is workable, having about 4dB of loss in the fibre 14. However, when the loss of the fibre 14 is so small other reflections may need to be considered.

An analysis from a user's end yields a very similar result, with the combined cross-talk being dominated by the reflection at the connector 44 in the station 30, a typical value being 25dB. Thus with the typical values for the losses men¬ tioned above, the simple system shown in Figure 1 does have limitations particularly regarding the number of users and the length of the line 14.

The problems of cross-talk can to a large extent be avoided by operating the system with a burst mode operation.

Consider now, the operation of this system with burst transmissions:

Type 1 The exchange transmitter 2 transmits to the first station 30 and then awaits its reply; the exchange then transmits to the second station and then awaits its reply; etc.

Type 2 The exchange transmitter 2 transmits to all user stations 30 in a TDM mode, and then the user stations reply in sequence with appropriate guard times.

Type 3 The exchange transmitter 2 transmits all the time, and the users reply in sequence at prede¬ termined times.

For a Type 1 system, when the exchange transmitter 2 is transmitting, the effect of the reflection from the connector 16 is to return a signal to the exchange receiver 4 which is typically about 25dB lower than the transmitted signal. As there is no desired receiver signal at this time, this signal is of little conse¬ quence because it can be eliminated in the receiver 4. The minimum attenuation of the exchange transmission signal reflected back to the exchange by all the reflections at the connectors 44 is:-

^A pi ^{* RL}P ^{+ A}0 ^{+ A}F ^{+ A}E

where RL = the return-loss of one connector 44,

- = the end-to-end attenuation of the optical system, including all couplers,

A_p, = the end-to-end attenuation of the fibre 14, excluding all couplers;

A_ = the excess loss of the multiport. coupler 18.

This" combined reflected signal will mostly coincide in time with a user's transmission from its transmitter 32. If it is assumed that transmitters with the same power are used in both the exchange and in the user station 30, then the signal to combined reflection ratio at the exchange receiver 4 is:-

Typical values are: R = 17dB, A_F = (5-29)dB, and A„ = 4dB. Hence,

which is adequate to ensure detection. When a user is transmitting the effect of its connector 44 is to return a fraction of that signal to its receiver 34. This signal may cause a problem, in that it may alter the phase of the clock in the user's receiver 32. For this reason, the receiver 34 is pre¬ ferably turned off during transmission, and for a short time after the transmission is completed to allow for the time taken for the reflection to return to the receiver 34. In most practical systems, however, station to station attenuation is so high that the station to station signals are lost in noise.

Figure 3 shows a typical frame structure for a communication with one user for a Type 1 burst-mode system. It will be appreciated of course that the frame structure is repeated for each station 30 connected, in the system.

If there are N stations 30, and the cycle time is Ts, then each station must complete its communication (burst from the exchange and a burst to the exchange) in T/N s. The number of bits of information in a burst is T 144000 (as there are typically 144000bit/s in each direction in a channel) . With the headers shown in Fig. 3; the exchange to station burst is (64+T144000)bits, and the station to exchange burst is (16+Tl4400Q)bits.

The time allocated to each station is therefore made up of the time to transmit information, and the time taken for that information to travel through the system. The transmission time Ta, is:- 64 + T x 144000 16 + T x 144000 ^Ta ⁼ 8.448X10^{6 +} 8.448 x 10⁶

The travel time, Tt, is:-

2L ZL t = ^V = 2 x 10⁵ = L x 10^"5

where L is the length of the fibre 14 plus the length of a fibre tail 26,

₅ V is the velocity of propagation m fibre (2 x 10 km/s) .

Now, Tt + Ta = T/N

L x lθ^"5 + (80 + T x 288 x 10³) /8.448xl0⁶ = T/N

where N = 16, and T = 4 ms:

L x lθ^"5 + (80 + 4 x 288)/8.448xl0⁶ = 4 x lθ^"3/16

L = 10.42 km

The distance between the exchange and the fur- therest station is strongly dependent on the cycle time - the time between successive communications with a station. If this time is very short then there is a high proportion of the total time spent waiting for signals to arrive at their destination; the longer the cycle time, the longer the burst length and a higher proportion of the total time is spent in active trans¬ mission. The cycle time is also the delay introduced into the local loop and adds directly to the delay for talker-echo calculations. As a result, it is necessary to keep this cycle time short, 1ms to 4ms being typically acceptable. _

Figures 2A to 2C show in graphical form typical results of calculations for performance characteristics of Type 1 burst mode systems having signal structures as shown in Figure 3. In these graphs, it is assumed that the line system operates at 8.448 M bits per second, there being a one millisecond loop delay in Figure 2A, a two millisecond delay in Figure 2B and a four milli¬ second delay in Figure 2C. The circles and crosses represent values for 8-way and 16-way systems respectively, i.e. the number of user stations 30 connected to the coupler 18.

