Abstract:
An optical network unit having an environmentally sealed printed circuit board arrangement within a first compartment and a signal in/out arrangement in a second compartment. The compartments are provided by two housings. Instead of replacing individual circuit boards in the case of breakdown or malfunction, replacement of the whole of the printed circuit board arrangement is necessary thereby eliminating human error and minimizing downtime. Two housings provide the two compartments. These housings are preferably mounted upon a housing mount and a carrier for the optical network unit is also preferably mounted upon the housing mount. This housing and carrier arrangement simplifies assembly and disassembly of parts of the unit for maintenance and replacement purposes.

Description:
This application claims the benefit of Provisional Application No. 60/341,220, filed Dec. 20, 2001. 

   FIELD OF THE INVENTION 
   This invention relates to optical network units. 
   BACKGROUND OF THE INVENTION 
   Optical network units are conventionally placed close to customers&#39; premises, e.g. businesses or homes. These units are connected to central offices by optical fibers and/or copper pairs. Signals received from these fibers and copper pairs are converted into electrical signals which are then transmitted along relatively short electrical conductors to the customers&#39; premises. Electrical signals, in the frequency range above the voice band, received from the customers are converted by the optical network units into optical signals which are transmitted by the optical fibers to the central office. Electrical signals in the voice band received from the customer are transmitted to the central office on the copper pairs. Normally it is expected that an optical network unit is situated adjacent to equipment known by various terms such as a joint wiring interconnect, outside wiring interface, serving area interconnect or feeder distribution interface. This equipment connects the optical network unit to the customer&#39;s line to transfer the voice, data, video, high definition broad band and XDSL signals to the customer. 
   Optical network units are located outside customers&#39; premises whereby they are subject to ambient temperature, wind and humidity conditions. For protection against these conditions and against vandalism, a housing is required to provide suitable protection to the circuitry, connections and any other features within the housing and upon which an optical network unit depends for operation. 
   In order to provide for their functional requirements, there are various designs of optical network units. In one particular design, e.g. as shown in U.S. Pat. Nos. 5,828,807 and 5,982,972, a frame has two compartments each having an opening. One compartment houses the electronics and optics of the optical network unit, and the second compartment houses a drop terminal block or blocks of the unit. Doors are provided for covering and sealing the openings when the unit is in use. The doors have a gull-wing design which enables them to be rotated upwards when the interior of the optical network is to be accessed. Catches or struts are provided to hold the doors in open positions, thereby freeing the hands of a technician working on the unit with the unit mounted horizontally, in which position the doors rotate by horizontal axes. With the doors open, each door thus overhangs its opening with the intention of preventing rain from entering its associated compartment. 
   One problem that may exist with the structure as described in the above patents, is that the internal environment for the electronics and optical components, is not controlled. Since field replaceable line cards are intended to be serviced through manual exchange through the opening to the appropriate compartment, humidity that enters the compartment, when the door is opened, is trapped inside the compartment after the door is closed, to seal the compartment. This humidity may affect the performance and reliability of the product, as the humidity, either as airborne mist or as heat-condensed droplets of water, may cause corrosion of electronic components and/or shorting out of the power supply. This would interfere with electrical signalling and/or power distribution. Even when new line cards are inserted into the compartment, the expected operating time, as required by industry standards, may be compromised due to the undesirable humid internal environment. In addition, the direction of wind-driven rain may be subject to change under certain uncontrollable environmental conditions. As a result, the doors when in open condition may not always prevent rain from entering the compartment. Wind-driven rain entering the electronics and optical compartment during service has similar potential for compromising the reliability and performance of the unit through corrosion and shorting out, as does trapped humidity. 
   A further problem that may exist with the above type of design relates to how the design affords the opportunity for human error to occur during service. Extended down time may be due to the exchange of line cards, which do not require exchange, and which are mistakenly identified as inoperable line cards. This would result in unnecessary delays in returning an optical network unit to its desirable function. It has been shown that in different types of electronic equipment, unnecessarily exchanging correctly operating line cards, where incorrectly operating cards should be exchanged, is a real and practical problem. Upon such mistakes being made, the problem may become intensified, thereby leading to further delay, when upon providing power to the unit once again, it is still found to be operating incorrectly. Such unnecessary delays, apart from economical considerations, and wastage of technicians&#39; time, are likely to lead to customers&#39; dissatisfaction due to extended lack of service. 
