WO2004019578A1

WO2004019578A1 - Encoding signaling information at a phaysical network layer by using code violations of an nb/mb line block code

Info

Publication number: WO2004019578A1
Application number: PCT/EP2002/008858
Authority: WO
Inventors: Simone Benvenuti; César ESTEBAN CASTRO
Original assignee: Pirelli & C. S.P.A.
Priority date: 2002-08-08
Filing date: 2002-08-08
Publication date: 2004-03-04
Also published as: AU2002331214A1

Abstract

A method of exchanging service information between devices (103a, 103b) in a data communication network (101) in which exchanged data are encoded into code words according to a prescribed nB/mB block coding scheme at a physical layer (107), the block coding scheme specifying invalid code words not defined for use for the exchange of data and control information, and non-invalid code words. Any service information always an invalid code word for providing a clock pulse (CK) to the invalid or a non-invalid code word, depending on the logical state (301) of the service information bit. The number of codewords with the clock and payload pulses CK and DT may be greater than one to relax operating speed requirements at th receiving side. This way, signaling service information can be encoded and multiplexed with the main data at a physical layer using the same physical channel Applications are in Fast or Gigabit Ethernet an Fiber Distributed Data interface networks.

Description

ENCODING SIGNALING INFORMATION AT A PHYSICAL NETWORK LAYER BY USING CODE VIOLATIONS OF AN NB/MB LINE BLOCK CODE

The present invention relates in general to data communication networks, and particularly to a method of exchanging service information between devices in a data communication network.

Network protocols enable communication between devices in data communication networks, for example, by defining procedures, data elements, and objects to be used to communicate between the devices .

A variety of network protocols are nowadays used. For example, network protocols exist which are specific to particular types of networks, such as Local Area Networks (LANs) , Metropolitan Area Networks (MANs) , Wide-Area Networks (WANs) .

A network protocol may define prescriptions specific to one or more layers of the Open System Interconnection (OSI) network layering model, promulgated by the International Organization for Standardization (ISO) . For example, some protocols, such as the Transport Control Protocol (TCP) /Internet Protocol (IP) suite of protocols and the Point-to-Point Protocol (PPP) , are specific to higher layers of the OSI network model, such as the network, transport and session layers. Other protocols, for example, Synchronous Optical Network (SONET) and Ethernet- based protocols such as protocols for Ethernet, Fast Ethernet and Gigabit Ethernet, are specific to lower layers of the OSI network model, including the physical layer and the data link layer.

In particular, Ethernet, Fast Ethernet and Gigabit Ethernet are protocols for enabling communication between devices over a LAN. Ethernet-based protocols have been standardized by the IEEE Standard 802.3; specifically, the Fast Ethernet protocol (for communication speeds up to 100

Megabits per second - Mbps) is codified in the IEEE

Standard Specification 802.3u, while the Gigabit Ethernet protocol is codified in the IEEE Standard Specifications 802.3z (for communication speeds up to 1 Gbps) and 802.3ae

(for speeds up to 10 Gbps) .

Several network protocols, for example those complying with the IEEE Standard 802.3, define block coding schemes for encoding data to be exchanged between network devices. Typically, these encoding schemes are implemented at the physical layer of the network protocol. Generally speaking, a block coding scheme is a scheme for encoding blocks of data bits into respective code words including more bits than the original blocks of data bits. For this reason, these encoding schemes are also referred to as redundant coding schemes .

Several block coding schemes are known, including the 4B/5B (four bits into five bits) scheme adopted in the Fast Ethernet protocol, the 8B/10B (eight bits into ten bits) scheme specified for the 1 Gbps Gigabit Ethernet protocol and the 64B/66B (sixty-four bits into sixty-six bits) scheme adopted in the 10 Gbps Gigabit Ethernet protocol. In particular, the IEEE Standard specifications for these protocols prescribe that the block coding scheme be applied at the physical layer of the OSI model.

Considering by way of example, and for the sake of simplicity, the Fast Ethernet protocol, blocks of four data bits (data nibbles) are encoded into code words of five bits by a known and invertible function. Specifically, letting x denote a sequence of four input bits, then a function y=h(x) is applied { e . g. , by an encoder) to x to produce y, where y denotes the five output bits; h(x) is a fully invertible function, i.e., a function such that an inverse function d (y) is defined; applying the inverse function d(y) to the function h(x) ( e . g. , by a decoder), the original sequence of bits x can be recovered (d(h(x))=x) .

The block coding schemes defined by the IEEE Standard specification are such as to balance the number of ones and zeros in a data stream, and to provide a sufficient transition density for clock recovery. In general, in these block coding schemes some code words are defined for use for exchanging data, some code words are defined for use for controlling the exchange of data, and the remaining code words are not defined for use for exchanging data or controlling the data exchange. By way of example, FIG. 4 shows a table of the code words specified in the 4B/5B coding scheme. Only sixteen code words are used to encode blocks of four data bits; of the remaining sixteen code words, five are defined for encoding extra controls (frame delineation, e . g. start of frame, end of frame, network idle state) , and eleven are not defined for use; these latter will be hereinafter referred to as invalid code words .

The choice of the invalid code words may be based on a Hamming weight criterion, as for example in the 8B/10B block coding scheme prescribed for the 1 Gbps Gigabit Ethernet protocol, in which only code words having a Hamming weight equal to four, five or six are defined for use .

