WO2000021257A2

WO2000021257A2 - Apparatus for routing data packets in a dtm network

Info

Publication number: WO2000021257A2
Application number: PCT/SE1999/001800
Authority: WO
Inventors: Per Lindgren; Christer Bohm; Bengt J. Olsson
Original assignee: Net Insight Ab
Priority date: 1998-10-07
Filing date: 1999-10-07
Publication date: 2000-04-13
Also published as: WO2000021257A3; SE513509C2; SE9803419D0; SE9803419L; EP1127432A2

Abstract

The present invention refers to a circuit board to be connected to a switch core. According to the invention, the circuit board comprises: an interface (111) for receiving one or more input DTM channels from said switch core and for transmitting one or more output DTM channels to said switch core; means (115) for deriving at least a portion of a data packet received, divided into DTM time slots, in one of said input DTM channels; routing means (117) for selecting, based upon information provided in said at least a portion of a data packet, if said data packet is to be transmitted in one or more of said output DTM channels and, if so, which one or more of said output DTM channels said data packet is to be transmitted in; and output means (116) for providing one or more output DTM channels with said data packet, divided into DTM time slots, in accordance with the selection of output DTM channels made by said routing means.

Description

APPARATUS FOR ROUTING DATA PACKETS IN A DTM NETWORK

Technical Field of Invention

The present invention refers to the field of circuit switched coinmunication networks, and, more specifically, to the field of routing data packets in a DTM network.

Background of the Invention

Today, new types of circuit-switched communication networks are being developed for the transfer of information using synchronous time division multiplexed bit- streams. Within this field, a new technology, referred to as DTM (Dynamic synchronous Transfer Mode) , is currently being developed, primarily addressing the problem of providing quality of service to users of real-time, broadband applications. The structure of a DTM network has been described in, e.g., "The DTM Gigabit Network", Christer Boh , Per Lindgren, Lars Ramfelt, and Peter Sjδdin, Journal of High Speed Networks, 3 (2) : 109-126, 1994, and in "Multi-gigabit networking based on DTM", Lars Gauffin, Lars Hakansson, and Bjorn Pehrson, Computer networks and ISDN Systems, 24 (2) -119-139, April 1992.

The basic topology of a DTM network is preferably a bus with two unidirectional, multi-access optical fibers connecting a number of nodes. However, the topology may just as well be any other kind of structures, e.g. a ring structure or a hub structure.

The bandwidth of each wavelength on the bus, i.e. each bitstream on each fiber, is divided into recurrent essentially fixed size frames, which in turn are divided into fixed size time slots. The number of slots in a frame thus depends on the network's bit-rate. The time slots are divided into two groups, control slots and data slots. Control slots are typically used for transferring of signaling messages between said nodes for the net- work's internal operation. The data slots are used for the transfer of payload data between users connected to the different nodes.

Each node is arranged to dynamically establish, terminate, and modify DTM channels by dynamically allocating time slots thereto.

When a DTM channel is used for transferring asynchronous traffic, such as TCP/IP packets, a mechanism for providing routing of said packets through the DTM network is often needed. In prior art, such a mechanism is typi- cally provided by the addition of a dedicated router station, either directly connected to the DTM network or indirectly connected to the DTM network via, e.g. an Ethernet link connecting the router station to a DTM access device. In this context, the use of a dedicated router station of course implies additional costs and increased network complexity. An object of the invention is therefore to provide a routing solution in a DTM network that reduces the cost for incorporation of dedicated router stations.

Summary of the invention

The above mentioned and other objects are achieved by the invention as defined in the accompanying claims. According to a first aspect of the invention, there is provided a circuit board to be connected to a switch core and being provided with an interface for receiving one or more input DTM channels from said switch core and for transmitting one or more output DTM channels to said switch core. Furthermore, said circuit board comprises means for deriving at least a portion of a data packet received, divided into DTM time slots, in one of said input DTM channels. Also, said circuit board is provided with routing means for selecting, based upon information provided in said at least a portion of a data packet, if said data packet is to be transmitted in one or more of said output DTM channels and, if so, which one or more of said output DTM channels said data packet is to be transmitted in, and with output means for providing one or more output DTM channels with said data packet, divided into DTM time slots, in accordance with the selection of output DTM channels made by said routing means.