   SUMMARY OF THE INVENTION 
   The present invention seeks to provide an optical network unit in use of which the above problems are avoided or minimised. 
   According to one aspect of the present invention, there is provided an optical network unit comprising: a housed environmentally sealed printed circuit board arrangement; a housed telecommunications signal in/out arrangement interconnected for signal transmission to the printed circuit board arrangement; and means to provide access to each of the printed circuit board and signal in/out arrangements. 
   Units according to the invention defined above are not subject to humidity effects suffered by the optical and electronics components because the surrounding environment is unable to contact such components, because they are environmentally sealed. Thus any moisture present or rain driven into contact with the environmentally sealed arrangement can have no effect upon its performance. 
   In addition, upon failure of any of the optical or electronic components in the sealed arrangement, human error cannot occur during servicing. This is because printed circuit boards (or line cards) are not changed individually so that an incorrect circuit board cannot be replaced in times of breakdown in service. With the present invention, any breakdown in service of the printed circuit board arrangement necessarily requires its complete exchange for a new environmentally sealed printed board arrangement. Thus delays and expense caused by human error in returning to customer service after breakdown, are prevented from occurring. 
   In a preferred arrangement, a first housing member provides at least part of a first compartment containing the printed circuit board arrangement, and a second housing member provides at least part of a second compartment containing the signal in/out arrangement. It is envisaged that the first and second compartments may be defined within housings which are completely separate and may be spaced from each other. However, the preferred arrangement comprises a housing mount to which each housing member is detachably assembled. It is also to be preferred that the printed circuit board arrangement is also detachably assembled to the housing mount. This allows for a simplified structure. It is also advantageous that the first housing member defines sides and one end of the first compartment, the other end of which is defined by the housing mount. Hence, upon removal of the first housing member from the housing mount, the printed circuit board arrangement is fully exposed extending from the housing mount to enable it to be removed and replaced. 
   Further, it is convenient for the second housing member to have a closure element across an access opening and to enable securing means holding the housing members to the housing mount to be accessible through the opening for removal of the housing members. 
   It is also preferable to provide vertically spaced air vents in the first housing member to allow cooling air to flow upwards through a passage defined between that housing member and the printed circuit board arrangement for cooling purposes. This avoids the possible necessity of having electrically driven fan arrangements for cooling purposes and which could be subject to break down. 
   The invention further includes an assembly of an optical network unit as defined according to the invention above and a carrier member for the optical network unit. The carrier member enables the optical network unit to be mounted in a fixed position to some rigid structure. In this further form of the invention, the carrier member is also detachably assembled to the housing mount which also assists in providing a simple structure, in which the housing mount provides the base unit for all of the components. 
   The invention also includes an optical network unit in which the first and second compartments are upper and lower compartments respectively. In these structures the environmentally sealed printed circuit board arrangement is located in the upper compartment, and any heat generated by this arrangement not only tends to move in an upward direction, but is also driven upwards by the convection airflow, thereby moving the heat away from the signal in/out arrangement. 