The IEEE Standard 802.3 defines a physical layer, which corresponds to the OSI model physical layer, and a Media Access Control (MAC) layer, corresponding to a sub- layer of the data link layer of the OSI model.

The functionality specified by the MAC layer of the IEEE Standard 802.3 and by higher-layer protocols such as the session, transport and network layers specified by OSI model, may include for example delineating a data packet, checking the source and destination address of a packet, verifying the integrity of a packet using the Frame Check Sequences (FCS) of the packet header, discarding corrupt packets, and invoking flow control, if necessary. The functionality specified by the IEEE Standard 802.3 for the physical layer includes encoding blocks of data bits as code words (in accordance with the block coding schemes previously mentioned) , converting the code words into a serial stream of electrical or optical signals, transmitting such signals onto a network physical communication medium, receiving such signals from the network physical communication medium, converting such received signals into code words, and decoding the code words into the original blocks of data bits. Some lower-layer network protocols, for example, SONET, provide a mechanism for exchanging service information between network devices, in addition to data. For example, SONET provides Data Communication Channel (DCC) overhead bytes that can be used to exchange service information between devices in the network.

Service information is meant to include information useful for controlling a channel on which data is being exchanged between two network devices, e . g. , creating the channel, destroying the channel, changing channel parameters.

Other lower-layer network protocols, for example Ethernet-based protocols, do not provide a mechanism for exchanging service information between network devices. In other words, such protocols do not reserve any byte to exchange service information.

Typically, for lower-layer network protocols that do not provide a mechanism for exchanging service information, higher-layer protocols, for example, network, transport and session layer protocols, are used to exchange the service information between the network devices. Accordingly, the network devices need to be configured to implement the high layer protocols. The network devices may be configured to use one or more of the higher-layer protocols to define packets encapsulating service information and define packets encapsulating data. These packets may be multiplexed together on a first physical medium connecting the two devices, or sent on separate physical media.

There are many cases in which some or even most of the devices in a network need not to be configured to implement higher-layer protocols in order to exchange data. For example, for data exchanged between network devices located external to an Optical Transport Network (OTN) , referred to herein as User Devices (UDs) , the data is transmitted across the OTN. An OTN is a network in which all of the network transmission links between network devices are optical transmission links, for example, fiber optic cables, although one or more of the network devices, for example, Optical Cross-Connects (OXCs) and Add/Drop Multiplexers (ADMs) , may process the transmitted signals non-optically.

To transmit the data across the OTN, the data is transmitted to and between one or more devices included as part of the OTN, referred to herein as Transport Network Devices (TNDs) . Typically, a TND is configured to implement merely the physical layer functions of any protocol, for example, Fast Ethernet, used to exchange data between the two UDs, and is not configured to implement the higher layers of any of the protocols, including the data link (e.g., the MAC layer of the Fast Ethernet protocol), network, transport and session layers.

Thus, in order to enable a typical TND to exchange service information with a UD or other TNDs, the TND needs to be re-configured to implement more than merely the physical layer of the protocol used to exchange data, for example, at least the data link layer of the protocol.

Further, if the data link layer of the protocol used to exchange data does not provide bytes for specifying service information, then it may be necessary to further reconfigure the TND to implement other higher-layer protocols to exchange the service information, as described above .

These problems have been considered in US-A1- 2001/0024457, which proposes a method for encoding signalling information (i.e., service information) at a physical layer of an Ethernet-based network protocol, particularly the Fast Ethernet and the Gigabit Ethernet protocols . Specifically, that document suggests to use the invalid code words in the block coding schemes defined for such protocols for encoding blocks of bits making up the signalling information. The signalling information is divided into sequences (blocks) of a prescribed number of bits by means of a signal divider, and each block of signalling information bits is converted into a respective invalid code word, by means of an encoder and a look-up table.

The invalid code words that encode blocks of signalling information bits are then multiplexed with the valid code words that encode blocks of data bits or the extra controls (referred to as K-characters in that document) .

When the physical layer of a network device receives an invalid code word, instead of generating as usual a code word violation message or a running disparity error message, that device sends the invalid code word to a decoder that, based on a look-up table identical to that used to encode the block of signalling information bits, decodes the invalid code word to recover the original block of signalling information bits.

A drawback of the solution proposed in the cited document resides in the fact that the components implementing the physical layer of the network devices need to feature functionalities that are not strictly required by the standards. For example, signal dividers for dividing the signalling information into blocks of bits, encoders and look-up tables for encoding the blocks of signalling information bits into prescribed invalid code words, and decoders and look-up tables for decoding the invalid code words to recover the original blocks of signalling information bits are required at the physical layer of the network devices. Due to this, it is not possible to exploit commercially available, low cost physical layer components.

This problem is particularly felt when signal repeaters are necessary, as it is often the case in networks; also the signal repeaters should feature the same capabilities of decoding the received invalid code words to recover the original blocks of signalling information bits, dividing the signalling information into blocks of bits, encoding the blocks of bits into prescribed invalid code words .

In view of the state of the art outlined, it has been an object of the present invention to enable the exchange of service information in a network exploiting the functionalities of commercially available physical layer components complying with the network protocol standard.

