SWITCH FOR MULTIPLEXES

Abstract: The invention concerns an add/drop multiplex comprising a matrix switch (30) the drop side of which is connected via buses (31, 32) to respective time slot assignment function circuits (33, 34) and characterised in that the time slot assignment function circuits (33, 34) are interconnected by at least one bus (35) capable of transferring traffic between the add and the drop functions of the multiplex.

Description: SWITCH FOR MULTIPLEXES This invention relates to an improvement in add-drop multiplexes, also known in Europe as drop-and-insert multiplexes. Such multiplexes are a particular form of combined multiplexer/ demultiplexer for digital signals, optimised for the low cost insertion or extraction of a proportion of the "through" traffic capacity of the multiplex. A digital multiplex combines multiple digital signals streams into a composite stream of digits, each of the digits in the original streams conventionally being allocated to a unique timeslot in the frame structure of the composite stream. Following recognition of the inserted frame start signal at the demultiplexer, the demultiplexer separates the multiple signals so as to restore them to their original form. In a terminal digital multiplex, all of the capacity of the composite stream is accessible in the form of multiple individual streams; by contrast, in an add-drop multiplex this is commonly not the case, and there are other differences which are outlined below. It is gradually becoming a common feature of add-drop multiplexes that they incorporate a matrix switch. This switch has the ability to control the routing of traffic in small increments, each of which is a small fraction of the total traffic passing through the multiplex. SUBSTITUTESHEET Where this fraction is particularly small , the switch has a correspondingly hi gh compl exity, because of the large number of such increments to be manipulated, and this invention relates to a proposed simplification of the switch. Accordingly the present invention provides an add-drop multi pl ex comprising a matrix switch, a pair of time slot assignment function circuits connected to the drop-side of the matrix switch, and one or more data buses interconnecting the timeslot assigment function circuits. In order that the present invention may be more read ly understood an embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which; Figure 1 a block diagram showing the basic functions of an add-drop multiplex; Figure 2 shows conventional add-drop multiplexes , Figure 3 shows a network incorporating add-drop multiplexes, and Figure 4 is a bl ock diagram of an add-drop multiplex according to the present invention. Referring now to Figure 1 of the drawings, Figure 1 shows how an add-drop multipl ex is conventional ly arranged. Figure 1 shows the basic function of an add-drop Multiplex. In the embodiment shown in this fi gure the traffic is demultipl exed at the terminal multiplexes 1 and 2 and sent between the multiplexes via wi re l inks 3. In accordance with requirements the traffic can pass between the multiplexes untouched or can be routed so that it is added to or dropped. Tributaries for adding or dropping traffic are shown at 4. This arrangement has the restriction that it invol ves the expense of two ful l terminal multipl exes and requires manual arrangement of the cabl es each time there is to be a change in routing of any component of the traffic. Referring now to Figure 2 of the drawings this shows a conventional add-drop multi pl ex system where routing flexibil ity is given by an interchange or matrix switch 5. Optical fibre lines are shown at 6 along with associated optical terminals 7. The output tributaries carrying data to be dropped or added are shown at 8. SUBSTITUTE SHEET The functionality of the interchange or matrix switch varies between designs; in particular it can vary in the following parameters: a) the size of the smallest increment of traffic which can be routed, and whether such an increment is fixed in size or is variable in the operation of the multiplex. b) the routing options which are available, e.g. whether the increments can be transferred between timeslots as they pass through the multiplex, or merely be accessed by the add-drop tributary ports (it is not uncommon for selection of the accessible times!ots to be the only degree of routing control which the multiplex offers; where there is no switching function, this control is implemented by physically associating each plug-in card with certain timeslots so that they are accessed automatically if the appropriate card is inserted). c) whether increments of traffic can be selectively sent back in the direction from which they came, possibly in a different timeslot, in line with item (b) above. d) the number of ports on the multiplex which are able to support its full ''through" traffic capacity. Combining the full functions of (a) and (b) allows the management of bandwidth in a chain of add-drop multiplexes. This is intended to be done by opening up a gap in the traffic, freeing an associated group of timeslots by transferring their previous contents to other timeslots, a process referred to as "repacking". As a result, the later insertion of a wideband data channel becomes possible. A wideband channel is conventionally specified so that it must occupy such an associated group of timeslots, rather than being spread piecemeal in a manner similar to that commonly associated with the writing of data to a partially filled computer disc drive. Item (c) is not normally required of an add-drop multiplex, this aspect of network flexibility being conventionally restricted to cross-connects, which have a much larger traffic capacity than is normal in an add-drop multiplex. This restriction is because at such a facility there is a much smaller probability of the wanted path being unavailable because of being already occupied. SUBSTITUTESHEET Ite (d) is i ll ustrated in fi gure 3, which shows two types of add-drop multi pl ex; these are slave and ring master multi pl exes 20, 21. The ring master connects a ring or chain of add-drop multi plexes to the network, and provides the gateway for management control and for network timing to support the operation of the add-drop multipl exes. The remaini ng multi pl exes are slave ones. In an add-drop port capacity on the l ink side is by definition equal to the capacity of the add-drop multiplex. The