   With the above latter structure, the housing members lie on opposite sides of the housing mount. However, it is envisioned that in other structures the housing mount may carry, on one of its sides only, both of the housing members, together with the enviromentally sealed printed circuit board arrangement and the signal in/out arrangement. The housing members may or may not lie side-by-side or one above another in these other structures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  is a front isometric view of an assembly of an optical network unit of the embodiment and a carrier member; 
       FIG. 2  is a view similar to  FIG. 1  with an upper housing member sectioned and a lower housing member open; 
       FIG. 3  is a view showing the righthand side of the assembly with the lower housing member open and in section to show inside detail; 
       FIG. 4  is a view similar to  FIG. 3  of the lefthand side of the assembly; 
       FIG. 5  is an exploded front isometric view of the assembly from the side opposite to  FIG. 1 ; 
       FIG. 6  is a scrap cross-sectional view of the assembly taken along line Vl—Vl in  FIG. 1  with inside features omitted for clarity; 
       FIG. 7  is a plan view of a housing mount of the optical network unit; 
       FIG. 8  is a front isometric view of a sub-assembly of the upper housing member and a housing mount; 
       FIG. 9  is an exploded front isometric view of the sub-assembly of  FIG. 8 ; 
       FIG. 10  is a front isometric view of a sub-assembly of an environmentally sealed printed circuit board arrangement and the housing mount; 
       FIG. 11  is an exploded front isometric view of the sub-assembly of  FIG. 10 ; 
       FIG. 12  is a front isometric view of a sub-assembly of the lower housing member and the housing amount; 
       FIG. 13  is an exploded front isometric view of the sub-assembly of  FIG. 12 ; 
       FIG. 14  is a front isometric view of a sub-assembly of the housing mount and the carrier member; 
       FIG. 15  is an exploded front isometric view of the sub-assembly of  FIG. 14 ; 
       FIG. 16  is a view similar to  FIG. 3  and showing optical transmission features removed from within the lower housing member; 
       FIG. 17  is a view similar to  FIG. 4  and showing electrical features removed from within the lower housing member; 
       FIG. 18  is a view similar to  FIG. 2  of the assembly and with the upper housing member removed; 
       FIG. 19  is a cross-sectional view through the environmentally sealed printed circuit board arrangement, taken along line XlX—XlX in  FIG. 11 ; 
       FIG. 20  is an isometric exploded view according to one modification of the embodiment; 
       FIG. 21  is an enlarged scrap isometric view of the modified part of the unit of  FIG. 20 ; and 
       FIG. 22  is a view similar to  FIG. 2  of a modification to the assembly. 
   

   DESCRIPTION OF PREFERRED EMBODIMENT 
   Basically, as shown by  FIGS. 1 and 2 , an optical network unit  10  has a housing comprising an upper housing member  12  and a lower housing member  14 . As shown particularly by  FIG. 5 , the upper housing member  12  has four vertically elongate sides  16  and a upper closed end  18  which define part of an upper compartment of the unit. A lower end of the compartment is defined by a housing mount in the form of a horizontal planar plate  20  to which the upper housing member is secured, as will be described. Also as shown by  FIG. 5  and other Figures, the upper compartment contains a environmentally sealed printed circuit board arrangement  22 , to be further discussed. 
   The lower housing member  14  is also detachably secured to the plate  20 , as is a carrier member  24 . 
   More specifically, as shown for instance, by  FIGS. 1 ,  2 ,  5 ,  12  and  13  the lower housing member  14  has three sides  26  which, together with a closure element in the form of a multi-sided door  28 , define a lower compartment of the optical network unit. The door  28  is hinged at its bottom edge about a horizontal axis, with the hinge position  30  being inset relative to the upper plan view of the lower housing member so that a relatively small lower end  32  is provided for this housing member. The upper end of the housing member  14  has four inwardly extending flanges  34  provided with screw clearance holes  36  to correspond in position with sixteen screwthreaded holes  38  arranged in four straight lines in the plate  20  ( FIG. 7 ). The lower housing member is detachably assembled to the plate  20  ( FIG. 12 ) by screws  40  ( FIG. 13 ) which extend upwardly through the flanges  34  and into the plate  20 . 
   As shown in  FIGS. 8 and 9 , the upper housing member  12 , is detachably secured to the plate  20  by four upwardly extending screws  42  (three being shown), which pass through clearance holes in the plate and into threaded holes (not shown) in flanges  44  which are turned inwardly from the sides  16  of the upper housing member ( FIG. 6 ). As shown by  FIG. 6 , the flanges  44  and  34  overlie each other and the flanges  34  are necessarily relieved at positions  46  ( FIG. 13 ) to provide clearance for the heads of the screws  42  into contact with the plate  20 . Two circular front clearance holes  48  in the plate  20  are provided to accept screws  42 , whereas the rear clearance holes  50  are keyhole shaped. This allows for the two rear screws  42  to be partly inserted into holes in the upper housing member  12 , for location and registration purposes through the wide parts of the holes  50  during mounting, and before securing the mounting member in position. 
