Abstract:
Switch and MAC layer components are located at a headend and PHY layer components for connecting a plurality of end-user devices are located remotely at nodes. Using SSMII technology, MAC layer ports can communicate with an equal number of PHY layer interface ports serially. Thus, the MAC layer connects to the PHY layer via fiber links, a separate link being used for each direction of traffic data flow.  
     Information data is encoded along with a frame sync signal and a clock signal into a serial stream for transmission across the network. The serial stream is decoded at the other end, and the frame sync signal is extracted to provided timing functionality. This allows full duplex operation with the MAC layer separated from the PHY layer at distances greater than a few inches. Also, user device status may be monitored at the single switch location.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit of priority under 35 U.S.C. 119(e) to the filing date of Bione, U.S. provisional patent application No. 60/342,988 entitled “Ethernet Switch Interface For Use In Optical Nodes”, which was filed Dec. 22, 2001, and is incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates, generally, to communication networks and, more particularly, to increasing the distance between the physical layer and the switch components of an Ethernet switch.  
         BACKGROUND  
         [0003]    As digital communications networks become more advanced, various chip makers and equipment maker&#39;s continue to improve and advance the devices, methods and systems used to facilitate higher and higher data transfer rates using smaller and less costly equipment and devices.  
           [0004]    For example, Cisco Systems, Inc. has developed an improvement to the media independent interface (“MII”) specification, which is known in the art for allowing a media access control (“MAC”) layer to control and interact with the physical interface (“PHY”) layer regardless of the type of physical media being controlled. The improvement is known in the art and defined by the Serial-MII (“SMII”) specification.  
           [0005]    SMII specifies that instead of using a conventional seven-wire arrangement for transferring Ethernet data between MAC and a corresponding PHY layer components, time division multiplexing (“TDM”) techniques can be used to transport the same amount of data over two wires serially. This is accomplished by using a global clock signal to provide timing to a plurality of MACs and corresponding PHYs. In addition, a global sync signal is sent to the MACs and PHYs. Thus, each group (typically comprising eight MAC-PHY sets) of components need only have 4 pins/wires instead of the nine per MAC-PHY set used in a conventional Ethernet system.  
           [0006]    While fewer pins and wires are required to connect the MACs to the PHYs under the SMII specification, the MACs and PHYs are inherently required to be located proximate one another, approximately within 1.5 ns. In other words, using SMII, MAC and corresponding PHY components should realistically be located on the same printed circuit board (“PCB”). This is due to trace delay caused by propagation characteristics of the connecting medium, such as copper.  
           [0007]    To allow greater distances separation distances between the MAC and the PHY layers, a dedicated set of clock and sync signals may be used for the transmit direction and a separate set of dedicated signals may be used for the receive direction. This allows separation distances of the MAC layer devices from the PHY layer devices greater than the trace delay inherent in the SMII specification, while providing full duplex capability as well. This specification using separate signal sets for the transmit and receive directions respectively is known in the art as source synchronous serial media independent interface (“SSMII”).  
           [0008]    Application of an SSMII system may be useful in computer network systems, telephony systems or any other type of system that transmits and receives digital data using the Ethernet format. As shown in FIG. 1, a typical Ethernet system  2  may comprise a plurality of computers  4 A-n, which are connected together through network  6 , typically an optical fiber network. Each computer  4 A-n typically interfaces through nodes (interface devices)  8 A and  8 B. It will be appreciated that network  2  may comprise many more computers  4  and interfaces  8  than shown in the figure. Each of the interface devices  8  typically comprises a PHY  10 , a MAC  12  and a switch  14 . PHY  10  is typically selected to provide an interface between the MAC, an electrical device, and the computer  4 , which may connect electrically, optically or wirelessly, to the network  6 . Switch  14  typically performs routing and signal flow functionality, i.e., which computer to route incoming signals to, and manage which connected computer (or other device) provides an outgoing signal at a given time. For example, if computer  4 A is at a head end and computers  4 B-n are subscribers, computer  4 B may not be allowed to communicate directly with computer  4 C, the communication there-between being routed through network  6  back to the head end computer  4 A. Thus, computer  4 A can be used to provide security and monitoring, and other management functions. These management functions are often performed by a management computer  16  at headend  15  with computer  4 A functioning as a data server. Whatever the management arrangement, each switch at each computer  4 A-n is managed independently of the others. In addition to signal flow control it is often desirable to be able to determine whether a particular customer or subscriber has a computer (or other network device) connected to the network and to be able to determine whether that subscriber device is transmitting or receiving a signal. When an apparent malfunction has occurred and a customer needs assistance, it is often necessary for service provider personnel to physically drive to the node location that houses interface device  8 B to perform basic diagnostic routines, such as visually checking to see whether one of computers  4 B-n are plugged into the network and/or are transmitting/receiving when they are supposed to be. In addition, each switch, MAC and PHY device, typically comprising integrated circuits mounted on a PCB, has a cost associated with it.  
