Abstract:
A network device includes a group of high speed redundant transmission lines and a switch. The switch is configured to select one of the high speed redundant transmission lines. The switch causes reflections and frequency dependent dispersions in the selected high speed redundant transmission line. The network device further includes a transmitting device that is configured to adjust signals transmitted over the selected high speed redundant transmission line so as to reduce the reflections and frequency dependent dispersions.

Description:
RELATED APPLICATION 
       [0001]    The present application is a Continuation-in-Part of U.S. patent application Ser. No. 09/521,281, filed Mar. 7, 2000, the contents of which are incorporated by reference in their entirety herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to data transfer, and more particularly, to systems and methods for reducing distortions in data transferred over high speed links. 
         [0004]    2. Description of Related Art 
         [0005]    A communications bus can be used to couple electrical components in a network device. Optimally, the communications bus should be transparent to the components that it interconnects. A source synchronous communications bus can be used to couple a transmitting component to one or more receiving components. In a source synchronous communications link, the transmitting component provides a source clock signal that can be used by the receiving component to synchronize the reading of data from the communications link. 
         [0006]    When the network device is to be used in mission critical environments (i.e., environments where the continuous operability of the network device is critical), redundancy may be built into the network device. Previous redundant source synchronous links that use switches for redundancy typically maintain controlled lengths between the transmitting component and the switch and between the switch and the receiving component to compensate for the effects of voltage standing waves that occur from reflections caused by the switch. Such redundant source synchronous links are limited to medium speed operation (e.g., 250 megabits per second). These redundant source synchronous link designs are inadequate for operations in the 1 gigabit per second (or greater) range. 
         [0007]    Accordingly, it is desirable to improve high speed signal transmissions in a network device. 
       SUMMARY OF THE INVENTION 
       [0008]    Systems and methods consistent with the principles of the invention address this and other needs by providing a network device that uses pre-emphasis to compensate for signal distortions caused by the implementation of a redundant field effect transmitter (FET) switch in a high speed channel. 
         [0009]    One aspect consistent with principles of the invention is directed to a method for performing pre-emphasis in a channel that includes high speed redundant links. The method includes characterizing the channel in the time domain to identify impedance discontinuities, characterizing the channel in the frequency domain to identify loss due to frequency dependent attenuations and dispersions, and performing pre-emphasis to compensate for the identified impedance discontinuities and frequency dependent attenuations and dispersions. 
         [0010]    A second aspect consistent with principles of the invention is directed to a system that includes a receiving device, redundant drivers that are configured to transmit signals to the receiving device, and a switch. The switch is connected to the receiving device and the redundant drivers via high speed links and is configured to transmit signals from one of the redundant drivers based on a control signal. The switch causes distortions in the high speed links. Each of the redundant drivers is configured to compensate for the distortions caused by the switch. 
         [0011]    A third aspect consistent with principles of the invention is directed to a network device. The network device includes a group of high speed redundant links and means for switching between the high speed redundant links. The means for switching causes distortions to signals transmitted over the high speed redundant links. The network device further includes means for compensating for the distortions prior to the signals being transmitted over the high speed redundant links. 
         [0012]    A fourth aspect consistent with the principles of the invention is directed to a network device that includes a group of high speed redundant transmission lines and a switch that is configured to select a high speed redundant transmission line from the group of high speed redundant transmission lines. The switch causes reflections and frequency dependent dispersions in the selected high speed transmission line. The network device further includes a transmitting device that is configured to adjust signals transmitted over the selected high speed transmission line so as to reduce the reflections and frequency dependent dispersions. 