   The carrier member  24  has a vertical rear wall  52  and two opposing vertically extending flanges  54  (see particularly  FIG. 5 ). In the assembly, the plate  20 , together with the upper housing member  12 , lies between these flanges  54 , as shown particularly in  FIGS. 1 ,  3  and  4 . The rear wall  52  and the flanges  54  have inwardly turned lower ends  56  which lie beneath marginal edge regions of the plate  20  to carry the weight of the optical network unit ( FIGS. 6 and 14 ). The flanges  54  are secured to the plate  20  adjacent to the flanges  54  by horizontally extending tamper-proof screws  58 , which pass through the flanges and into screw-threaded holes  60  in the edges of the plate (see  FIGS. 14 and 15 ). 
   As may be seen from the above, the housing and carrier member structure is particularly simple in design with both the housing members  12  and  14  and the carrier member  24  detachably assembled onto a single feature, i.e. the plate  20 . To detach either housing member from the plate  20 , the door  28  is lowered to its position shown, for instance, by  FIGS. 2 and 13 , thereby providing an access opening  61  in the lower housing member  14  for manual removal of the appropriate screws  40  and  42 . 
   A particularly important aspect of the present invention is the environmentally sealed printed circuit board arrangement. In this embodiment, the arrangement  22  is provided for receiving incoming optical and electrical signals from a central office (by means to be described), and for transmitting corresponding electrical signals through a joint wiring interconnect (not shown) to customers&#39; premises. It also serves the function of receiving electrical signals from the customers&#39; premises to return these as corresponding electrical and optical signals to the central office for dialled distribution. 
   The arrangement  22  comprises a housing  62  ( FIGS. 5 ,  10 ,  11  and particularly  FIG. 19 ) within which a plurality of printed circuit boards or line cards, to be described, are environmentally protected. The housing  62  comprises two housing halves  63  each of which has a finned main wall  64 , the fins  65  of which are vertical heat exchange fins. From one side of the main wall  64  extend two long vertical side walls  66  and two relatively short top and bottom walls  67  and  67   a,  which extend between and are joined to the side walls  66 . Free ends of the walls define a rectangular opening and terminate in a continuous outwards flange  68  surrounding the opening. 
   The two housing halves are secured together at the free ends of side, top and bottom walls, with an environmental and EMI seal  69  disposed between free ends of the walls and between the flanges  68 . The seal  69  is a composite environmental and EMI seal. However, as is known generally in the art for sealing purposes, separate EMI and environmental seals may be provided. Screws (not shown) pass from flange to flange  68  to hold the housing halves  63  assembled with the seal  69  compressed between the housing halves. An environmentally sealed chamber  70  is thus formed between the housing halves. 
   The chamber  70  houses four printed circuit boards  71  and  73  to provide the required electronic function of the unit  10 . The printed circuit boards are assembled on to the two housing halves  63  before these are secured together. A printed circuit board  71  is secured to the main wall  64  of each housing half  63  by heat conductive screws  75 . These pass through the printed circuit board  71  in heat conductive relationship to a ground plane (not shown) of the board and into the associated wall  64  of the housing half. Each printed circuit board  71  is spaced from its associated wall  64  by heat conductive (e.g. copper) spacers which surround the screws  75 . The screws  75  are disposed in horizontally spaced vertical rows across the board  71 , one vertical row of three screws being shown for each board  71  in  FIG. 19 . All of these screws are aligned, each vertically with an associated fin  65  and extend, into the bases of the fins to shorten the heat exchange paths into the fins. 
   Each printed circuit board  71  has a spaced and parallel printed circuit board  73  attached to it, each board  71  joined to its board  73  by connectors  77  between top and bottom end regions of the boards. 
   A flexible ribbon cable or flex circuit  79  connects top end regions of the printed circuit boards  71  and a power board  81  is secured to the main wall  64  in lower regions of one of the housing halves  63 , heat conductive screws  83  and spacers  85  being used for this purpose. These screws extend into bases of the fins  65  as shown in  FIG. 19 . 
