ATM cell transmission system

Abstract: In an ATM cell transmission system having an ATM layer device (1), a data path interface (3) and a plurality of normal PHY (Physical) layer devices (2-0 to 2-M) according to Utopia Level 2 specification, the ATM layer device (1) comprises: a cell buffer (4); FIFO memories (5-0 to 5-M) each corresponding to each of the normal PHY layer devices (2-0 to 2-M); an output controller (5') for controlling the cell buffer (4) to output an ATM cell to be transmitted through one of the normal PHY layer devices (2-0 to 2-M) into corresponding one of the FIFO memories (5-0 to 5-M) on condition that the ATM cell is stored in the cell buffer (4) and the corresponding one of the FIFO memories (5-0 to 5-M) is not full; and a cell transmission controller (10) for performing polling of the normal PHY layer devices (2-0 to 2-M), designating a selected PHY layer device among the normal PHY layer devices (2-0 to 2-M) which have returned the HIGH level of the cell transmission allowance signal (TxClav) to the polling and whereof corresponding FIFO memories (5-0 to 5-M) are not empty, selecting one of the FIFO memories (5-0 to 5-M) corresponding to the selected PHY layer device as a next sender of cell data to be transmitted through the data path interface (3), and designating the selected PHY layer device at an end of the current transmission cycle as a next receiver of the cell data.

Description: The present invention relates to an ATM (Asynchronous Transfer Mode) cell transmission system having an ATM layer device and more than one PHY (physical) layer devices connected to the ATM layer device by way of a data path interface according to Utopia (Universal Test & Operations PHY interface for ATM) Level 2 specification for transmitting ATM cells through the PHY layer devices. Specifications of interface between the ATM layer and the physical layer for absorbing diversity of PHY layer devices are studied in The ATM Forum as the Utopia. A data path interface definied for interfacing one ATM layer device with one PHY layer device is the Utopia Level 1 interface, and that defined for interfacing one ATM layer device with more than one PHY layer devices is the Utopia Level 2 interface, which is described in "Utopia, An ATM-PHY Interface Specification", pp. 1 to 66, Utopia Level 2, v1.0 (af-phy-0039.000), published by The ATM Forum Technical Committee. US 5,485,456 discloses a conventional ATM interface system coupling several PHY layers to one ATM layer, each PHY layer employing its receive and transmit queues. Along with the Utopia protocol, an appended extra ID byte is transmitted for identification which physical layer is to receive or transmit a data cell. Bi-directional data cell communication is provided via an external programmable logic device. Timing and priority issues for a required quality of service might be difficult to maintain for all transmit and receive members in the system as individual buffers are to be kept filled even under operating conditions with differing data rates among the PHY layers. FIG. 3 is a block diagram illustrating a basic configuration of an ATM cell transmission system having an ATM layer device 1, a plurality of PHY layer devices 2-0 to 2-M and a data path interface 3 according to the Utopia Level 2 for interfacing the ATM layer device 1 with the PHY layer devices 2-0 to 2-M for transmitting ATM cells through the PHY layer devices 2-0 to 2-M. Addresses 0 to M are assigned to the PHY layer devices 2-0 to 2-M, respectively. FIG. 4 is a timing chart for illustrating operation of the data path interface 3, which is described in page 19 of the "Utopia, An ATM-PHY Interface Specification". In FIGs. 3 and 4, TxClk denotes a transmission clock having a clock cycle T, which is delivered from the ATM layer device 1 to each of the PHY layer devices 2-0 to 2-M. TxAddr denotes an address signal whereby addresses of the PHY layer devices 2-0 to 2-M are transmitted from the ATM layer device 1 to the PHY layer devices 2-0 to 2-M for polling and selecting one of the PHY layer devices 2-0 to 2-M. Each of the addresses is represented with five-bit data and an address '11111' ('1F' in hexadecimal) represents a null PHY port assigned to neither of the PHY layer devices 2-0 to 2-M. TxClav (Transmission Cell Available) denotes a cell transmission allowance signal having three statuses transmitted from one of the PHY layer devices 2-0 to 2-M to the ATM layer device 1. When a PHY layer device is polled with its address transmitted by the address signal TxAddr at a clock cycle, the designated PHY layer device makes HIGH the cell transmission allowance signal TxClav for next one clock cycle on condition that the PHY layer device can accept one more ATM cell as a whole, and makes LOW the cell transmission allowance signal TxClav for a period of the next one clock when the PHY layer device cannot accept the whole ATM cell, as shown in FIG. 16, wherein the cell transmission allowance signal TxClav is made HIGH by the PHY layer devices 2-(N - 3), 2-(N + 3) and 2-N having addresses N - 