The invention is thus based upon the idea of providing, in a DTM network, a routing mechanism in a circuit board designed to be received as a disconnectable module ^" in a switch apparatus and to communicate via the switch core thereof. Thus, a network operator may add a routing mechanism to a DTM network by merely incorporating an electronic circuit board according to the invention in a typically already existing switch of the network.

Consequently, according to a second aspect of the invention, there is provided an apparatus for switching data in a communication network, said apparatus comprising: a switch core; one or more circuit boards, each providing access to one or more network links; one or more electronic circuit boards of the above mentioned kind; and means for receiving said circuit boards and for providing connectivity between said circuit boards and said switch core.

Furthermore, additional advantageous aspects has been found when designing the interface between the switch core and the electronic circuit board in such a way that complete DTM frames are exchanged.

Thus, according to a preferred embodiment according to said first aspect of the invention, said the interface of the circuit board comprises: means for receiving sequential input DTM frames from said switch core and for transmitting sequential output DTM frames to said switch core; and means for determining the existence of one or more input DTM channels transferred in said input DTM frames, and of one or more output DTM channels trans- ferred in said output DTM frames, and said output means comprise frame generating means for generating said sequential output DTM frames and for providing DTM time slots thereof, defining an output DTM channel, with said data packet, divided into DTM time slots, in accordance with the selection of output DTM channels made by said routing means. Similarly, said switch core is preferably arranged to provide time and space switching between DTM ports and wherein said one or more circuit boards, which provide access to said network links, each comprises an interface for receiving sequential input DTM frames from said switch core and for transmitting sequential output DTM frames to said switch core.

One advantage of using such an interface between the circuit board and the switch core is that the protocols used to handle DTM frames at said interface may, if so desired, be designed very similar to the protocols used at any other DTM interface.

For definition, as referred to herein, a DTM network" is a circuit switched time division multiplexed network of the kind wherein information is transferred between nodes of the network on bitstreams. Each bit- stream is divided into regularly recurrent, essentially fixed size frames, so called "DTM frames", each comprising a number of fixed size time slots, said time slots being separated into control slots and data slots. Cont- rol slots are used for control signaling between nodes of the network, and data slots are used for the transfer of user data (sometimes often referred to as payload data) .

Furthermore, in a DTM network, write access to the time slots of a DTM frame is distributed among nodes being attached to the bitstream carrying said DTM frame, each node typically having write access to a respective at least one control slot and a respective dynamically adjustable set of data slots within each recurrent frame. Moreover, having write access to a time slot position in a frame means having write access to said time slot position within each recurrent frame. In a DTM network, a node will use the data slots it has write access to for establishing so called "DTM channels" by allocating one or more of said data slots to each respective DTM channel. Hence, as referred to here- in, a DTM channel is defined by one or more time slots occupying the same time slot position within each DTM frame of the bitstream upon which said DTM channel is carried. However, if a DTM channel reaches, for example, - over two bitstrea s, the channel may of course be defined by a different set of time slot positions on the two bitstreams. Also, a DTM channel may be either a control channel or a data channel, depending on whether control or data slots that is allocated to said channel. Furthermore, a DTM channel may be uni-, multi- or broadcast. As the demand for network capacity changes, DTM channels may be dynamically established, terminated, or modified, the latter by changing the number of time slots allocated to a DTM channel. Also, the distribution of write access to time slot among different nodes may be dynamically modified as different nodes develop different needs for control signaling and data transfer.

Consequently, a circuit board according to the invention is typically provided with means for determining which input and output channels that are to be handled by said routing processor and which that are to be bypassed, i.e. not routed via the routing processor.

The above-mentioned and other aspects and features of the invention, such as the use of a switch core memory shared by all switch ports and the use of a router memory shared by all channels accessed by the router means, will be more fully understood from the following description of embodiments thereof.

Brief Description of the Drawings Exemplifying embodiments of the invention will now be described with reference to the accompanying drawings, wherein: Fig. 1 schematically shows en example of the structure of a DTM frame of a bitstream in a DTM network;

Fig. 2 schematically shows transfer of asynchronous traffic in one of the DTM channels shown in Fig. 1; Fig. 3 schematically shows a switch equipped with a circuit board according to the invention;

Fig. 4 schematically shows an exemplifying embodiment of a circuit board according to the invention;

Fig. 5 schematically shows a switch core connected to a circuit board according to an embodiment of the invention; and

Fig. 6 schematically shows another switch core connected to a circuit board according to another embodiment of the invention.