The Applicant has devised a method by means of which service information are exchanged between devices in a network by simply relying on the presence or the absence of invalid code words, without the complications inherent to the method disclosed in US-A1-2001/0024457.

According to a first aspect of the present invention, there is provided a method as set forth in claim 1 of exchanging service information between devices in a data communication network in which exchanged data are encoded into code words according to a prescribed block coding scheme at a physical layer, the block coding scheme specifying invalid code words not defined for use for the exchange of data and control information, and non-invalid code words .

Summarising, the method comprises associating a first logic state of an exchanged service information unit with a presence of at least one of the invalid code words, and associating a second logic state of the exchanged service information unit with a presence of at least one of the non-invalid code words. Preferred features and alternatives of the method are set forth in claims 2 to 11.

In brief, in an embodiment of the present invention said at least one invalid code word is any one of the invalid code words specified by the block coding scheme. The service information and the data can be multiplexed and exchanged over a same communication physical medium.

In particular, exchanging service information comprises transmitting the service information. Transmitting the service information includes engaging a physical layer of a transmitter device in the network for: forcing transmission of at least one invalid code word for transmitting the first logic state of a service information unit; and not forcing transmission of at least one invalid code word for transmitting the second logic state of the service information unit.

In one embodiment of the invention, said transmitting the service information includes conditioning the transmission of the service information to an absence of transmission of data by the transmitter device. A service information channel is thus set up which has a lower priority than a data channel . Alternatively, a transmission of data by the transmitter device is conditioned to an absence of transmission of the service information by the transmitter device. In this way, the service information channel has priority over the data channel . Receiving the service information includes: exploiting a physical layer of a receiver device in the network to detect the presence or the absence of invalid code words.

In a preferred embodiment of the present invention, the presence of at least one invalid code word and the presence of at least one non-invalid code word are combined with the presence of at least one additional invalid code word, whereby the first logic state is associated with the presence of at least two invalid code words and the second logic state is associated with the presence of a number of invalid code words equal to that associated with the first logic state less at least one.

Preferably, the service information is formatted according to a prescribed service information protocol. The method according to the present invention can in particular be applied to Ethernet networks or Fiber Distributed Data Interface networks. The block coding scheme may be one among a 4B/5B, a 8B/10B, a 64B/66B block coding scheme . According to a second aspect of the present invention, there is provided a system as set forth in claim 12 for exchanging service information between devices in a data communication network in which data exchanged between the devices are encoded into code words according to a prescribed block coding scheme at a physical layer of the devices, the block coding scheme specifying invalid code words not defined for use for the exchange of data and control information, and non-invalid code words. In brief, the system includes a physical layer device and a physical layer control device in the devices of the network .

The physical layer device and the physical layer control device associate a first logic state of an exchanged service information unit with a presence of at least one of the invalid code words, and associate a second logic state of the exchanged service information unit with a presence of at least one of the non-invalid code words.

In particular, in a service information transmitter device of the network, the physical layer control device engages the physical layer of the transmitter device for: forcing transmission of at least one invalid code word for transmitting the first logic state of a service information unit, and not forcing transmission of at least one invalid code word for transmitting the second logic state of the service information unit.

In one embodiment of the invention, the operation of the physical layer control device is conditioned to presence of data to be transmitted over the network.

Alternatively, the physical layer control device interrupts the transmission of data over the network when service information needs to be transmitted.

In one embodiment of the invention, the physical layer control device implements a media access control layer, that controls the transmitter device access to a communication physical medium of the network for the transmission of the service information.

In a service information receiver device of the network, the physical layer control device associates the presence of invalid and non-invalid code words detected by the physical layer of the receiver device with the first and second logic state of a service information unit, respectively. The physical layer control device may be part of a media access control layer of the receiver device.

The network may be an Ethernet network or a Fiber Distributed Data Interface network, and the block coding scheme may be one among a 4B/5B, a 8B/10B, a 64B/66B block coding scheme.

According to a third aspect of the present invention, there is provided a device as set forth in claim 21, configured for use in a data communication network in which data exchanged between devices in the network are encoded into code words according to a prescribed block coding scheme, the block coding scheme specifying invalid code words not defined for use for the exchange of data and control information, and non-invalid code words. The network device is capable of exchanging service information with other devices in the network.

In summary, the network device comprises a physical layer device, and a physical layer control device.

These and other features and advantages of the present invention will be made apparent by the following detailed description of a possible embodiment thereof, provided merely by way of non-limitative example, which will be made in connection with the attached drawings, wherein:

FIG. 1 schematically shows two devices of a data communication network exchanging data and service information, according to an embodiment of the present invention;

FIG. 2 is a more detailed view of a data link layer of the devices, according to an embodiment of the present invention;

FIG. 3 shows the structure of one bit of a service message carrying the service information, in an embodiment of the present invention; FIG. 4 shows a table of code words specified in one standardised block coding scheme, particularly the 4B/5B block coding scheme prescribed by the IEEE 802.3u Standard specification (Fast Ethernet) ;

FIG. 5 is a simplified flow chart illustrating how the service messages are transmitted, according to an exemplary embodiment of the present invention;

FIG. 6 shows a service message frame in a service message communication protocol according to an embodiment of the present invention. With reference to the drawings, FIG. 1 schematically shows a data communication network, globally identified as 101, which in the following will be simply referred to as the network 101. Two generic devices of the network 101 (in the following referred to as network devices) 103a, 103b are shown. The two network devices 103a, 103b communicate with each other over a communication physical medium 105 of the network 101.

