Abstract:
A media access controller to adapt a rate of an output signal to a rate of an output medium is provided. The media access controller includes a register configured to output data to an external device, said register comprising a first input configured to control an output of the register and a second input configured to control an input to said register. The media access controller also includes a receiver configured to accept a signal from an external clock over the output medium and to provide said external clock signal to said first input of said register. An internal clock in the media access controller is configured to provide an internal clock signal from said internal clock to said second input of said register.

Description:
RELATED APPLICATIONS 
       [0001]    The present application is a divisional application of and claims priority under 35 U.S.C. §121 to U.S. Ser. No. 10/180,509, filed on Jun. 27, 2002, the subject matter of which is hereby incorporated by reference in full. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to systems and methods for adapting the output speed of a device to the speed of the output medium. More particularly, this invention relates to systems and methods for adapting the output rate of an Ethernet network device to the speed of the line to which it is connected. 
         [0004]    2. Description of the Related Art 
         [0005]    Over the last several years, the proliferation of the Internet has had a significant impact on many industries, especially the computer industry. The Internet has grown into an enormous network to which virtually any large or small computer network may be connected. In fact, it is now commonplace for the vast majority of people in free, industrialized nations to have access to the Internet, either through their business or work and/or through personal accounts. This connectivity has allowed many businesses, universities, governments, etc. to expand to provide their services via the Internet. 
         [0006]    Most people or businesses receive their Internet access via an Internet Service Providers (ISPs). ISPs provide access to the Internet for their customers usually through membership subscriptions. ISPs make at least a portion of their income on service fees such as subscription fees, on-demand provisioning of services, etc. ISPs often regulate the amount of Internet bandwidth (i.e., data speed) that a customer is entitled. 
         [0007]    Today&#39;s networks primarily support data services. However, new applications, such as network games, voice telephony and live video will elevate the data network infrastructure as an integral component of the next generation network. A typical home in the near future could have two phones, one or two personal computers and/or Personal Digital Assistants (PDA&#39;s) and general television sets. Applications will strain network bandwidth availability unless end-to-end quality of service (QOS) is available to guarantee real time voice and video quality as well as data integrity. 
         [0008]    QOS can be affected by flow control issues and line mismatch that can cause delays in packet delivery and unintended congestion problems. For example, referring to  FIG. 1 , two devices are shown connected by a connection medium such as fiber, coaxial cable, twisted pair, etc. The network devices may be, for example, network switches, routers, hubs, etc. Device  1  includes a buffer  106   a , a media access controller (MAC)  104   a , and a physical interconnect (PHY)  102   a . When the buffer of device  1  is full, the ingress rate control subsystem (e.g., ARL) of the device  1  MAC will send a PAUSE frame to prevent device  2  from sending additional frames. This protocol provides what is called back pressure to the traffic flow. In the meantime, device  2  may still be sending frames before the device  2  MAC receives the PAUSE frame. Device  1 , therefore, will need to reserve enough buffer space to account for the line delay, which is based on the type of medium and the length of the line. Furthermore, each PHY and each device MII layer (not shown) may also add delay to the system. For example, a Reed-Solomon (R/S) interleaver may be included in the MII layer to interleave data packets and perform error correction. The R/S interleaver therefore adds delay to data packets traveling between the MAC and the PHY. This delay may be variable based on the size and type of data being interleaved. 
         [0009]    Rate mismatch may occur especially when a VDSL PHY is connected to an Ethernet port. As an attempt to solve rate adaptation issues, some network device implementations use a buffer in the PHY to account for the rate mismatch. For example, referring to  FIG. 2 , shown is a network device which includes a buffer  106 , a MAC  104  and a PHY  102 . In between the PHY  102  and the MAC  104  in the physical layer are an input buffer  108   a  and an output buffer  108   b . The input buffer  108   a  can account for the rate mismatch in outputting data from the PHY  102  to the medium, and the output buffer  108   b  can account for the rate mismatch between the PHY  102  and the MAC  104 . In this proposed scenario, when the input buffer  108   a  is becoming full, a carrier sense (CRS) signal may be sent to the MAC  104  to defer transmission to the input buffer  108   a . When a carrier sense (CRS) signal is asserted, the output buffer  108   b  can still continue to empty the buffer. Of course, in this scenario, the MII must be working in half duplex mode. However, problems may occur when there is an actual collision. When there is an actual collision, the MAC will back off sending packets, and part of the packet that is currently being sent becomes a fragment. In the meantime, the packet transmitted by the buffer is anticipated to be a fragment and gets dumped by the MAC. When the MAC dumps this packet, the packet will be lost since the buffer has not backed off and resent the packet. PHY to PHY backpressure is also needed (as well as buffer back-off), if the switch is rate controlling the port. Since in this mode the PAUSE frame is not available, the only mode of operation to avoid a subsequent collision is through jamming of the line. 