   The housing  62  is formed from a suitable heat exchange material, in this embodiment aluminum, but other heat exchange materials may be used. To preserve the surface of the housing  61  in all types of environmental conditions, the housing is subjected to the known irriditing process followed by a protective paint coating such as polyurethane. 
   In the assembled housing  62 , the fins  65  extend from both front and rear surfaces of the housing (see  FIGS. 10 ,  11  and  19 ). These fins  65  extend into airflow passages  87  defined between the fins and the inside surfaces of sides  16  of the upper housing member  12  ( FIG. 19 ). The vertical side walls  66  are also spaced from the sides  16  (this not being shown). 
   In use, heat generated by the printed circuit boards is to be conducted away by the fins to dissipate this heat into the atmosphere. To create a necessary convection air flow up through the passages  87 , the sides  16  of the upper housing member  12  are provided with vertically spaced air vents, i.e. upper and lower air vents  72  and  74 . In use the cool air will enter the lower vents  72  and pass upwardly through the airflow passages  87  to extract heat, the heated air then passing into ambient atmosphere through the upper vents  74 . 
   To assist in ensuring that the printed circuit board arrangement  22  does not become overheated because of ambient temperature conditions or exposure to direct sunlight, the housing member  12  is desirably made with low heat conducting characteristics and thus forms a solar shield. Such a structure may be made by any suitable molding or fabrication process, applicable to the material from which it is being made, which may be certain plastics or metals. An appropriate plastic foam material is envisaged which may include a glass fiber reinforcement. Such a structure complies with GR-950 CORE and GR-487 CORE. 
   The printed circuit board arrangement  22  is also assembled in detachable fashion to the plate  20 . This is as shown by  FIGS. 10 and 11  in which the lower end of the arrangement  22  has a downwards extension  76  (see also  FIG. 19 ) which registers within a rectangular orifice  78  within the plate  20  with the end  80  of the arrangement  22  supported by the plate  20  within a well defined by edges  82  of the plate. Upwardly extending screws  84  extend through clearance holes  86  in the plate  20  and into the end  80  of the arrangement  22  for securing purposes. These screws  84  are also accessible from within the lower compartment with the door  28  in the open position, for instance as shown by  FIGS. 12 and 13 . 
   As may be seen from the above description, each of the housing members  12  and  14 , the carrier member  24  and the printed circuit board arrangement  22  are detachably secured to the housing mount, i.e. plate  20 . This is a conveniently designed structure for the following reasons. 
   For initial installation, the optical network unit  10  is factory assembled, as shown in  FIG. 1 . In this condition, ( FIG. 11 ) the printed circuit board arrangement  22  has incoming and outgoing short optical cable, 88 , two short electrical transmission cables  90  and a short electrical power cable  92 , extending in environmentally sealed relationship between the bottom walls  67   a  of the housing halves  63  and into the environmentally sealed chamber  70 . As shown by  FIG. 19 , the power cable  92  is connected to the power board  81 , the optical cable  88  is connected to one of the printed circuit boards  71  and the electrical transmission cables  90  are connected one to each of the printed circuit boards  73 . 
   After installation, within the lower compartment, a signal in/out arrangement is provided. This comprises passive electrical and optical components. These include protectors  94  and  96  for preventing electrical surges from reaching the printed circuit boards  62 , and a slack fiber management device  98  for optical cable ( FIGS. 2 ,  3 ,  4 ,  16  and  17 ). The short telecommunications electrical cables  90  are interconnected by environmentally sealed connectors  100  to other cable lengths  102  which extend from the protectors  94 . A sealed connector  104  ( FIG. 17 ) also connects the power cable  92  to a power cable  106  extending from its protector  96 . Short cable lengths  110 ,  112 , extend from the other sides of environmentally sealed protectors  94  and  96 , to be joined by environmentally sealed connectors  114  to power and electrical transmission cables  116  and  118 , respectively, (see also  FIG. 5 ). The cables  116  and  118  pass through and are environmentally sealed to a rectangular base plate  120  of the housing member  14  ( FIG. 5 ), the base plate  120  being secured in position and environmentally sealing a rectangular aperture  122  at the lower end of housing member  14 . 