Detailed Description of an Exemplifying Embodiment

An example of the structure of a DTM frame of a bit- stream in a DTM network will now be described with reference to Fig. 1. As shown in Fig. 1, in a DTM network, a bitstream B, interconnecting at least two bitstream access units, is divided into recurrent, essentially fixed sized DTM frames, wherein the start of each DTM frame is defined by a frame synchronization time slot F. Each DTM frame will typically have a nominal duration of 125 μs .

Each DTM frame is further divided into a plurality of fixed sized, typically 64 bit, time slots. When using said frame length of 125 μs, a time slot size of 64 bits, and a bit rate of 2 Gbps, the total number of time slots within each frame will be approximately 3900.

The time slots are divided into control slots Cl, C2, C3, and C4, and data slots Dl, D2, D3, and D4. The control slots are used for control signaling between the nodes of the network, whereas the data slots are used for the transfer of payload data. Each node connected to the bitstream B is typically allocated at least one control slot, i.e. each node will have write access to at least one control slot. Furthermore, write access to data slots are distributed among the nodes connected to the bit- stream. As an example, a first node (connected to the bitstream B) will have access to a control slot Cl and a set of data slots Dl within each DTM frame of the bit- stream, another node (also connected to the bitstream) will have access to a control slot C2 and a set of data slots D2 within each DTM frame of the bitstream, and so on. The set of slots allocated to a node as control slot(s) and/or data slot(s) occupy the same respective slot positions within each DTM frame of the bitstream. Hence, in the example, said first node's control slot Cl will occupy the second time slot within each DTM frame of the bitstream. During network operation, each node may increase or decrease its access to control slots and/or data slots, thereby re-distributing the access to control slots and/or data slots among the nodes. For example, a node having a low transfer capacity demand may give away its access to data slots to a node having a higher transfer capacity demand. Furthermore, the slots allocated to a node need not be consecutive slots, but may reside anywhere within the DTM frame.

Also, note that each DTM frame typically begins with said frame synchronization time slot, defining the frame rate on the bitstream, and ends with one or more guard band time slots G.

In Fig. 1 at (c) , it is furthermore assumed that said second node, having access to its control slot C2 and its range of data slots D2, has established four channels CHI, CH2, CH3, and CH4 on the bitstream. As shown, each channel is allocated a respective set of slots. In the example, the transfer capacity of channel CHI is larger than the transfer capacity of channel 2, since the number of time slots allocated to channel CHI is larger than the number of time slots allocated to channel CH2. The time slots allocated to a channel occupy the same time slot positions within each recurrent DTM frame of the bitstream.

An example of the transfer of asynchronous traffic in one of the isochronous channels carried by the bit- stream B shown in Fig. 1 will now be described with reference to Fig. 2. In Fig. 2, it is assumed that the channel CH3 shown in Fig. 1 is established to carry asynchronous traffic in the form of sequentially transmitted variable size data packets, which for example could be TCP/IP packets or Ethernet frames.

Fig. 2 shows three data packets transmitted in channel CH3. Each data packet is encapsulated according to a predefined encapsulation protocol. In Fig. 2, it is assumed that the encapsulation protocol defines that each data packet shall be divided into 64 bit data blocks (corresponding to the size of a time slot) , that a start_of_packet slot S is to be added to the start of each data packet, and that an end_of_packet slot E is to be added to the end of each data packet, thereby forming encapsulated data packets PI, P2, and P3. In case of gaps between packets, the bitstream is provided with so called idle slots, identifying said gaps as not providing valid data. Note that Fig. 2 only shows the sequence of sequential time slots transmitted within the channel CH3. Since Fig. 1 schematically indicates that channel CH3 comprises seven time slots within each DTM frame on bitstream B, the first seven time slots transmitted in the channel CH3, i.e. the first seven time slots in Fig. 2, will be transmitted in one DTM frame, the next seven time slots will be transmitted in the next DTM frame, and so on.

A switch equipped with a circuit board according to the invention will now be described with reference to Fig. 3. In Fig. 3, a switch apparatus 50 is shown comprising a switch casing 52, a switch power and control unit 54, an electronic circuit board 56 providing a routing mechanism according to the invention, and a DTM network interface card 58 providing access to a DTM network link. As schematically illustrated in Fig. 3, the circuit board 56 and the DTM network interface card 58 is releasably connected to a switch core (not shown) arranged inside the switch casing 52. An embodiment of a circuit board 110 according to an embodiment of the invention will now be described with reference to Fig. 4. In Fig. 4, the electronic circuit board 110 comprises a port 111, which in turn comprises an incoming channel interface 113 and an outgoing channel interface 114 receiving and transmitting, respectively, time slot data in DTM channels from/to a switch core (not shown) . The incoming and outgoing channel interfaces will provide for synchronization of the operation of the circuit board 110 in relation to the DTM frame frequency accordance with the switch core.