In an exemplary embodiment of the invention, the network 101 is an Ethernet network, complying with the IEEE Standard 802.3, particularly a Fast Ethernet or a Gigabit

Ethernet network. Ethernet networks are commonly used in LANs. More generally, the network 101 is any network complying to standards that prescribe block coding of data at the physical layer of the OSI layer model, such as a Fiber Distributed Data Interface (FDDI) network.

The physical medium 105 may comprise fiber optic cables, electrical cables ( e . g. , twisted-pair wires or coaxial cables) , the ether, or a combination of these media .

The number of network devices may be, and normally is, higher than two .

Each network device 103a, 103b is structured according to the OSI layer model, and includes a plurality of layers, comprising a physical layer 107, a data link layer 109, and higher layers of the OSI model, not shown in detail and globally indicated as 111.

The data link layer 109 includes a Media Access

Control (MAC) sub-layer 113 (in the following simply referred to as MAC layer) , implementing a media access protocol used by the network devices 103a, 103b for sharing the communication physical medium 105.

In order to interact with the MAC layer 113, the physical layer 107 includes a Media Independent Interface (Mil) 115. The Mil 115 enables the interaction between the

MAC layer 115 and units of the physical layer 107 responsible of managing the low-level ("physical") details of the communication over the communication physical medium

105. In the exemplary embodiment of the invention discussed herein, such units of the physical layer 107 includes a transmitting unit section and a receiving unit section. The transmitting unit section comprises a block coding unit 117 and a transmitter unit 119. The receiving section unit comprises in turn a receiver unit 121 and a block decoding unit 123.

The block coding unit 117 receives blocks 125 of a prescribed number of data bits ( e . g. , four bits) from the Mil 115, and transforms them into prescribed code words 127 including more bits than the data bit blocks ( e . g. , five bits) . The code words 127 are fed to the transmitter unit 119, which manages the physical details of the transmission of the code words 127 over the physical medium 105. The receiver unit 121 manages the physical details of the reception of code words from the communication physical medium 105, and feeds the received code words 129 to the block decoding unit 123. The block decoding unit 123 decodes the received code words 129 (of five bits, in the example) , converting them into the corresponding original data bit blocks (of four bits) , and feeds the data bit blocks 131 to the Mil 115.

The table shown merely by way of example in FIG. 4 provides the correspondence between blocks of data bits and code words as prescribed by the 4B/5B block coding scheme specified by the IEEE Standard 802.3u (Fast Ethernet). In network devices belonging to a network complying with that standard, the physical layer 107, particularly the block coding unit 117 and the block decoding unit 123, operates according to this 4B/5B block coding scheme.

The transmitter unit 119 may include a parallel/serial converter, for producing a serial stream of bits, an NRZ/NRZI encoder, a pulse shaper, and a driver of the physical medium. The receiver unit 121 may in turn include an NRZ/NRZI decoder, a serial/parallel converter, a clock recovery circuit .

Components implementing the physical layer 107 are commercially available; an example of a commercially available physical layer component is the KS8737 Mil physical layer transceiver produced by KENDIN.

The MAC layer 113 and the physical layer 107 exchanges signals and data, through the Mil 115. In particular, the MAC layer 113 and the physical layer 107 exchange transmit data TXD, receive data RXD, a transmit data enable signal TXEN, a receive data valid signal RXDV, a transmit error signal TXER and a receive error signal RXER.

The transmit data TXD represents data that the MAC layer 113 sends to the physical layer 107 for transmission over the physical medium 105. Conversely, the receive data RXD represents data received over the physical medium 105, that the physical layer 107 sends to the MAC layer 113. The transmit data enable signal TXEN is asserted by the MAC layer 113 for enabling the physical layer 107 to transmit the data over the physical medium 105. The receive data valid signal RXDV is a signal asserted by the physical layer 107 when the data received over the physical medium are ascertained to be valid. The receive data valid signal RXDV is for example asserted after a prescribed extra control code word is received, for example a "START OF FRAME" control code word (in the example shown in FIG. 4, a code word "11000" followed by a code word "10001") , signalling the beginning of a data frame, and is kept asserted until another prescribed extra control code word is received, for example an "END OF FRAME" control code word (in the example of FIG. 4, a code word "01101" followed by a code word "00111") signalling the end of the data frame, or after a prescribed time out.

The receive error signal RXER is asserted by the physical layer 107 when an invalid code word is received, i.e., one of the set of code words not defined for use in the prescribed block coding scheme for the exchange of data or for control information; as schematically shown in FIG. 1, when this occurs the block decoding unit 123 asserts a code word error signal 135 to the Mil 115.

The transmit error signal TXER is asserted by the MAC layer 113 to force the physical layer 107 transmitting an invalid code word; schematically, the Mil 115 asserts a force invalid code word signal 133 to the block coding unit 117, which in turn feeds one (any one) of the invalid code words to the transmitter unit 119.

FIG. 2 is a more detailed view of the data link layer 109 of the network devices 103a, 103b, in an embodiment of the present invention. Schematically, the MAC layer 113 includes two MAC layers 201a and 201b.