For a Type 2 system the exchange sends 'out a TDM burst which contains information for all stations 30; the stations then reply in sequence at specified time intervals after the reception of the burst from the exchange.

The analysis of the reflections is identical to that of the Type 1 system described above.

A suitable exchange to station burst is as shown in Figure 4A, and a station to exchange burst is shown in Figure 4B.

Station 1 can reply immediately it registers the end of an exchange burst. Station 2 must than wait until the burst from station 1 has cleared the multiport coupler 18 before its information arrives at the multi¬ port coupler. The wait time is therefore dependent on the burst length from station 1, and also on the delays in the fibre tails 26 to station 1 and station 2. The burst length if fixed at 160bit, assuming that.the cycle time is 1ms. Now assuming a maximum length difference in the fibre tails 26 of 1km (5 microsecond or about 40 bit at 8.448Mbit/s) , station 2 must count 200 bit inter¬ vals before transmitting. Station 3 must count 400, station 4 counts 600, etc. As a result, a Type 2 system with 16 stations and with a 1ms maximum delay in the local loop can have a maximum fibre length of 32 km. This is in excess of what is required for most applica¬ tions, and as a result, it may be of value to trade-off some of the span for extra capacity and thereby provide additional services.

A Type 3 system is preferably utilized in conjunction with a system arrangement as diagram- matically shown in Figure 5. The system of Figure 5 is essentially the same as shown in Figure 1 except that at the exchange a filter 46 is interposed between the transmitter and the coupler 10. Further, the trans¬ mitter 2 comprises a laser diode which provides an optical power of say 0 dBm at the coupler 10. In a typical system, the receiver power at the stations 30 will be about -35 dBm. Assuming that there is a minimum of 25 dB isolation between the user station receiver and transmitter 32 and 34, the output power from the user transmitter 34 would have to be less than -20 dBm to ensure that the signal from the exchange at the receiver 34 is 10 dB above the signal from the transmitter 32, and therefore ensure satisfactory detection. If the signal level from the transmitter 32 is -20 dBm, then the level at the exchange receiver 4 will be -55 dBm, and the NEXT at the exchange end therefore has to be greater than 65 dB to ensure an adequate margin for detection. Accordingly, the filter 46 at the exchange should provide at least 40 dB attenuation at the exchange transmit wavelength. Figure 6A diagrammatically shows the output 48 at the transmitted wavelength from the exchange generator 2. Figure 6B shows the wavelength response of the filter 46, the response having a notch 50 at the transmitter wavelength. Figure 6C shows a typical output 52 from aτ LED transmitter which would comprise the transmitter 32 at the user station. Figure 6D shows schematically a typical received output 54 from the filter 46 which is passed to the exchange receiver 4. The effect of the filtering is small in the output 54; it selectively filters out the signal from the exchange transmitter 2.

This system, could operate at 34 Mbits/sec down¬ stream, i.e. from the exchange to the user stations 30 and 8.5 Mbits/sec. upstream. The downstream information rate is such that it could provide for stereo hi-fi broadcast transmissions and possibly other services.

The frame structure for the system of Figure 5 is widely variable since the exchange transmitter 2 trans¬ mits continuously.

A difference between the Type 1 and the Type 2 systems is that there are less waiting periods in the Type 2 system. It therefore has a greater capacity for a given system span, and can be configured to provide clock information at the receivers 32 for a greater fraction of the time than that possibly with the Type 1 system. The clock information received at the stations 30 is more bursty in the Type 2 system than in either the Type 1 or Type 3 systems, as users only receive one burst of clock information in each cycle. A line code with a very high clock content should therefore be used to help maintain the stability of the user's clocks. Fig. 7 shows one possible line code. An alternative would be the use of CMI.