           [0009]    Thus, there is a need for a method and system for implementing an Ethernet network using SSMII technology that reduces the complexity of managing the signal flow through the switches, that reduces the need for personnel having to physically go to a site to perform rudimentary diagnostic functions, and that maintains low cost of the system by using off-the-shelf parts.  
         SUMMARY  
         [0010]    It is an object to provide a method and system for implementing a network using Ethernet technology wherein an Ethernet switch can be located at a central location and a plurality of PHY interface devices associated with the switch—each corresponding to an individual user—can be remotely located, the separation between the switch/MAC layer and the PHY devices being on the order of miles.  
           [0011]    As discussed above, SMII Ethernet switch technology is used to reduce the number of connections between the MAC devices associated with the switch, and the PHY devices. The SSMII specification facilitates the extending of the separation distance between the switch/MAC and PHY layers up to approximately twelve inches, so that they may not be required to be mounted on the same PCB. To extend the distance between the switch/MAC and the PHY to distances on the order of miles, interface components are used. Thus, the MAC layer components and associated switch components can be located at a headend, for example, and the PHY layer components can be located remotely at a node that is near an end user.  
           [0012]    An aspect of the invention provides an interface between the MAC layer and the PHY layer components so that each of these layers behaves as if it is located on the same PCB as the other, or at least within the same enclosure, such as a node housing, for example. Thus, instead of being limited to transfer between MAC and PHY components being proximately located, data can be transferred between MAC layer components and PHY layer components over a port-to-port network infrastructure spread out over a campus or even a metropolitan area. Accordingly, a full complement of components including a switch, MAC layer components and PHY layer components are not needed at both a headend, or other central location, and at the remote nodes.  
           [0013]    Instead, the number of components used to implement a network architecture is reduced, as the node only has PHY layer components for interfacing with a user&#39;s device, such as a computer or other device for transmitting, receiving and processing information data. Moreover, the more expensive switch and MAC layer components are only located at the headend. Thus, material costs and complexity are reduced and the network is easier to manage.  
           [0014]    To reduce costs even further, off-the-shelf components may be used to implement the architecture, as a channel normally used for transferring information data related to a particular user is used to transport clock and other timing signals. This reduces the need for additional links between the headend and node for transporting the timing signals, as the information signals are all transported together serially using SSMII technology. At each location of the network architecture pertinent to the invention described herein, these locations being referred to herein as the centrally located headend and the remotely located (with respect to the headend) nodes, transmit and receive circuitry and devices are used to provide interface between the MAC and PHY layers, and the network, preferably an optical fiber network. It will be appreciated that other network transport technologies may be used including copper gigabit backplane technology For the transmit direction, an encoder is used to encode eight channels of data, seven being information data and the other used for the timing signals referred to earlier. Thus, commonly available octal devices (such as an integrated circuit comprising eight MAC layer components or eight PHY layer components) can be used without the need for customized components. The encoded data is multiplexed using a serial transmitter into a typically 1.25 Gbps signal. This signal is then fed to a transmitting device, typically a laser, for transport across the network, which preferably comprises optical fiber.  
           [0015]    For the receive direction, a detector device, preferably a photodiode used in the optical network scenario, receives a transmitted signal and feeds it to a serial receiver, typically operating at a frequency of 1.25 Gbps. The serial receiver demultiplexes the received serial signal, which is fed to a decoder that performs the opposite operation of the encoder in the transmit portion. The decoded data is then output as seven information channels of data and one timing channel of data. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0016]    [0016]FIG. 1 illustrates a schematic of a network architecture using SSMII Ethernet technology at each node of the network.  
         [0017]    [0017]FIG. 2 illustrates a block diagram of a network architecture using SSMII Ethernet technology where one node has part of the SSMII components and the other node has the other components.  