The MAC layer 201a is a conventional MAC layer, responsible of controlling the access to the physical medium 105 for the exchange of data between the network devices; in the context of this description, the channel on which data are exchanged between network devices according to the network protocol standard will be referred to as primary channel CHI, and the MAC layer 201a as primary channel MAC layer. The MAC layer 201b is instead responsible of controlling the access to the physical medium 105 for the exchange of service messages, other than the data exchanged on the primary channel, between the network devices; in the context of this description, the channel on which the service messages are exchanged between the network devices will be referred to as secondary channel CH2, and the MAC layer 201b as secondary channel MAC layer.

In the context of the present description, service information is intended to mean any information useful for, e . g. , controlling a channel on which data is being exchanged between two or more network devices (for example, creating the channel, destroying the channel, negotiating parameters for the communication over the channel) , monitoring the channel status, monitoring one or more network devices from one or more remote network device, from example from a network central, or enabling voice or data communication between installation or maintenance operators located at different sites of the network.

Conventionally, the primary channel MAC layer 201a delivers the transmit data TXD and receives the receive data RXD to/from the physical layer 107, through the Mil 115; additionally, the primary channel MAC layer 201a receives from the physical layer 107 the receive data valid signal RXDV and the receive error signal RXER.

The secondary channel MAC layer 201b interacts with the physical layer 107, through the Mil 115, via the transmit data enable signal TXEN, the transmit error signal TXER and the receive error signal RXER. In particular, the secondary channel MAC layer 201b controls the access to the physical medium 105 for the transmission of service messages over the secondary channel by means of the transmit data enable signal TXEN and the transmission error signal TXER, while for receiving the service messages from the secondary channel the receive error signal RXER is exploited, as will be described in greater detail later on.

In the embodiment of the invention shown in FIG. 2, the secondary channel MAC layer 201b is designed to have a lower priority than the primary channel MAC layer 201a for accessing the physical medium 105. In other words, the secondary channel MAC layer 201b may access the physical medium 105 for transmitting service messages only if no data are to be transmitted on the primary channel . The secondary channel MAC layer includes a microcontroller 203, configured to receive and sense a transmit primary channel data enable signal TXEN' generated under the control of the primary channel MAC layer 201a, and asserted whenever the primary channel MAC layer 201a needs to transmit data over the primary channel ; the state of the transmit primary channel data enable signal TXEN⁷ is sensed by the microcontroller 203 to ascertain whether data are to be transmitted over the primary channel . An output TXSM of the microcontroller 203 provides the service message to be transmitted over the secondary channel, in the form of a serial stream of bits. The transmit data enable signal TXEN to be fed to the physical layer 107 is the result of a logic OR operation on the transmit primary channel data enable signal TXEN' , produced by the primary channel MAC layer 201a, and the transmit service message output TXSM of the microcontroller 203. The transmit service message output TXSM of the microcontroller 203 and the transmit primary channel data enable signal TXEN' are also logically combined to produce the transmit error signal TXER. In particular, a logic complement of the transmit primary channel data enable signal TXEN' is put in logical AND with the state of transmit service message output TXSM; the assertion of the transmit error signal TXER is thus entrusted to the microcontroller 203 only if the transmit primary channel data enable signal TXEN' is deasserted, i.e., only if no data are to be transmitted over the primary channel. As shown in the drawing, the OR and AND logic operations are for example performed by an OR logic gate 205 and an AND logic gate 207, respectively; to this purpose, commercially available logic integrated circuits can be exploited. It is pointed out that, in principle, the signals TXEN and TXER could be controlled directly by the microcontroller 203, without the need of logically combining the output TXSM of the microcontroller with the signal TXEN'; in other words, the microcontroller, sensing the status of the signal TXEN' , could directly control the status of the signals TXEN and TXER; however, the arrangement shown in the drawing ensures a substantially immediate response to the assertion of the signal TXEN' , causing a substantially immediate assertion of the signal

TXEN and deassertion of the signal TXER when the signal TXEN' is asserted by the primary channel MAC layer 201a; in this way, it is guaranteed that no primary channel data are lost, for example due to delays caused by the microcontroller 203.

The receive error signal RXER is fed to an input of the microprocessor 203, in particular an interrupt input.

Preferably, the signal RXER is fed to a pulse generator 209, for example a monostable circuit, producing a pulse of a prescribed time length whenever the signal RXER is asserted.

The primary channel MAC layer 201a and the secondary channel MAC layer 201b interact with a MAC control and a Logical Link Control (LLC) layers 211, which in turn interact with the higher layers 111 of the OSI model.

Concerning the secondary channel, the MAC control layer, the LLC layer and the higher layers may be implemented by means of the microcontroller 203. Preferably, the secondary channel MAC layer 201b implements a service message queuing mechanism for queuing service messages waiting to be transmitted. A similar queuing mechanism may also be implemented for the received service messages .

It is pointed out that the provision of a microcontroller for controlling the components implementing the physical layer is not at all rare in conventional network devices, and is also suggested by the producers of such components. The functions of the microcontroller 203 can thus be carried out by the microcontroller already provided for controlling the components implementing the physical layer.