The incoming channel interface and the outgoing channel interface are connected to an incoming channel manager 115 and an outgoing channel manager 116, respectively. The incoming channel manager 115 and the outgoing channel manager 116 are both connected to a routing processor 117, a shared memory 119, and a buffer manager 120. The routing processor 117 is connected to a routing memory 118. Furthermore, a controller 121 is connected to the incoming channel manager 115 as well as the outgoing channel manager 116.

In operation, the incoming channel interface 113 will receive (arrow 1) data packets, for example TCP/IP packets, from the channels monitored by said interface. Each data packet is typically encapsulated according to a predefined protocol, as described with reference to Fig. 2, and will typically be received as a set of consecutive sequential 64 bit data blocks.

The incoming channel interface 113 will then forward, with preserved sequential order, each received data block to the incoming channel manager 115 (arrow 2) . Each data block forwarded to the incoming channel manager 115 is accompanied by a channel identifier, designating the channel from which it was received.

Having received sufficiently many data blocks at the head end of a data packet to be able to derive informa- tion designating the size of the data packet, the incoming channel manager will send a request (arrow 3) , containing the size of the data packet, to the buffer manager 120. The request will thereby inform the buffer manager 120 that the incoming channel manager 115 needs to store a data packet of the designated size in the shared memory 119.

The buffer manager 120 will then allocate an address space of the shared memory 119 to said data packet, the size of the allocated address space not being smaller than the size of said data packet. The buffer manager 120 will then answer the request by returning (arrow 4) a start address corresponding to the start of said address space to the incoming channel manager 115.

Having received said start address from the buffer manager 120, the incoming channel manager will start writing the data blocks forming the associated data packet into the shared memory 119 (arrow 5) , starting at the start address received from the buffer manager data and incrementing the address one step for each data block written into the shared memory 119.

At the same time, the incoming channel manager 115 will send the start address received from the buffer manager 120, along with the address designated in the header of the data packet, to the routing processor 117 (arrow 6) .

Using the routing memory 118 (arrow 7), the routing processor 117 will, based upon the address received from the incoming channel interface 115, determine whether or not the associated data packet is to be transmitted from the outgoing channel interface 114 and, if so, which outgoing channel that is to be used when transmitting said data packet. Having determined an outgoing channel for the data packet, the routing processor 117 will transmit a signal to the outgoing channel manager 116 (arrow 8), containing a channel identifier and the start address received from the incoming channel manager. The channel identifier identifies the outgoing channel to be used when transmitting the associated data packet address, and the start address designates where to read the associated data packet from in the shared memory 120. Having received the outgoing channel identifier and the start address from the routing processor 117, the outgoing channel manager 116 will access the shared memory (arrow 9) and start reading (arrow 10) data blocks forming the associated data packet from the shared memory 119, beginning at the start address received from the routing processor 117 and incrementing the address one step for each data block read from the shared memory 119.

At the same time, the outgoing channel manager 116 will continuously receive requests (arrow 11) for data blocks for respective outgoing channels from the outgoing channel interface 114, said request being sent from the outgoing channel interface at the rate as DTM frames is requested for transmission to the switch core connected to the outgoing channel interface 114. As triggered by said requests for data blocks, when said requests relates to a channel identified by the a channel identifier received form the routing processor 117, the outgoing channel manager 116 will forward (arrow 12) , with preserved sequential order, each data block of the associated data packet, as read from the shared memory 119 starting at the designated start address, to the outgoing channel interface 114. The outgoing channel interface 114 will then, in turn, forward (arrow 13) the received data blocks to the respective channels on the outgoing bitstream.

Having read the ^' last data block of a data packet from the shared memory 119, the outgoing channel manager 120 will return (arrow 14) the associated start address, which was received from the routing processor 117, to the buffer manager 120. This will inform the buffer manager that the processing of the data packet stored at the address space associated with said start address is complete and that the buffer manager is now free to allocate said address space to a new data packet received via the incoming channel interface.