         [0010]    Jamming is intended to cause the colliding stations to reschedule their next transmission attempts at different intervals in order to avoid subsequent collisions. Note that the jamming will have to pass through the buffer in the PHY, and there will be added latency caused by the buffer. To prevent large numbers of packet losses, a larger buffer must be used. However, a large buffer creates even larger latency. Additional control between the PHY and the MAC might be required. 
         [0011]    In view of the problems described above, a solution is desired for adapting to the line rate and accounting for line delay and delays in the Mil layer which does not include PHY buffers. Accordingly, there is a need for new systems and methods for rate adaptation in a data network. 
       SUMMARY OF THE INVENTION 
       [0012]    According to one embodiment of the invention, provided is a media access controller, which includes an output buffer and a clock controller. The output buffer includes a first and second clock input. The first clock is configured to control data input into the buffer, and the second clock is configured to control data output from the buffer. The clock controller is coupled to the output buffer and configured to regulate a first clock signal input into the first clock input to control the data input into the buffer. 
         [0013]    According to another embodiment of the invention, provided is a method of adapting a rate of an output signal to a rate of an output medium. The method includes providing a media access controller inputting an external clock signal into the media access controller, providing a register at an output of the media access controller, providing the external clock signal to a first input of the register, and providing a clock signal to a second input of the register. 
         [0014]    According to another embodiment of the invention, provided is a network device. The network device includes a media access controller, a physical interconnect, an output buffer and a clock controller unit. The output buffer is coupled with an output of the media access controller and an input of the physical interconnect. The clock controller unit is coupled with the output buffer, and is configured to regulate a first clock signal being input to the first clock input in order to control the data input into the buffer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The objects and features of the invention will be more readily understood with reference to the following description and the attached drawings, wherein: 
           [0016]      FIG. 1  is a block diagram of a network including two network devices illustrating back pressure and line delays; 
           [0017]      FIG. 2  is a block diagram of a network device which includes a PHY buffer; 
           [0018]      FIG. 3  is a block diagram of a network device supporting rate adaptation in accordance with an embodiment of the present invention; and 
           [0019]      FIG. 4  is flowchart of a method for aggregated rate control according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    The present invention provides rate adaptation to virtually any line speed and accounts for MII layer delays and line delays. 
         [0021]    The following exemplary configuration is discussed in terms of an Ethernet switch for a VDSL (Very High Rate Bit-Rate DSL) application. Although certain aspects of the configuration may be specific to VDSL, the invention is not meant to be limited as such. One having ordinary skill in the art will readily understand that the present invention may be applicable to many other network device configurations. 
         [0022]      FIG. 3  is a block diagram of a portion of a network device such as an Ethernet switch. The network device may include a PHY  302 , an R/S interleaver  304 , a high-level data link controller (HDLC)  306 , and a MAC  308 . The HDLC  306  and the R/S interleaver  304  are generally referred to as being part of the MII layer. PHY  302  may be, for example, a 100-Base-T PHY for VDSL over a phone line. Accordingly, PHY  302  may use quadrature amplitude modulation (QAM) or DMT to modulate the line. Therefore, an HDLC is required in order to encapsulate the data packets into a format compatible with VDSL for Ethernet. 
         [0023]    The R/S interleaver interleaves the data by spreading out the data bits to reduce the impact of hit errors. Furthermore, the R/S interleaver can perform error correction. Accordingly, the R/S interleaver may cause delays in data output based on the type of data (i.e., video versus data). Rather than buffering the data at the HDLC  306 , the HDLC  306  may be integrated into the MAC  308  as shown in  FIG. 4 . Alternatively, the network device may be configured so that the HDLC  306  is not integrated within and the MAC  308 . 
         [0024]      FIG. 4  is a block diagram of a MAC according to an embodiment of the present invention. MAC  400  includes a buffer  402 , an HDLC  306  and a First-in-First-out register (FIFO)  308 . Buffer  402  may be a common or shared memory pool and is configured to buffer data packets during switching or routing. MAC  400  performs well-known switching functions and may be coupled with other subsystems of the switch, such as an ARL, a CPU, etc. There may be one MAC per PHY, or one MAC for many PHYs in a device. As data packets are routed to a PHY (or port), data packets are first encapsulated by HDLC  306 . That is, an HDLC header and footer may be added to the packet, bit stuffing may be performed and other steps are performed to encapsulate the data packet in the format for high-level data link control. Once encapsulated, the data packet may be delivered to the PHY (not shown). 