3, N + 3 and N, respectively, at clock cycles #4, #10 and #14, polled at clock cycles #3, #9 and #13, respectively, and made LOW by the PHY layer devices 2-(N + 2), 2-(N - 2), 2-(N -1), 2-(N + 1) and again by the PHY layer device 2-(N + 1). An ATM cell consists of a cell header of 5 octets (bytes) H1 to H5 and a payload of 48 octets P1 to P48, whereof a part P35 to P48 of an ATM cell and H1 to H5 of another ATM cell is depicted in FIG. 4 as transmission data TxData. The PHY layer device polled with its address determines to make HIGH or LOW the cell transmission allowance signal TxClav according to whether the PHY layer device has a room or not for receiving the 53-octet data H1 to H5 and P1 to P48 of the ATM cell as a whole. TxEnb (Transmission Enable) denotes a transmission enabling signal delivered from the ATM layer device 1 to the PHY layer devices 2-0 to 2-M for notifying transmission of cell data, indicating a selected PHY layer device whereto the cell data is to be forwarded by putting address thereof on the address signal TxAddr at the same clock cycle. TxData (Transmission Data) denotes cell data delivered from the ATM layer device 1 to the selected PHY layer device, wherein cell header of five octets H1 to H5 followed by payload of 48 octets P1 to P48 are transmitted octet by octet in synchronization with the transmission clock TxClk, and TxSOC (Transmission Start Of Cell) denotes a transmission start signal indicating beginning of the cell data, that is, a first header octet H1 of an ATM cell to be transmitted. As above described, the ATM layer device 1 polls status of each one of the PHY layer devices 2-0 to 2-M by putting its address on the address signal TxAddr one by one, and a PHY layer device designated by a polling address at a clock cycle drives the cell transmission allowance signal TxClav to HIGH or LOW for a period of one clock cycle just following the cloclc cycle of the polling address. Then, selecting an appropriate PHY layer device among PHY layer devices which have made HIGH the cell transmission allowance signal TxClav, the ATM layer device 1 notifies the selection by asserting the transmission enabling signal TxEnb for a clock cycle and putting address of the selected PHY layer device on the address signal TxAddr at the same time. When the transmission enabling signal TxEnb is made HIGH, every of the PHY layer devices 2-0 to 2-M checks the address on the address signal TxAddr, and the selected PHY layer device acknowledges the selection by driving the cell transmission allowance signal TxClav to HIGH at the next clock cycle. The selection remains valid until the transmission enabling signal TxEnb is again made HIGH. In the example of FIG. 4, payload octets P35 to P48 are transmitted to a selected PHY layer device 2-N at clock cycles #1 to #14 as the transmission data TxData, which represents a part of a cell transmission cycle consisting of 54 clock cycles, that is, 53 clock .cycles to #14 for transmitting five header octets H1 to H5, 48 payload octets P1 to P48, and one clock cycle #15 for designating a next selection. For preparing the next selection, polling of the PHY layer devices 2-0 to 2-M is performed in parallel with the data transmission during second to 53-th clock cycles of a cell transmission cycle, as shown in FIG. 4, wherein polling for selecting the PHY layer device 2-(N + 3) is performed until clock cycle #14 where the last payload octet P48 is transmitted, and polling for selecting a next PHY layer device begins at clock cycle #17 where the second header octet H2 is transmitted. In the example, the cell transmission allowance signal TxClav indicates that the PHY layer devices 2-(N - 3), 2-(N + 3) and 2-N having respective addresses N-3, N+3 and N can accept data of an ATM cell as a whole. The PHY layer device 2-N, which is under receiving cell data at the cell transmission cycle, drives the cell transmission allowance signal TxClav to HIGH on condition that it can accept another whole ATM cell besides cell data actually receiving. In the following paragraphs, PHY layer devices which drives the cell transmission allowance signal TxClav to HIGH when it can accept a whole ATM cell besides cell data actually receiving, if there is any, are to be called the normal PHY layer devices according to a normal specification defined in the Utopia Level 2 specification. Returning to the example of FIG. 4, selecting the PHY layer device 2-(N + 3) among the above PHY layer devices 2-(N - 3), 2-(N + 3) and 2-N, the ATM layer device 1 notifies the selection by putting the address N+3 on the address signal TxAddr at clock cycle #15, and begins to transmit cell data of a next ATM cell at clock cycle #16 from a top octet, namely, the first header octet H1 as the transmission data TxData to the PHY layer device 2-(N+3), followed by octets H2, H3, ... transmitted at clock cycles #17, #18, .... Here, the ATM layer device 1 may select and designate another one of the PHY layer devices 2-(N - 3), 2-(N + 3) and 2-N at the clock cycle #15 in place of the PHY layer device 2-(N + 3), of