The exchange has no need to derive a clock from the user transmissions, as the clock in the exchange is used to send the data to the users. There is however a need to determine the phase of the received signal in order to receive the data from the user stations. This can be done by the phase detector circuit 57 shown in Fig. 8. This system has an incoming signal input 56 which is over-sampled (possibly eight times the exchange clock frequency) . The appropriate phase of the exchange clock is determined by analysing the sampled received signal during the training period, (as shown in the frame structures of Figures 3 and 4B) . The sampling is accomplished using a clock from a clock generator 58 inputted to an analogue to digital converter 60. Output from the analogue to digital converter 60 is clocked into a eight stage shift register 62 the outputs A and B of the first and second stages of which are coupled to a logic circuit 64. Output from the circuit 64 is coupled to a divide by 3 counter 66 and to a divide by eight counter 68. Output from the counter 68 is coupled to the fourth stage of the register 62. In- this circuit, the most recent sampled output from the analogue to digital converter 60 is always compared with the previous sampled output. If three consecutive comparisons produce a "less than result" then the fourth sample must have been the maximum sampled value and therefore close to the peak of the received waveform. Once the maximum sampled value is identified in the training sequence subsequent data bits can be established by taking the values at every eighth sample. This technique is graphically illustrated in Figure 9 which shows the input 56 to the converter 60 and the typical sampling instants. When the signal to noise ratio is high, the circuit shown in Fig. 8 will provide appropriate phase detection from the eight phases of the exchange clock.

Many modifications will be apparent to those skilled in the art without departing from the spirit and scope of the invention.

Claims

1. An optical distribution system for two-way commu¬ nication from an exchange to up to N user stations (30) , said system comprising an exchange transmitter (2) , an exchange receiver (4) , an exchange coupler (10) for coupling said transmitter (2) and receiver (4) to an optical fibre communication line (14) , a passive N-port directional coupler (18) having one fibre lead (20) coupled to said fibre communications line (14) and N fibre tails (26) extending to respective user stations (30) which are required in the system, each user station including a user transmitter (32) , a user receiver (34) , and a station coupler (36) for coupling the fibre tail (26) to the user transmitter (32) and user receiver (34).

2. A system as claimed in claim 1 wherein the exchange and station couplers (10, 36) comprise 3-port couplers and wherein one input/output port (12) of the exchange coupler is coupled to the line 14 by a connector 16.

3. A system as claimed in claim 1 or 2 wherein said one fibre lead (20) is coupled to the line 14 by a fused connection 22 and the N-port coupler 18 has selected output ports (24) coupled to tails (26) by fused connec¬ tions (28) .

4. A system as claimed in any preceding claim where the tails (26) are coupled to the user stations (30) by connectors (44) . 5. A system as claimed in any preceding claim wherein the exchange transmitter (2) includes a laser or a LED wherein the station transmitters (32) include lasers or LEDs.

6. A system as claimed in any preceding claim wherein the exchange transmitter (2) transmits to each user station 30 and awaits a reply form the transmitter 32 of that station before transmitting to the next station.

7. A system as claimed in claim 6 wherein the trans¬ mission to and from each station (30) is in a signal frame which has first and second portions, the first portion being for transmission to the station and including a multibit training segment, a station address segment and a data transmission segment, and the second portion being for transmission from the station and including, a multi-bit framing segment, a station address segment and a data transmission segment.

8. A system as claimed in any one of claims 1 to 5 wherein the exchange transmitter (2) transmits to the stations in a TDM mode and the transmitters (32) of the stations transmit to the exchange in a predetermined sequence.

9. A system as claimed in claim 8 wherein transmission to each station (30) is in a signal frame which includes a first training segment and N sub-frames, each sub-frame including a station address segment and a data transmission segment and wherein transmissions from the stations occur in bursts, each burst commencing with a training segment and including station address and data transmission segments. 10. A system as claimed in any one of claims 1 to 5 wherein the exchange transmitter (2) transmits to all stations simultaneously and the transmitters (32) of the stations transmit to the exchange in a predetermined sequence.

li. A system as claimed in claim 10 wherein the exchange transmitter (2) includes a laser diode which produces output at wavelength and wherein a filter (46) having its notch at wavelength V_^is coupled betweer the exchange receiver (4) and the exchange coupler (10) _

12. A system as claimed in any one of the above claims wherein a phase detector (57) is provided at the exchange for detecting phase shifts of transmissions received at the exchange relative to a synchronizing signal transmitted from the exchange.