         [0025]    FIFO  308  is provided between the HDLC  306  and the output of the MAC. The FIFO  308  receives two input clocks, a first input clock (internal clock) for controlling input into the FIFO  308  and a second input clock (external clock) for controlling output from the FIFO  308 . The first input clock (internal clock) may be a high-speed clock which is used to establish the rate at which the packets are transferred out of buffer  402  into FIFO  308 . The second input clock (external clock) connects to each port to control the rate of forwarding of the packets out of FIFO  308  to the PHY. In order to match the speed of the output from the HDLC  306  with the PHY the input clock (internal clock) is controlled to match the speed of the output clock (external clock). For example, VDSL line speeds may vary from 1 megabyte per second (Mbps) to 25 Mbps. Assuming that the internal clock speed of a switch chip is at least 166 megahertz, the internal clock of the MAC  400  is assumed to be much faster than the external clock. Therefore, data will always be accepted into FIFO  308  faster than data can be output from FIFO  308 . Instead of buffering the data as is commonly done, the internal clock input into FIFO  308  is controlled to match the speed of the line. Therefore, a clock control unit  310  is added in line with the internal clock to the first clock input of FIFO  308 . The clock control unit  310  may include a watermark device, a state control device, or other means for regulating the internal clock input to the first clock input of FIFO  308 . It is preferred that the clock control unit operates as fast as possible in order to improve quality of service. 
         [0026]    Based on the FIFO size, as FIFO  308  fills up to a predetermined level, the clock control unit  310  may be configured to stop the internal clock signal from being input into the first clock input of FIFO  308 . However, the external clock signal is still input into the second clock input of FIFO  308  such that the output from the MAC continues at the rate of the external clock signal. Namely, MAC  400  may be configured so that, when the FIFO reaches a predetermined level (threshold), clock control unit  310  may substitute the internal clock signal with the external clock signal. Thus, the external clock signal will take over the input rate of the packets into FIFO  308  so that the input speed of the packets into FIFO  308  will match the line speed. In this mode of operation, the external clock signal will simultaneously control both the input rate into and the output rate out of FIFO  308 . The external clock signal may be taken directly off the board (PCB), from the PHY, or from the line in order to match the output speed of MAC  400  with the line speed exactly, or it may be desired to match the external clock with the output speed of a R/S interleaver (or the MII layer). When the packets within FIFO  308  falls below a predetermined level, the clock control unit  310  will switch control of the input rate from the external clock back to the internal clock so that the internal clock will resume control of the input of the packets to FIFO  308 . 
         [0027]    Thus, by substituting the input clock into FIFO  308  with the external clock, there is no need to provide a separate buffer in the MAC or in the HDLC  306  because reducing the rate of input into FIFO  308  to match line speed will prevent an overflow of the FIFO. Since the HDLC (or any parts of the MII layer before the R/S interleaver or before a delay causing subsystem) may be integrated into the MAC, the internal clock may be used to drive the data rather than the clock of the line. Accordingly, data may be driven faster than the clock of the line. However, if the internal clock signal is substituted with the external clock signal, the data will be outputted at the speed of the clock of the line (external clock signal), and the speed will be matched perfectly. 
         [0028]    One having ordinary skill in the art will readily understand that the external clock may be set to any speed, and may also be variable. Therefore, by dynamically starting and stopping (i.e., substituting or swapping out) the internal clock to FIFO  308 , the output of the MAC  400  may be matched with the line speed or any desired data rate. 
         [0029]      FIG. 5  is a flowchart of a method for rate adaptation according to an embodiment of the present invention. At Step S 5 - 1 , within a network device such as a switch, certain devices or subsystems may delay the output of data. For example, as described above within a DSLAM switch for a VDSL, there may be a delay between an HDLC and the PHY. There may be a speed mismatch between an HDLC and the R/S interleaver. According to the present invention, the HDLC may be incorporated into the MAC. Next, at step S 5 - 2 , a FIFO is provided at the output of the MAC having two separate input clocks: a first input clock for controlling data packets input into the FIFO and a second input clock for controlling the output from the FIFO. The first input clock may receive the internal clock, while the second input clock may receive an external clock. The external clock may be matched with the line or matched with the subsystems of the device (e.g., R/S interleaver), as desired. At Step S 5 - 3 , the internal clock or the first input clock is controlled in order to control input into the FIFO. Based on the FIFO size, the first input clock may be turned ON and OFF so that the FIFO does not overflow and so there are no packet drops. As described above, a clock control device such as a state control device, a watermark device, etc., may be placed in line with the first input clock signal to control the clock. The clock control unit may control the first input clock signal based on FIFO size and FIFO utilization. Accordingly, the clock control unit may be connected to the FIFO to receive this information. Alternatively or conjunctively, the clock control unit may be configured to directly receive the external clock signal. Thus, by the above configuration, the output speed of a network device may be matched with a line, or internal subsystems of the device may be matched with external subsystems, such as the transceivers or PHYs. Processing ends at step S 5 - 4 . 
         [0030]    One having ordinary skill in the art will understand that a network device may be configured to perform the above-described method either in silicon, in discrete components, or in software. Accordingly, one will understand that the switching configurations described herein are merely exemplary. Although the invention has been described based upon these preferred embodiments, it would be apparent to those of skilled in the art that certain modifications, variations, and alternative constructions would be apparent, while remaining within the spirit and scope of the invention. For example, any network device may implement the configuration of the present invention. In order to determine the metes and bounds of the invention, therefore, reference should be made to the appended claims.