course, and it is to be noted that the cell transmission allowance signal TxClav made HIGH by a PHY layer device actually receiving cell data, that is, the PHY layer device 2-N, in the example, is defined to be not valid when it is driven more than five clock cycles before transmission of the concerning cell data ends, according to the Utopia Level 2 specification. Hence, polling of the PHY layer device 2-N is performed by the ATM layer device 1 lastly at clock cycle #13, in FIG. 4. The ATM layer device 1 begins again polling of PHY layer devices 2-0 to 2-M at second clock cycle #17 of the following cell transmission cycle forwarded to the PHY layer device 2-(N+3), for selecting a PHY layer device for a next cell transmission cycle. In the example of FIG. 4, an address N+1 is put on the address signal TxAddr at clock cycle #17 for polling the PHY layer device 2-(N+1), which is followed by the address '1F' of the null PHY port put on the address signal TxAddr at clock cycle #18. Thus, one PHY layer device is polled at every two clock cycles, in the example, and 26 PHY layer devices can be polled at maximum in one cell transmission cycle. FIG. 5 is a flowchart illustrating driving operation of the cell transmission allowance signal TxClav performed by a normal PHY layer device defined in the Utopia Level 2 specification, that is, a PHY layer device which makes HIGH the cell transmission allowance signal TxClav when it can accept a whole ATM cell besides cell data actually receiving. In the flowchart of FIG. 5, a normal PHY layer device 2- n assigned with an address n is assumed to be polled at a clock cycle a . At the clock cycle a , the PHY layer device 2- n checks whether the polling address on the address signal TxAddr is n or not (at step S1). When the polling address is n, the PHY layer device 2- n checks whether it can accept one more whole ATM cell or not (at step S2). When the check result is TRUE, the PHY layer device 2- n drives the cell transmission allowance signal TxClav to HIGH at the following clock cycle ( a + 1) (at step S3), and drives the cell transmission allowance signal TxClav to LOW at the following clock cycle ( a + 1) (at step S4) when the check result is FALSE. When the polling address on the address signal TxAddr is found to be not n at step S1, the PHY layer device 2- n ends the procedure, leaving the cell transmission allowance signal TxClav to be driven by another PHY layer device. As heretofore described, necessary conditions of the normal PHY layer device and the ATM layer device are defined in the Utopia Level 2 specification together with details of the interface signals TxAddr and so on. However, no concrete configuration of the ATM layer device has been disclosed until now. Therefore, a primary object of the present invention is to provide a concrete configuration example of an ATM layer device which can realize an ATM cell transmission system having high data transmission efficiency cooperating with the normal PHY layer devices according to the Utopia Level 2 specification. Another object of the invention is to provide an ATM cell transmission system wherein the configuration of the ATM layer device is simplified by employing enhanced PHY layer devices defined in the Utopia Level 2 specification in place of the normal PHY layer devices. Still another object of the invention is to provide an ATM cell transmission system having high data transmission efficiency with the simple configuration making use of the normal PHY layer devices and the enhanced PHY layer devices in parallel. In an ATM cell transmission system according to the invention having an ATM layer device, a data path interface and a plurality of PHY layer devices whereof each is connected to the ATM layer device by way of the data path interface, and each is either one of a normal PHY layer device which returns a HIGH level of a cell transmission allowance signal on condition said each can accept one more whole ATM cell when said each is polled by the ATM layer device through the data path interface and an enhanced PHY layer device which returns the HIGH level of the cell transmission allowance signal on condition said each can accept K more whole ATM cells when said each is polled by the ATM layer device through the data path interface, K being an integer more than one; the ATM layer device comprises: a cell buffer for storing ATM cells to be transmitted through the PHY layer devices; a FIFO memory; a cell transmission controller for performing polling of the PHY layer devices, designating a selected PHY layer device among the PHY layer devices which have returned the HIGH level of the cell transmission allowance signal to the polling and satisfy a first condition that at least one ATM cell to be forwarded thereto is stored in the cell buffer and a second condition that addresses thereof are not registered as one of D selected port numbers, D being a number of cell transmission cycles needed for outputting an ATM cell from the cell buffer to the FIFO memory according to the polling after the polling is performed, controlling