According to the present invention, service information is exchanged over the secondary channel CH2 in terms of presence or absence of invalid code words, i.e., code words not defined for use for the exchange of data and controls in the block coding scheme specified for the network protocol for the exchange of data over the primary channel CHI. In particular, a first logic state ( e . g. , a logic "1") of a service information unit (a bit of a service message) is associated with the presence of at least one invalid code word, while a second logic state (a logic "0") is associated with the absence of invalid code words, i.e., the presence of at least one non-invalid code word.

FIG. 3 shows the structure of one bit of the service messages, in an embodiment of the present invention, not at all limitative. Any service message bit comprises two consecutive code words. A first code word is always an invalid code word, and provides a service message bit clock pulse CK for synchronising the microcontroller 203 on the side of a network device receiving the service message. A second code word carries the information, and forms a service message bit payload pulse DT for the microcontroller 203 in the network device receiving the service message; this second code word may be either an invalid or a non-invalid code word, depending on the logic state of the service message bit. A bit formed by two consecutive invalid code words is associated with a first binary state, for example a logic "1", while a bit formed by an invalid code word followed by a non-invalid code word is associated with a second binary state, for example a logic "0".

The number of invalid code words to be associated with the clock pulse CK may be higher than one, as well as the number of invalid or non-invalid code words to be associated with the payload pulse; in this way, the operating speed requirements for the microcontroller 203 can be reasonably relaxed. Let it be assumed that a network device, e . g. the network device 103a, needs to transmit a service message over the secondary channel, for example a service message directed to the network device 103b; it is pointed out that, in principle, a service message may even consist of a single bit .

As schematically shown in FIG. 5, the secondary channel MAC layer 201b of the network device 103a ascertains that a service message is to be transmitted by checking the service message queue (block 501) ; if messages are present in the queue, one message is read from the queue (block 503) , for example on a first-in first-out basis .

Before transmitting the service message, the network device 103a checks whether it is currently transmitting data over the primary channel (block 505) . To this purpose, the secondary channel MAC layer 201b in the network device 103a checks whether the primary channel MAC layer 201a has the control of the physical layer 107, by sensing the status of the signal TXEN' (which is asserted when the primary channel MAC layer 201a needs to transmit data on the primary channel) . If the signal TXEN' is asserted, the transmit data enable signal TXEN is asserted, the transmit error signal TXER is deasserted, the data TXD supplied by the primary channel MAC layer 201a are transmitted by the physical layer 107 over the primary channel CHI, and the microcontroller 203 waits until the primary channel MAC layer 201a leaves physical layer 107 free, a situation corresponding to the deassertion of the signal TXEN' . When the primary channel MAC layer 201a deasserts the signal TXEN', the signal TXEN is deasserted. In this condition, the physical layer 107 continuously transmits a prescribed control code word indicating an idle state of the primary channel CHI; for example, as shown in FIG. 4, in the 4B/5B block coding scheme the code word corresponding to the idle state of the primary channel is "11111" . It is pointed out that this code word (as well as all the control code words) is a non-invalid code word, i.e. a code word which, when received by a network device, does not cause the assertion of the receive error signal

RXER.

The secondary channel MAC layer 201b, detecting that data are no more transmitted over the primary channel, can now take control of the physical layer 107 and start transmitting the first message present in the queue of service messages. The microcontroller 203 transforms the service message into a serial stream of bits, and supplies the service message to the physical layer 107 one bit at a time (block 507) . As discussed in the foregoing, in the exemplary embodiment of service message bit structure shown in FIG. 3, transmission of any bit of the service message involves the transmission of an invalid code word (the clock pulse CK) , followed by the transmission of an invalid or non-invalid code word (the payload pulse DT) , depending on the logic state of that bit . In order to transmit the clock pulse CK, the microcontroller 203 asserts the output TXSM. This causes the assertion of the signals TXEN and TXER. The assertion of the signal TXER forces the physical layer 107 to transmit one (any one) of the invalid code words specified in the block coding scheme. For transmitting the payload pulse DT, the microcontroller output TXSM is either asserted, in which case, similarly to the clock pulse CK, another invalid code word is transmitted, or deasserted, in which case the signals TXEN and TXER are deasserted, and the physical layer 107 transmits the non-invalid control code word corresponding to the idle state of the primary channel CHI. This is repeated for each bit of the service message until all the bits are transmitted (block 509) . After the complete message has been transmitted, the message is deleted from the service message queue (block 513) ; if other messages are present in the queue waiting to be transmitted (block 517) , they are read from the queue and transmitted in a similar way.

The transmission of the service message can be at any time interrupted by the primary channel MAC layer 201a, if data need to be transmitted over the primary channel CHI . In particular, by asserting the signal TXEN' , the primary channel MAC layer 201a immediately takes the control of the physical layer 107; by sensing the signal TXEN' (block 511) , the secondary channel MAC layer 201b detects that the primary channel MAC layer 201a has taken the control of the physical layer 107, and interrupts the transmission of the service message, leaving the message in the queue (block 515) ; the secondary channel MAC layer 201b waits for the physical layer 107 to be left free by the primary channel MAC layer, and then tries to retransmit the complete service message.

Each time the physical layer 107 of the network device 103b receives an invalid code word, the receive error signal RXER is asserted. This is detected by the secondary channel MAC layer 201b of the network device 103b. In particular, when the receive error signal RXER is asserted, a pulse of a prescribed time length is produced by the monostable 209 and fed to the interrupt input of the microcontroller 203. The microcontroller 203 launches an interrupt service routine: the interrupt is temporarily masked, and the interrupt input is checked to ascertain whether the signal RXER is again asserted after a prescribed time. If the signal RXER is again asserted after the prescribed time, reception of a logic "1" service message bit is declared, otherwise reception of a logic "0" service message bit is declared received. The interrupt is again enabled, and the microcontroller 203 waits for the next assertion of the signal RXER.