   One of the transmission cables  90  is connected to the central office while the other cable  90  is interconnected into the joint wiring interconnect (not shown) for interconnection to customers&#39; premises. 
   The optical cable  88  ( FIGS. 3 ,  11  and  16 ) extends into the slack fiber management device  98  to be joined by connector  124  to a coiled superfluous length optical cable  126 , joined by connector  128  to optical cable  130 . This cable extends from the device  98  to be interconnected to the central office. The cable  130  is also environmentally sealed into the base plate  120 , as shown by  FIG. 5 . 
   During installation of the unit  10 , the carrier member  24  ( FIG. 5 ) is secured by screws (not shown) passing through keyhole shaped clearance holes  132  ( FIG. 14 ) into an upstanding pedestal  134 , or horizontal support (not shown), the clearance holes assisting in screw location. After installation of the factory assembled unit  10  using screws  58  through the carrier member  24  and into plate  20  ( FIGS. 14 and 15 ), the passive electrical and optical components are then added into the housing member  14  to complete the assembly and interconnect the arrangement  22  in the desired manner to power, central office and customers&#39; premises. To successfully connect all cables together within the housing member  14 , the protectors  94  and  96  and slack fiber management device  98  need to be outside and in front of the housing member  14  for access purposes, as shown in  FIGS. 16 and 17  with the door  28  open. Hence, although not shown in the drawings (particularly  FIGS. 3 and 4 ), the cables  88 ,  90 ,  102 ,  106 ,  110  and  112  need to be sufficiently long for their ends to be drawn from within the housing member  14  for connection purposes. After connections are made, the protectors  94  and  96 , and the fiber management device  98 , are moved into stowed positions ( FIGS. 3 and 4 ) upon slides (not shown) within the housing member  14 . The door  28  is then closed and secured with a tamper-proof lock  136  ( FIG. 1 ). 
   During use, in the event of a breakdown or malfunction of one or more of the printed circuit boards  71 ,  73  and  81 , any problem deciding which board is at fault or in removing a correctly operating printed circuit board, instead of a faulty one, is avoided. This is because the printed circuit board arrangement  22  with its environmentally sealed housing  62 , denies direct access to any of the printed circuit boards for removal and exchange purposes. Thus, increase in downtime due to human error is avoided. Instead, with structures according to the invention and as exemplified by the embodiment, the whole of the printed circuit board arrangement  22  needs to be replaced by a new arrangement  22 . As will be appreciated, such removal and replacement is a rapid, simple and positive operation. 
   A pre-built factory assembly may be made of the arrangement  22 , the plate  20  and the lower housing member  14  which contains the cables and cable lengths such as items  90 ,  92 ,  102  and  130 , referred to above, and protectors  94  and  96 , the slack fiber management device  98  and the connectors such as items  100 ,  114 ,  124  and  128 . The lower housing member  14  may thus be sealed under factory controlled conditions to provide environmental sealing to the lower compartment, or lower chamber. For this sealing purpose, seals, for instance gaskets, are provided between mating surfaces. In the embodiment these include a gasket  137  beneath end  80  of the arrangement  22  ( FIG. 11 ), gasket  139  between flanges  34  of housing member  14  and the plate  20  ( FIG. 6 ), gasket  141  around the opening to door  28  ( FIG. 12 ) and gasket  143  between lower end  32  of the lower housing member and base plate  120  ( FIG. 5 ). All of the gaskets are partially shown only. This factory installed environmental sealing also involves relatively short lengths of cables  116 ,  118  and  130 , extending from the sealed lower compartment for connection to appropriate longer lengths (not shown) outside the unit  10  for signal transmission and power input to the unit as desired. 