the cell buffer to output an ATM cell to be transmitted through the selected PHY layer device into the FIFO memory together with destination information of the ATM cell, replacing one of the D selected port numbers which is registered earliest with an address of the selected PHY layer device when the selected PHY layer device is the normal PHY layer device and with an invalid address when the selected PHY layer device is the enhanced PHY layer device, controlling the FIFO memory to transmit cell data of an ATM cell through the data path at a next cell transmission cycle when the FIFO memory is not empty, and designating a PHY layer device indicated by the destination information of the ATM cell at an end of the current transmission cycle as a next receiver of the cell data to be transmitted through the data path interface at the next cell transmission cycle. Therefore, an ATM cell transmission system having a high transmission efficiency can be realized with a single FIFO memory according to the fourth embodiment of the invention, even when normal PHY layer devices defined in the Utopia Level 2 specification are included in the PHY layer devices connected to the data path interface. The foregoing, further objects, features, and advantages of this invention will become apparent from a consideration of the following description, the appended claims, and the accompanying drawings wherein the same numerals indicate the same or the corresponding parts. In the drawings: FIG. 6 is a flowchart illustrating operation of an enhanced PHY layer device 2- n assigned an address n performed in response to the polling from the cell transmission controller 10'''; FIG. 1 is a block diagram illustrating an ATM cell transmission system according to a fourth embodiment of the invention; FIG. 2 is a flowchart illustrating operation of the cell transmission controller 10''' of FIG. 13; FIG. 3 is a block diagram illustrating a basic configuration of an ATM cell transmission system according to the Utopia Level 2 specification; FIG. 4 is a timing chart for illustrating operation of the data path interface 3 of FIG. 3 described in the "Utopia, An ATM-PHY Interface Specification"; and FIG. 5 is a flowchart illustrating driving operation of the cell transmission allowance signal TxClav performed by a normal PHY layer device defined in the Utopia Level 2. Now, an embodiment of the present invention will be described in connection with the drawings. FIG. 1 is a block diagram illustrating an ATM cell transmission system according to an embodiment of the invention. FIG. 1 is a block diagram illustrating an ATM cell transmission system according to an embodiment of the invention. The ATM transmission system of FIG. 1 comprises an ATM layer device 1 such as an ATM switch, for example, PHY (physical) layer devices 2''-0 to 2''-M each provided for each one of a plurality of output ports OP-0 to OP-M of the ATM cell transmission system, and a data path interface 3 for connecting the ATM layer device 1 and the PHY layer devices 2''-0 to 2''-M, in the same way with the basic configuration of FIG. 3. The ATM layer device 1 comprises a single FIFO memory 5, a cell buffer 4 and a cell transmission controller 10''' connected to the cell buffer 4 and the FIFO memory 5. In the ATM layer device 1, the cell buffering signal 7 from the cell buffer f4 is delivered to the cell transmission controller 10''', and the cell buffer 4 outputs an ATM cell having a destination indicated in the cell output command 6 to the FIFO 5 together with destination information of the ATM cell, that is, a port number of one of the output ports OP-0 to OP-M each belonging respective one of the output ports OP-0 to OP-M each belonging respective one of the enhanced PHY layer devices 2''-0 to 2''-M, in accordance with the cell output command 6 delivered from the cell transmission controller 10'''. The ATM cell inputted to the FIFO memory 5 is outputted together with its destination information, whereof cell data is transmitted through the data path interface 3 as the transmission cell data TxData, and the destination information is inputted to the cell transmission controller 10''' as an output port number signal 12 when the empty signal 11 is disabled. The cell buffer 4 delivers the cell buffering signal 7 to the cell transmission controller 10''', and the FIFO memory 5 enables the empty signal 11 when it becomes empty. The cell transmission controller 10''' knows presence of ATM cells in the cell buffer 4 to be forwarded to each one of the output ports OP-0 to OP-M from the cell buffering signal 7, and presence of cell data of an ATM cell prepared in the FIFO memory 5 to be transmitted through the data path interface 3 as the transmission cell data TxData from the empty signal 11. Further, the cell transmission controller 10''' knows enhanced PHY layer devices which can accept at least K more ATM cells by performing polling of the enhanced PHY layer devices 2''-0 to 2''-M. FIG. 6 is a flowchart illustrating operation of an enhanced PHY layer device 2''- n assigned an address