The physical layer 107 is capable of detecting that an invalid code word has been received, but may be not capable «-,4 interpreting the service message received over the secondary channel. In particular, the physical layer 107 may be not capable of discriminating whether the received invalid code word is part of a bit of a service message, or has been caused by an error occurred during transmission of data over the primary channel. In both cases, typically the physical layer 107 merely detects the invalid code word and asserts the receive error signal RXER.

A suitable service message protocol can be implemented in the higher OSI layers for interpreting the data received over the secondary channel. For example, the service message protocol can be implemented at the data link layer 109, or at higher layers, by the microcontroller 203. In other words, the microcontroller 203 is programmed to implement the service message protocol .

The choice of the service message protocol to be implemented is extremely open, depending for example on the type, number, length of service messages to be exchanged, and the desire of implementing error detection and correction schemes for increasing the reliability of the secondary channel .

FIG. 6 merely provides an example of a service message protocol that can be implemented for regulating the exchange of service messages over the secondary channel CH2. In particular, FIG. 6 shows an example of a service message frame. In this example of service message protocol, the service message frames have a fixed length of forty bits (five bytes) , any bit having the structure shown in FIG. 3, comprising a clock pulse CK followed by a payload pulse DT.

The service message frame includes several fields.

A first field of the frame is a preamble field, formed of a group of five preamble bits P0 - P4 carrying a predetermined digital pattern. In particular, all the preamble bits are logic "l"s. Any network device stores (e. g. , in a non-volatile memory of the microcontroller 203) the predetermined preamble pattern, and can establish that a new service message frame is about to be received by comparing the first five bits received from the secondary channel CH2 to the predetermined pattern. In this way, the network devices can discriminate between received invalid code words carrying the service message and received invalid code words caused by corruption of primary channel data.

A second field of the frame comprises three reserved bits R0 , Rl and R2 that can be used in future upgrades of the protocol. By default, the three reserved bits R0, Rl, R2 are logic "0"s. In order to ensure the compatibility of any protocol upgrade with previous protocol versions, a different logic state of the three reserved bits R0 , Rl, R2 shall not cause a received frame error.

A fourth field of the frame is a service message type field comprising eight bits TO - T7. The service message type field describes the type of service message that is transmitted in the current frame. For example, assuming that the secondary channel CH2 is used to exchange values of operating parameters detected by the network devices (e.g. , temperature of the environment) , the service message type field in the frame specifies which parameter is being transmitted by the current frame . The service message type field is also used for discriminating between different types of service information, e . g. operating parameter values and commands, e . g. auto-negotiation of primary channel parameters between different network devices. For example, a prescribed pattern of "l"s and "0"s in the service message type field may be used to indicate that the frame carries an auto-negotiation information.

A fifth field of the frame is a service message payload field, comprising sixteen bits DO - D15 carrying the service information, e . g. the value of the parameter specified in the service message type field, or the auto- negotiation information. A sixth field of the frame is an error checksum byte (bits CO - C7) , in which the checksum of the previous four bytes is contained. The provision of this field ensures the reliability of the data transmitted over the secondary channel . The service message frame always ends with a clock pulse CK, enabling the receiver network device to establish whether or not the last bit of a service message frame has been received.

The service message protocol described in the foregoing enables a network device to distinguish between received invalid code words carrying the service message and received invalid code words corresponding to corrupted primary channel data, and to quickly detect an interruption in the transmission of a service message. This second aspect is particularly important when the primary channel is assigned a higher priority than the secondary channel, as in the exemplary embodiment described herein.

Clearly, the service message frame shown in FIG. 6 is merely an example, and more complex service message frames may be devised. For example, in order to increase the flexibility and enable the exchange of more complex service messages over the secondary channel, service message protocols with variable-length frames may be developed. The service message frame may in this case include a further field specifying the frame length in, e . g. , bytes. Also, a policy of acknowledgement of the received service message frames may be implemented, for example exploiting one or more of the reserved bits R0 - R2 , or providing additional bits in the frame. It can be appreciated that, thanks to the present invention, a secondary channel can be created for exchanging service messages between devices of a data communication network. In particular, the secondary channel can be multiplexed with the primary, data exchange channel over a same physical communication medium. A distinctive advantage of the present invention is that the secondary channel exploits the functions of a standard physical layer, without requiring changes or customisations . Commercially available components complying with the minimum prescriptions of the network standards can thus be straightforwardly used.

Although the present invention has been disclosed and described by way of some embodiments, it is apparent to those skilled in the art that several modifications to the described embodiments, as well as other embodiments of the present invention are possible without departing from the scope thereof as defined in the appended claims.

In particular, and by way of example only, should the network operate in half-duplex mode, the secondary channel MAC layer may be designed so that before attempting to transmit service messages over the secondary channel, a check that no primary channel data are currently being received is performed. This can for example be attained by sensing the status of the receive data valid signal RXDV.