Also, as is understood, the purpose of the control- ler 121 is, among others, to determine, based upon information provided in control signaling received in a channel from the incoming channel manager, which channels that are to be handled by the incoming channel manager 115 and the outgoing channel manager 116, i.e. which channels that are to be directed to/from the routing processor 117. If there exists a channel that is received by the incoming channel manager but is not to be provided to the routing processor, said channel is bypassed at the input/output interface 113, 114. A switch core 203 connected to an circuit board 110 according to an embodiment of the invention will now be described with reference to Fig. 5. In Fig. 5, the switch core 203 receives time slot in DTM frames from two input ports 201a and 202a and transmits received time slot data to two output ports 201b and 202b.

Each input port 201a, 202a is arranged to write each received DTM frame into a respective frame buffer of a shared frame memory 204. The data blocks from the time slots of a DTM frame are written sequentially into corre- sponding time slot data fields of the respective frame buffer, i.e. one data field for each input time slot.

At the same time, two time slot selection units (not shown) are arranged to select time slot data blocks to be transmitted in output DTM frames by deciding, for each output time slot to be transmitted into the respective output DTM frame, which frame buffer, and from which time slot data entry thereof (i.e. among the presently stored time slot data blocks from both currently stored DTM frames) , time slot data slot is to be collected, or passed on, to the respective output DTM frame. Hence, each selection unit is connected to the frame memory for the selection and collection of data blocks therefrom. In order to know which frame buffer and entry or field thereof to be used for a specific output time slot, each selection unit has access to a respective slot mapp- ' ing table (not shown) that, for each time slot of the respective output DTM frame and at a respective entry, provides one field designating the entry or field of said memory to be used for collecting the given output time slot. Consequently, the selection unit will pick data blocks in given output order for each time slot of the output DTM frame to receive time slot data blocks. Of course, the switch will only transmit data blocks into those slots of the output DTM frame that are allocated for that purpose.

Furthermore, as schematically illustrated in Fig. 5, a circuit board 110 according to the invention, and thus being equipped with routing means, for example as described with reference to Fig. 4, is connected to the input/- output ports 201, 202 of the switch and is arranged to received DTM frames from, and transmit DTM frames to, the switch core 203. Also, the above mentioned selection unit will determine which time slots that go into the DTM frames delivered to the circuit board 110.

In the situation shown in Fig. 5, a channel defined by time slot 7 of DTM frames received on port 201a is read to the output port 202b, more specifically to the second time slot of the DTM frame delivered therefrom, and is received by the routing circuit board 110. Based upon routing decisions, the routing circuit board 110 will transmit data packets received on said time slot 2 of the DTM frame at port 202b to either a channel defined by time slot 2 or a channel defined by time slot 3 of the DTM frame on port 202a, said channels then being mapped into time slot 6 and 7, respectively, of the output DTM frame on port 201b. Consequently, a data packet received on the channel defined by time slot seven on port 201a will be routed to the channel defined by time slot 6 or the channel defined by time slot 7 on the output port 201b. (As is understood in Fig. 5, the routing circuit board will receive and transmit entire DTM frames from/to the switch core. However, it will typically only read data from and transmit data into time slot thereof that define channels that the routing circuit board 100 is currently configured to provide routing in relation to.) Also, in the situation shown in Fig. 5, the selection unit is set so that time slot seven of the DTM frame received on port 201b is also, in addition to what has been described above, mapped to time slot five on the DTM frame on port 201b. Thus, packets received on the channel defined by time slot seven at port 201a is always transmitted on the channel defined by time slot five at port 201b in a circuit switched manner, irrespective of the routing decisions made by the routing circuit board 110. As is understood, even though the channels described with reference to Fig. 5 is defined by one single time slot, they could just as well comprise any number of time slots within each frame, as dynamically selected in a DTM network.

Fig. 6 shows another embodiment solution wherein the switch core 203 of a switch is realized in form of a DTM ring/bitstream 205 connecting all ports, said ports acting as nodes on said internal DTM ring. As schemati- cally illustrated, each time slot of the input ports 201a and 201b is written into a respective time slot of the internal DTM bitstream frame. Each output port 201b, 202b then reads selected time slots of the internal DTM bit- stream 205 when transmitting output DTM frames. Moreover, the routing circuit board connected to the switch core is arranged to receive data packets from channels defined by time slots on the internal DTM bitstream 205 and to route said data packets to channels similarly defined by time slots on the internal DTM bitstream.