         [0018]    [0018]FIG. 3 illustrates an embodiment for increasing the distance between SSMII Ethernet layer components for use in node locations separate from one another. 
     
    
     DETAILED DESCRIPTION  
       [0019]    As a preliminary matter, it will be readily understood by those persons skilled in the art that the present invention is susceptible of broad utility and application. Many methods, embodiments and adaptations of the present invention other than those herein described, as well as many variations, modifications, and equivalent arrangements, will be apparent from or reasonably suggested by the present invention and the following description thereof, without departing from the substance or scope of the present invention.  
         [0020]    Accordingly, while the present invention has been described herein in detail in relation to preferred embodiments, it is to be understood that this disclosure is only illustrative and exemplary of the present invention and is made merely for the purposes of providing a full and enabling disclosure of the invention. The following disclosure is not intended nor is to be construed to limit the present invention or otherwise to exclude any such other embodiments, adaptations, variations, modifications and equivalent arrangements, the present invention being limited only by the claims appended hereto and the equivalents thereof. Furthermore, while some aspects of the present invention are described in detail herein, no specific cable type, conductor type, fiber type, connector, enclosure, circuit board arrangement, laser type, for example, is required to be used in the practicing of the present invention. Indeed, selection of such parts and components would be within the routine functions of a designer skilled in the art.  
         [0021]    Turning now to the figures, as discussed above, FIG. 1 illustrates an Ehternet system  2  that uses SSMII technology to transport data between a headend  15  and a plurality of remote nodes  8 . Each node  8  comprises an Ethernet switch  14 , a plurality of MAC layer components  12  and a plurality of PHY layer components  10  for providing an interface between fiber network  6  and user devices  4 . As switches  14  facilitate routing of information and other data signals to various parts of the network  2 , the central headend switch management components  16  manages each switching and routing function of the switches. Typically, the central office or headend  15  may comprise components  8 A and  4 A, as well as headend switch management components  16 . Management components  16  are shown in FIG. 1 as separate from components  8 A and  4 A in order to illustrate that the headend typically comprises MAC layer and PHY layer components, as well as the management and switching components. However, these components may also be remotely located, or at least reside on separate PCBs. Thus, they are illustrated separately, but collectively surrounded by dashed lines to indicate that headend switch management components  16 , interface device  8 A, and computer  4 A typically function as the headend  15 . It is noted that the inches of separation shown between PHY  10 A and MAC  12 A is applicable for node  8 B, as well as other nodes and Ethernet devices that are not shown for clarity, but would be referred to as  8 C- 8   n  if shown.  
         [0022]    Turning now to FIG. 2, a network  18  is illustrated that implements transport of data over fiber network  6  using SSMII technology, wherein switch  14  and MAC  12  components are located at headend  20  and PHY layer components  10  are remotely located at node  22 . It will be appreciated that multiple nodes may be served by headend  20 . For purposes of example and discussion, PHY  10  referred to herein is an octal device having eight PHY layer ports on a single integrated circuit. However, it will be appreciated that node  22  may comprise multiple octal PHY (more or less than eight ports may be used as well) integrated circuits  10 , and therefore may be capable of serving more than eight user devices  4 .  
         [0023]    As shown in the figure, a distance of miles rather than inches as shown in FIG. 1 separates the MAC components  12  and the PHY components  10 . To facilitate the separation of miles instead of merely a few inches, network interface devices  24  and  26  provide an interface at headend  10  and node(s)  22  so that MAC  12  and PHY  10  can interact with one another via network  6 , which may be spread out over many miles.  
         [0024]    Turning now to FIG. 3, a schematic diagram is shown illustrating the components of interfaces  24  and  26  that facilitate the spreading out of the MAC components  12  from the PHY components  10  located at headend  20  and node  22  respectively. Interface  24  comprises an  8 B/ 10 B encoder  28  which receives input signals from MAC layers  12 . Assuming that MAC  12  is an octal device having eight ports for data transport, eight transport links  30  feed information from the MAC to encoder  28 . In addition a clock signal is provided from MAC  12  to encoder  28 . Seven of the links  30  are used to feed information data signals from MAC  12  to encoder  28 . The eighth link  30  is used for a transmit frame synchronization signal to be used upon decoding at node  22 .  
         [0025]    Encoder  28  takes the signals received from links  30  and  32 , and encodes them into a 10-bit data stream that includes information data, frame sync data and a clock timing signal. The encoded signal is then fed to serial transmitter  34 , which multiplexes the incoming data into a serial data stream at a rate of 1.25 Gbps. Laser  36  sends the multiplexed serial signal across network  6  toward node  22 .  