n performed in response to the polling from the cell transmission controller 10'''. When, at a clock cycle a , an address is put on the address signal TxAddr, the enhance PHY layer device 2"- n checks (at step S51) whether the address is n or not. When the check result is TRUE, the enhanced PHY layer device 2"- n checks (at step S52) whether it can accept at least K more ATM cells or not, besides cell data actually receiving when it is receiving cell data. When the check result at step S52 is TRUE, the enhanced PHY layer device 2"- n drives the cell transmission allowance signal TxClav to HIGH level (at step S53), and otherwise drives it to LOW level (at step S54), at the next clock cycle a + 1. When the check result at step S51 is FALSE, the enhanced PHY layer device 2"- n ends the procedure leaving the cell transmission allowance signal TxClav driven by another PHY layer device. In the example of FIG. 6, the following conditions are assumed, that is, only two enhanced PHY layer devices 2"-0 and 2"-1 having output port OP-0 and OP-1, respectively, are comprised in the ATM cell transmission system, the cell buffer 4 has sufficient ATM cells to be forwarded to the output ports OP-0 and OP-1, and the checks at a step of if there is output port satisfying (C1 & C3)? and if FIFO is not empty? is performed by the cell transmission controller 10''' among last 6 clock cycles of each cell transmission cycle, that is, from a clock cycle whereby 44-th payload octet P44 is transmitted until the last clock cycle whereby next enhanced PHY layer device is designated. FIG. 2 is a flowchart illustrating operation of the cell transmission controller 10''' of FIG. 1. At every cell transmission cycle, the cell transmission controller 10''' performs (at step S61) polling of a plurality of PHY layer devices 2"-0 to 2"-M including the normal PHY layer devices and the enhanced PHY layer devices defined in the Utopia Level 2 specification. The normal PHY layer devices reply to the polling as described in connection with the flowchart of FIG. 5, the enhanced PHY layer devices reply to the polling as described in connection with the flowchart of FIG. 6, and the cell transmission controller knows addresses of the PHY layer devices returning HIGH level of the cell transmission allowance signal TxClav. The HIGH level of the cell transmission signal TxClav returned from the PHY layer device whereto cell data is actually transmitted is valid only when it is returned in the last five clocks of the cell transmission cycle. Therefore, the cell transmission controller 10''' polls the PHY layer devices so that the PHY layer device actually receiving the transmission cell data TxData can respond at the last five clock cycles of the cell transmission, in the same way with the other embodiments. In the cell transmission controller 10"' of the embodiment, D selected port numbers L D to L 1 are registered, D being a number of cell transmission cycles needed for an ATM cell is outputted from the cell buffer 4 into the FIFO memory 5 after polling at step S61 is performed. The selected port numbers L D to L 1 represent normal PHY layer devices whereto selected output ports designated in the last D cell transmission cycles belongs, as follows. Each of the selected port numbers L D to L 1 is the address of PHY layer device whereto the respective selected output port belongs when the respective output port belongs to one of the normal PHY layer devices, and the address '1F' of the null PHY port when the respective selected output port belongs one of the enhanced PHY layer devices. The selected port numbers L D to L 1 are registered in a shift register, for example, provided in the cell transmission controller 10"'. Returning to FIG. 2, the cell transmission controller 10''' checks (at step S62) for every of m = 0 to M if the output port OP- m satisfies a first condition that the address of the corresponding PHY layer device 2"- m is not registered as any of the selected port numbers L D to L 1 , a second condition that at least one ATM cell to be forwarded to the output port OP- m is stored in the cell buffer 4, and a third condition that the corresponding PHY layer device 2"- m whereto the output port OP- m belongs has returned HIGH level of the cell transmission allowance signal TxClav, at the same time. If there is found at least one output port which satisfies all of the first, the second and the third condition, the cell transmission controller 10''' designate one of them as a selected output port (at step S63) according to an appropriate algorithm, and indicates (at step S64) the cell buffer 4 to output an ATM cell to be forwarded to the selected output port into the FIFO memory 5, before proceeding to step S65. When no output port is found to satisfy all of the first, the second and the third condition at step S62, the cell transmission controller 10''' proceeds directly to the step S65 without performing steps S63 and S64. At the step S65, the cell transmission controller 10''' shifts the selected port numbers L D to L 2 by one cell transmission