As an example, in the situation illustrated in Fig. 6, an input channel defined by time slots five and six at port 202a is mapped into time slots 12 and 13 of the internal DTM bitstream 205 and is read by the routing circuit board 110. The circuit board 110 then routes said data packets to, for example, a channel defined by time slot 17 of the internal bitstream, said channel then being mapped into an output channel defined by time slot five as well as an output channel defined by time slot six of the output DTM frame at port 202b.

Even though the invention has been described above with reference to exemplifying embodiments thereof, these are not to be considered as limiting the scope of the invention. Consequently, as understood by those skilled in the art, different modifications, combinations and alterations may be made within the scope of the invention, which is defined by the accompanying claims.

Claims

1. A circuit board (56; 110) to be connected to a switch core, said circuit board comprising: an interface (111) for receiving one or more input DTM channels from said switch core and for transmitting one or more output DTM channels to said switch core; means (115) for deriving at least a portion of a data packet received, divided into DTM time slots, in one of said input DTM channels; routing means (117) for selecting, based upon information provided in said at least a portion of a data packet, if said data packet is to be transmitted in one or more of said output DTM channels and, if so, which one or more of said output DTM channels said data packet is to be transmitted in; and output means (116) for providing one or more output DTM channels with said data packet, divided into DTM time slots, in accordance with the selection of output DTM channels made by said routing means.

2. A circuit board as claimed in claim 1, wherein said interface comprises: means (113, 114) for receiving sequential input DTM frames from said switch core and for transmitting sequential output DTM frames to said switch core; and means (121) for determining the existence of one or more input DTM channels transferred in said input DTM frames, and of one or more output DTM channels trans- ferred in said output DTM frames, and wherein said output means comprise: frame generating means (114) for generating said sequential output DTM frames and for providing DTM time slots thereof, defining an output DTM channel, with said data packet, divided into DTM time slots, in accordance with the selection of output DTM channels made by said routing means.

3. A circuit board as claimed in claim 1 or 2, comprising a memory (119) for temporarily storing data packets at respective memory locations thereof, wherein said interface comprises means (115) for writing said data packet into an allocated memory location of said memory when receiving said data packet and wherein said frame generating means comprises means for reading said data packet from said allocated memory location for transmission in accordance with the selection of output DTM channels made by said routing means.

4. A circuit board as claimed in claim 3, further comprising a storage manager (120) arranged to tempora- rily allocate a memory location of said memory for storing said data packet and to provide said interface with information designating said memory location.

5. A circuit board as claimed in claim 4, wherein said memory location is allocated by said storage manager for storing said data packet as a result of a request made by said interface when receiving said data packet.

6. A circuit board as claimed in any one of the preceding claims, comprising means (121) for determining which input and output channels that are to be handled by said routing processor and which that are to be bypassed.

7. A circuit board as claimed in any one of the pre- ceding claims, wherein said data packet, when transmitted within said channel, is encapsulated according to a DTM encapsulation protocol.

8. An apparatus (50) for switching data in a commu- nication network, comprising: a switch core (203); one or more circuit boards (58), each providing access to one or more network links; one or more circuit boards (56; 110) as claimed in any one of the preceding claims; and means for receiving said circuit boards and providing connectivity between said circuit boards and said switch core.

9. An apparatus as claimed in claim 8, wherein said switch core is arranged to provide time and space switching with respect to DTM frames and wherein said one or more circuit boards, which provide access to said network- links, each comprises an interface for receiving sequen- tial DTM frames from said switch core and for transmitting sequential DTM frames to said switch core.

10. An apparatus as claimed in claim 9, wherein said switch core comprises a memory (204) having a number of memory locations, each being associated with a respective circuit board, and wherein said means for providing connectivity between said circuit boards and said switch core comprise DTM frame receiving and generating means, each associated with a respective circuit board and each having write access to a respective one of said memory locations, for writing DTM frames received from the respective circuit board thereinto, and having read access to all of said memory locations, for reading time slot data from selected DTM time slot fields thereof when generating DTM frames to be delivered to the respective circuit board.

11. An apparatus as claimed in claim 10, comprising means for establishing DTM channels between circuit boards by determining which memory fields of said memory that said DTM frame receiving and generating means are to read data from when generating DTM frames to be delivered to respective circuit boards.

12. An apparatus as claimed in any one of the preceding claims, wherein said switch core is circuit switched.