capacity on the drop side is conventionally asymptotic to the same value, although facilities to access the full capacity are commonly not fitted. As can be seen in Figure 3, the ring master 21 has 4 full capacity ports and each slave 20 has 3. This arises because the ring master has two different streams of "through" traffic, one for each side of the ring, while the slave has only one. In practice a proportion of the through traffic of the ring master may be passed along the ring rather than being accessed by the network as shown in fi gure 3. On a slave multiplex, the drop-side port may take the physical form of a number of relati vely l ow capacity tributaries. On a ring master the same may be true but these ports may al so take the form of a pair of high capacity (i .e. high bit rate) physical ports, as implied in figure 3. In the latter case the ring master has no capacity to spare to support a number of local ly ari sing circuits, i .e. it cannot provide the same function as the slave multiplex, whose entire drop-side capacity is dedicated to just this function. There are significant commercial situations in which a ri ng master may be required to provide this kind of drop-side access whi le still acting as a ring master. One possibl e sol ution to the problem is to provide a fifth port on the ring master, dedicated to the same role as the drop-side port in the sl ave multiplex. Thi s would result in there being three drop-side ports on the ring master. Such an arrangement suffers from the disadvantage that switches are commonly constructed from elements with an even number of ports , usually a power of 4, so that large switches can be SUBSTITUTE SHEET constructed by a simpl e inter-connection of a number of such el ements . For commonal ity across a range of products , the switchi ng el ement in an add-drop multi pl ex should fol l ow this rule also. An uneconomic alternati ve to providing a fi fth port is to use two add-drop multi plexes at the ring master location , one being the ring master and the other being a slave multi plex. A better scheme is to construct a 6-port switch from a pair of 4-port el ements inside the same add-drop multiplex, but this is again unattractive because it involves replicating one of the potential ly most expensive el ements in the add-drop multiplex in order to gain an increase in port capacity, and this increase is then wasteful ly l arge. A more economic sol ution can be provided by taking advantage of a further feature which an add-drop multi pl ex may be requi red to provide. This feature is consol idation . It arises because traffic streams accessed on the drop-side ports may be only partial ly fi lled with traffic. Since the total drop-side capacity of a slave multi pl ex is commonly equal to the "through" capacity of the multiplex, any partial fi l l ing of the tributaries on the drop side represents lost capacity for access. To overcome this probl em, at least one network operator has proposed that add-drop multi pl exes be fitted with an excess of tributary port capacity, larger possibly than the "through" capacity. The concept is that some internal concentration wil l be applied so as to pack the through path more efficiently than the partly loaded tributaries, the multi pl ex selectively accessing only those time slots which are loaded with traffic. The invention lies in providing the interchange or matrix switch with an extra pair of interconnected buses . This is shown in Figure 4. In this figure a 4-port matrix switch 30 occupies the position of the switch 5 in Figure 2 and al so acts as a ring master. The drop side of switch 30 has buses 31 , 32 connected to a pair of respective time slot assi gnment function circuits 33, 34; there being one time slot assignment function ci rcuit for each di rection of traffic. The time slot assignment function ci rcuits each have access to al l the drop-side capacity of switch 30 and each of the tributary ports can be al located a share of the total bus capacity. SUBSTITUTE SHEET The each buses are indicated at 35. Each timeslot assignment function now has the ability to allocate capacity on any bus (three for one direction of traffic, three for the other). The purpose of the new buses 35 is to transfer traffic between the add and drop functions, al l owing traffic entering or leaving on one tributary, not only to enter or leave via the l ink side, but alternati vely to do so via another tributary. This is a function normal ly reserved to a cross-connect and considered inappropriate in an add-drop multiplex, as described in paragraph C earlier in this specification. In this case however, the extra facil ity allows possibl e l ower rate tributaries serving l ocal circuits, to be connected either to the "through" path or to the high bit rate tributaries connected to the network, i .e. the facility of a slave multipl ex is provided. If not all of the traffic on the link side is to be connected to the network, possibly because it is connected to circulate on the ring, then traffic capacity inherently becomes free on the buses, al l owing the local tributaries to access that capacity and connect to the network via the high bit rate tributaries. The described arrangement provides the function of a fifth port, within the inherent capacity limitations of the add-drop multiplex, and with greater economy of circuits and devices than known alternatives. SUBSTITUTE SHEET

Claims: CLAIMS 1. An add/drop multiplex comprising a matrix switch (30) the drop side of which is connected via buses (31, 32) to respective time slot assignment function circuits (33, 34) and characterised in that the time slot assignment function circuits (33,34) are interconnected by at least one bus (35) capable of transferring traffic between the add and the drop functions of the multiplex. 2. An add/drop multiplex as claimed in Claim 1, and further characterised in that the matrix switch (30) acts as a ring master. 3. An add/drop multiplex as claimed in Claim 1 or claim 2, and further characterised in that the time slot assignment function circuits have access to all the drop-side capacity of switch (30) and each of the tributary ports can be allocated a share of the total bus capacity. SUBSTITUTE SHEET

Inventors: FERGUSON STEPHEN PATRICK, FERGUSON, STEPHEN, PATRICK
Applicants: PLESSEY TELECOMM, GEC PLESSEY TELECOMMUNICATIONS LIMITED
Application year: 1991
Main classification: H04J3
IPC codes: H04J3