As an alternative to the embodiment described in the foregoing, wherein the primary channel is assigned a higher priority than the secondary channel, so that the latter is transparent to the primary channel, the secondary channel may be assigned a higher priority than the primary channel. In this case, the primary channel MAC layer may be modified so that before attempting to engage the physical layer for transmitting the primary channel data, the state of the secondary channel is ascertained. This may be useful when the service information include important or essential information on the network, for example in order to quickly drop an existing link between two network devices.

Also, depending on the service information exchange needs, a network device may be configured so as to merely transmit service information, and another network device be configured so as to merely receive the service information.

Claims

1. A method of exchanging service information between devices (103a, 103b) in a data communication network (101) in which exchanged data are encoded into code words according to a prescribed block coding scheme at a physical layer (107) , the block coding scheme specifying invalid code words not defined for use for the exchange of data and control information, and non-invalid code words, characterised by comprising associating a first logic state of an exchanged service information unit (301) with a presence of at least one of the invalid code words, associating a second logic state of the exchanged service information unit with a presence of at least one of the non-invalid code words.

2. The method according to claim 1, in which said at least one invalid code word is any one of the invalid code words specified by the block coding scheme.

3. The method according to any one of the preceding claims, in which the service information and the data are multiplexed and exchanged over a same communication physical medium (105) .

4. The method according to any one of the preceding claims, in which said exchanging service information comprises transmitting the service information, the transmitting the service information including engaging a physical layer (107) of a transmitter device (103a, 103b) in the network for: forcing (TXER) transmission of at least one invalid code word for transmitting the first logic state of a service information unit; not forcing transmission of at least one invalid code word for transmitting the second logic state of the service information unit .

5. The method according to claim 4, in which said transmitting the service information includes conditioning the transmission of the service information to an absence of transmission of data by the transmitter device.

6. The method according to claim 4, in which a transmission of data by the transmitter device is conditioned to an absence of transmission of the service information by the transmitter device.

7. The method according to any one of the preceding claims, in which said exchanging comprises receiving the service information, the receiving the service information including: exploiting a physical layer (107) of a receiver device

(103a, 103b) in the network to detect (RXER) the presence or the absence of invalid code words.

8. The method according to any one of the preceding claims, comprising combining the presence of at least one invalid code word and the presence of at least one non- invalid code word with the presence of at least one additional invalid code word (CK) , whereby the first logic state is associated with the presence of at least two invalid code words and the second logic state is associated with the presence of a number of invalid code words equal to that associated with the first logic state less at least one .

9. The method according to any one of the preceding claims, comprising formatting the service information in accordance to a prescribed service information exchange protocol .

10. The method according to any one of the preceding claims, in which said network is an Ethernet network or a Fiber Distributed Data Interface network.

11. The method according to claim 10, in which said block coding scheme is one among a 4B/5B, a 8B/10B, a 64B/66B block coding scheme.

12. A system for exchanging service information between devices (103a, 103b) in a data communication network

(101) in which data exchanged between the devices are encoded into code words according to a prescribed block coding scheme at a physical layer (107) of the devices, the block coding scheme specifying invalid code words not defined for use for the exchange of data and control information, and non-invalid code words, characterised by comprising in the devices of the network: a physical layer device (107) , and a physical layer control device (201b) , the physical layer device and the physical layer control device associating a first logic state of an exchanged service information unit (301) with a presence of at least one of the invalid code words, and associating a second logic state of the exchanged service information unit with a presence of at least one of the non-invalid code words .

13. The system according to claim 12, in which, in a service information transmitter device (103a, 103b) of the network, the physical layer control device engages the physical layer (107) of the transmitter device for: forcing (TXER) transmission of at least one invalid code word for transmitting the first logic state of a service information unit, and not forcing transmission of at least one invalid code word for transmitting the second logic state of the service information unit.

14. The system according to claim 13, in which the operation of the physical layer control device is conditioned to presence of data to be transmitted over the network .

15. The system according to claim 13, in which the physical layer control device interrupts the transmission of data over the network.

16. The system according to claim 13, in which the physical layer control device implements a media access control layer for controlling access of the transmitter device to a communication physical medium (105) of the network for transmitting the service information.

17. The system according to claim 12, in which, in a service information receiver device (103a, 103b) of the network, the physical layer control device associates the presence of invalid and non-invalid code words detected (RXER) by the physical layer of the receiver device with the first and second logic state of a service information unit, respectively.

18. The system according to claim 17, in which the physical layer control device is part of a media access control layer of the receiver device.

19. The system according to any one of claims 12 to 18, in which the network is an Ethernet network or a Fiber

Distributed Data Interface network.

20. The system according to claim 19, in which said block coding scheme is one among a 4B/5B, a 8B/10B, a 64B/66B block coding scheme.

21. A device configured for use in a data communication network (101) in which data exchanged between devices (103a, 103b) in the network are encoded into code words according to a prescribed block coding scheme, the block coding scheme specifying invalid code words not defined for use for the exchange of data and control information, and non-invalid code words, the device being capable of exchanging service information with other devices in the network, characterised by comprising a physical layer device (107) , and a physical layer control device (201b) , the physical layer device and the physical layer control device associating a first logic state of an exchanged service information unit (301) with a presence of at least one of the invalid code words, and associating a second logic state of the exchanged service information unit with a presence of at least one of the non-invalid code words .