   Replacement of the arrangement  22  in the unit  10  is necessarily effected by first opening the door  28  to provide access through the opening  61 . After removal of the protectors  94  and  96  and the fiber management device  98  to the position shown in  FIGS. 16 and 17 , the cables  88 ,  90  and  92  are disconnected and the screws  42  are removed to allow for removal of the housing member  12  from the plate  20 . At this point, the arrangement  22  is fully exposed ( FIGS. 10 and 18 ) extending upwards from the plate  20  and is easily removed by firstly removing screws  84  ( FIG. 11 ). The arrangement  22  is then replaced with the new, unused and factory prebuilt environmentally sealed arrangement  22 . As may be seen, with this structure of the embodiment, customers&#39; service may be returned in the shortest possible time, while minimizing human error. 
   Further, with the environmentally sealed arrangement  22  in its compartment, apart from moving the passive elements forwards in the housing member  14  for access purposes, as discussed, further interference with these elements for removal of the arrangement  22  is avoided. Also, should any of the protectors  94 ,  96  require replacement, this may be done simply by opening the door  28  without any contact or interference with the contents of the housing member  12  being necessary. 
   In addition, the design of the optical network unit  10 , together with the carrier member  24 , is practical for other reasons. As the housing members  12  and  14 , and the carrier member  24  are detachably secured to the plate  20 , should the need arise to replace a housing or carrier member, perhaps because of damage thereto, this is easily effected. The complete unit  10  may be removed from the carrier member  24  merely by removing the screws  58  with an appropriate tool to allow for change of the carrier member. Alternatively, should it be necessary to replace either or both housing members  12  and  14 , manual access to the appropriate screws through opening  61  is easily effected with the plate  20  remaining secured to the carrier member  24 . Further, it is possible to replace either housing member, together with the plate  20  as a sub assembly, should the need arise. This is shown by the two sub-assemblies of plate  20  and housing members  12  and  14 , in  FIGS. 8 and 12 . 
   There is a further advantage with a design of a unit  10  having two housings, each assembled onto a housing mount or plate  20 . If, after insulation, advancements in design or technology enable a smaller hermetically enclosed printed circuit board arrangement to be provided to replace the arrangement  22  described, then the smaller unit may be used if its lower regions are suitable for securing into the well formed by edges  82  of the plate  20 . With this considered as a possibility, the housing member  20  may be replaced in one modification, by a housing member  136 , but made in sections as shown in  FIGS. 20 and 21 . The member  136  has side panels  138  and vent panels  140  which provide sides of the member when assembled together. Vertical corner elements  142  detachably secure the panels together to form the sides of the member  136 . The corner elements  142  have vertical slots  144  to receive narrow edge regions  146  of the panels. Screws  148  are then used to secure the edge regions  146  into the slots. A closed top  150  and an open bottom framework  152  hold the carrier elements  142  in position by screws  154 . With the housing member  136  of the same size and shape in plan view as the housing member  12 , the member  136  is attached to the plate by the screws  42  as in the embodiment. 
   Should a shorter design of printed circuit board arrangement be constructed and use as a replacement for the arrangement  22 , then the member  136  may be made appropriately shorter to decrease the total size of the optical network unit. This is done by dismantling the member  136  by either reducing the height of each panel  138  or by replacing it with a shorter panel and correspondingly reducing the length of each corner element  142 . 
   In a further embodiment, as shown by  FIG. 22 , in which the optical network unit is as described in the embodiment, the carrier member  24  is replaced by another carrier member  160  for mounting the unit  10  to a different support. The carrier member  160  has an upper element  162  similar to the carrier member  24  but having the holes  132  omitted. A lower element  164  is attached to the upper element  162  by screws (not shown). The lower element has three sides  166 , two of which have inwardly turned flanges  168  provided with bolt holes  170 . It is intended that bolts should be passed through the holes  170  to mount the carrier member  160  onto a concrete slab (not shown). 
   In yet a further embodiment (not shown), the environmentally sealed printed circuit board arrangement  22  has, at its lower end region, a plurality of visual indicators which face forwardly of the unit  10 , these indicators provided for the purpose of showing whether the sealed printed circuit boards are receiving power and whether the operational status of the product is in active mode or in failure mode. These indicators provide an interface which may accommodate wired and/or wireless communication interfaces for hand held terminal devices for the purpose of indicating product configuration and/or diagnostics through external administrative software. To reach the indicators, the front lower air vent  72  is provided as part of a hingeable door directly in line with the indicators.