         [0026]    At node  22 , receiver device  38 , such as a photodiode, receives the optical signal sent by laser  36  over network  6 , and converts the incoming data stream into an electrical signal. This electrical signal is then fed to serial receiver  40 , which demultiplexes the data stream from the 1.25 Gbps signal, and sends the demultiplexed signal to  10 B/ 8 B decoder  42 . Decoder  42  decodes the signal into seven different information data signals and a frame sync signal corresponding to the seven information data signals and the frame sync signal encoded by encoder  28  at headend  20 . These seven information data signals and one frame sync signal are provided to seven corresponding information data ports and a frame sync input respectively at PHY  10  on links  44 . The clock signal generated at headend  20  may be retrieved from decoder  42  and provided along link  46  to PHY  10 , or a phase locked loop circuit (“PLL”) may be used to generate a new clock signal based on the clock signal retrieved from the incoming serial data stream.  
         [0027]    For the direction of data being transmitted from node  22  to headend  20 , similar components as discussed above are used in interfaces  26  and  24 . Assuming that PHY  10  comprises an octal device having eight interface ports for connecting with eight user devices, only seven ports are used to actually connect user devices. Thus, only seven of the set of eight lines  48  are used to transport information from PHY  10  toward headend  20 . As with the transport of information in the other direction from headend  20  towards node(s)  22 , one of the eight links  48  is used for a frame sync signal. In addition to links  48 , a clock signal may be generated at node  22  and provided to interface device  26  via link  50 . Alternatively, the headend clock signal clock signal received at node  22  may be reused for the clock timing signal in the reverse direction for transport from the node toward the headend  20 . The information data and frame sync signal produced from output from PHY  10  on links  48 , along with the clock signal on link  50 , are encoded with encoder  52 , preferably an  8 B/ 10 B encoder known in the art. The encoded signal is then fed into serial transmitter  54 , which multiplexes the encoded signal into a 1.25 Gbps serial signal. The multiplexed serial data stream is then fed into transponder  56 , preferably a laser, for transmission to headend  20  via network  6 , preferably an optical fiber network. It will be appreciated that data flow in the two different directions is carried out on two separate serial data links, the serial data stream from headend  20  to node(s)  22  being transported on network link  58  and the data stream from node(s)  22  toward headend  20  over network link  60 . Thus, full duplex transport of data is facilitated.  
         [0028]    When the serial data stream from laser  56  reaches headend  20  via link  60 , transponder  62 , preferably an optical decoder device, such as, for example, a photodiode, converts the received signal into an electrical signal. Serial receiver  64  then demodulates the serial stream from the 1.25 Gbps signal, and feeds the demultiplexed signal to decoder  66 , preferably a  10 B/ 8 B decoder known in the art. Decoder  66  separates the information data from the sync data and provides the information data to MAC layer  12  via seven of eight links  68 . The frame sync signal is provided on the eighth link of links  68 . The clock signal is provided on link  70 , either directly from the decoded data stream, or generated by a PLL based on the incoming clock signal. Accordingly, full duplex communication between the headend  20  and nodes  22  is facilitated with a switch  14  and MAC layer  12  located at the headend, and the PHY layer at the node  22 .  
         [0029]    Furthermore, management is only required of one switch at the headend  20 , as opposed to both at the headend and at the node(s)  22 . This may reduce the number of occurrences when provider personnel must physically drive to the node location and perform diagnostics in the case of a malfunction. Indicators  72 , preferably LEDs, may be used to provide monitoring of the status at the node  22 . For example, if user devices are connected to only six of the seven ports served by PHY  10  (the eighth being unused as only seven links between interface device  26  and the PHY are used as discussed above), the six LEDs  72  corresponding to these users may be illuminated green with the other illuminated red. If trouble develops with one of the devices, or connection with PHY  10  related thereto, the corresponding LED  72  may be intermittently illuminated green, the flashing indicating to an observer at headend  20  that a problem may exist with a connected device.  
         [0030]    These and many other objects and advantages will be readily apparent to one skilled in the art from the foregoing specification when read in conjunction with the appended drawings. It is to be understood that the embodiments herein illustrated are examples only, and that the scope of the invention is to be defined solely by the claims when accorded a full range of equivalents.