cycle such as L 1 ← L 2 , L 2 ← L 3 , ..., L D -1 ← L D . Then, the cell transmission controller 10''' checks whether the selected output port belongs one of the enhanced PHY layer devices or not (at step S66). When it is TRUE, the cell transmission controller 10''' registers address '1F' of the null PHY port as the port number L D (at step S67), and otherwise, registers address of the corresponding normal PHY layer device as the port number L D (at step S68). Thus, addresses of the normal PHY layer devices whereto ATM cells are transmitted at preceding D cell transmission cycles are registered in the cell transmission controller 10'''. Then (at step S69), the cell transmission controller 10''' checks whether the empty signal 11 is enabled by the FIFO memory 5 or not. When it is enabled, the cell transmission controller 10''' ends its procedure at the current cell transmission cycle without performing nothing else. When it is not enabled, the cell transmission controller 10"' puts the address of the PHY layer device notified from the FIFO memory 5 by way of the output port number signal 12, on the address signal TxAddr at the last (54-th) clock cycle of the current cell transmission cycle (at step S70), for designating the PHY layer device as the receiver of the transmission cell data TxData of the next cell transmission cycle. As above described, the cell transmission controller 10''' checks the selected port numbers L D to L 1 , that is, port numbers of normal PHY layer devices whereto cell data transmission after the last polling is already scheduled, and omits them from candidates of the selected output port to be designated at step S63. Therefore, cell loss on the data path interface 3, which might occur when cell data is transmitted to an PHY layer device unable to accept a whole ATM cell, can be prevented as well in the ATM transmission system of FIG 2 according to an embodiment of the invention. As heretofore described, an ATM cell transmission system having a high transmission efficiency can be realized with a single FIFO memory 5 according to an embodiment of the invention, even when normal PHY layer devices defined in the Utopia Level 2 specification are included in the PHY layer devices connected to the data path interface 3.

Claims: ATM cell transmission system having an ATM layer device (1), a data path interface (3) and a plurality of PHY layer devices (2''-0 to 2''- M) whereof each is connected to the ATM layer device (1) by way of the data path interface (3), said ATM cell transmission system being characterized in that each of said PHY layer devices (2''-0 to 2''-M) is either one of a normal PHY layer device which returns a HIGH level of a cell transmission allowance signal (TxClav) on condition it can accept one more whole ATM cell when it is polled by the ATM layer device (1) through the data path interface (3), and an enhanced PHY layer device which returns the HIGH level of the cell transmission allowance signal (TxClav) on condition it can accept K more whole ATM cells when it is polled by the ATM layer device (1) through the data path interface (3), K being an integer more than one; and said ATM layer device (1) comprising: a cell buffer (4) for storing ATM cells to be transmitted through the PHY layer devices (2''-0 to 2''-M); a FIFO memory (5); a cell transmission controller (10''') for performing polling of the PHY layer devices (2''-0 to 2''-M), designating at least one PHY layer device returning HIGH level of the cell transmission allowance signal (TxClav) among PHY layer devices other than normal PHY layer devices which outputted a transmission ATM cell during the past D transmission cycles as a selected PHY layer device capable of transmitting, when a cell stored in the cell buffer (4) is an ATM cell to be transmitted to said selected PHY layer device, controlling the cell buffer (4) to output said cell to said FIFO memory as a transmission cell, and when said transmission cell is stored in said FIFO memory, transmitting said transmission cell stored in said FIFO memory selectively to said selected PHY layer device as a next transmission ATM cell through the data path interface (3), wherein the cell number K satisfies a condition K ≥ D + 1. ATM cell transmission system as recited in claim 1, wherein the data path interface (3) and the PHY layer devices (2''-0 to 2''-M) are designed according to a Utopia Level 2 specification. ATM cell transmission system as recited in claim 1, wherein the FIFO memory (5) outputs an empty signal (11) and an output port number signal (12) to the cell transmission controller (10'''), the empty signal (11) being disabled and the output port number signal (12) representing the destination information of an ATM cell data when data of the ATM cell is prepared to be transmitted in the FIFO memory (5).

Inventors: ISHIDA HIDEO, MIZUKOSHI NOBUYUKI, SATO NOBORU, ISHIDA, HIDEO, MIZUKOSHI, NOBUYUKI, SATO, NOBORU
Applicants: NEC ELECTRONICS CORP, NEC ELECTRONICS CORPORATION
Application year: 1998
Main classification: H04Q11
IPC codes: H04Q11, H04L12, H04Q3