         [0013]    A fifth aspect consistent with the principles of the invention is directed to a network device that includes a group of high speed, source synchronous buses, and a switch. The switch is configured to select one of the high speed, source synchronous buses and that switch causes reflections and frequency dependent dispersions in the selected high speed, source synchronous bus. The network device further includes a driver that is connected to the selected high speed, source synchronous bus and configured to transmit signals to a receiving device over the selected high speed, source synchronous bus. The driver is further configured to adjust the signals prior to transmission to compensate for the reflections and frequency dependent dispersions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an embodiment of the invention and, together with the description, explain the invention. In the drawings, 
           [0015]      FIG. 1  is a block diagram illustrating an exemplary routing system in which systems and methods consistent with the principles of the invention may be implemented; 
           [0016]      FIG. 2  is a detailed block diagram illustrating portions of the routing system of  FIG. 1 ; 
           [0017]      FIG. 3  is a block diagram illustrating an exemplary configuration of a channel connecting a pair of input/output interfaces (I/Os) to a packet processor in an implementation consistent with the principles of the invention; 
           [0018]      FIG. 4  is a diagram illustrating an exemplary configuration of a driver with two finite impulse response (FIR) filter taps in an implementation consistent with the principles of the invention; 
           [0019]      FIG. 5  illustrates an exemplary process for providing equalization in a redundant system in an implementation consistent with the principles of the invention; 
           [0020]      FIG. 6  is an exemplary graph of impedance versus time for the channel of  FIG. 3  in an implementation consistent with the principles of the invention; 
           [0021]      FIG. 7  is an exemplary graph of the scattering parameter S 21  versus frequency for the channel of  FIG. 3  in an implementation consistent with the principles of the invention; and 
           [0022]      FIG. 8  is an exemplary channel response in an implementation consistent with the principles of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    The following detailed description of the invention refers to the accompanying drawings. The same reference numbers in different drawings may identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims and equivalents. 
         [0024]    As described herein, a network device uses pre-emphasis to compensate for signal distortions caused by the implementation of a redundant FET switch in a high speed channel. 
       System Configuration 
       [0025]      FIG. 1  is a block diagram illustrating an exemplary routing system  100  in which systems and methods consistent with the principles of the invention may be implemented. System  100  receives one or more packet streams from a physical link, processes the packet stream(s) to determine destination information, and transmits the packet stream(s) out on a link in accordance with the destination information. System  100  may include a routing engine (RE)  110 , packet forwarding engines (PFEs)  120 A,  120 B, . . . ,  120 N (referred to collectively as “PFEs  120 ”), and a switch fabric  130 . 
         [0026]    RE  110  performs high level management functions for system  100 . For example, RE  110  maintains the connectivity and manages information and data necessary for performing routing by system  100 . RE  110  creates routing tables based on network topology information, creates forwarding tables based on the routing tables, and forwards the forwarding tables to PFEs  120 . PFEs  120  use the forwarding tables to perform route lookup for incoming packets and perform the forwarding functions for system  100 . RE  110  also performs other general control and monitoring functions for system  100 . 
         [0027]    PFEs  120  are each connected to RE  110  and switch fabric  130 . PFEs  120  receive packet data on physical links connected to a network, such as a wide area network (WAN) or a local area network (LAN). Each physical link could be one of many types of transport media, such as optical fiber or Ethernet cable. The data on the physical link is formatted according to one of several protocols, such as the synchronous optical network (SONET) standard, an asynchronous transfer mode (ATM) technology, or Ethernet. 
         [0028]    PFEs  120  may process incoming packet data prior to transmitting the data to another PFE or the network. PFEs  120  may also perform route lookup for the data using the forwarding table from RE  110  to determine destination information. If the destination indicates that the data should be sent out on a physical link connected to one of PFEs  120 , then the PFE prepares the data for transmission by, for example, adding any necessary headers, and transmits the data from the port associated with the physical link. If the destination indicates that the data should be sent to another PFE via switch fabric  130 , then PFE  120  prepares the data for transmission to the other PFE, if necessary, and sends the data to the other PFE via switch fabric  130 . 
         [0029]      FIG. 2  is a detailed block diagram illustrating portions of routing system  100 . PFEs  120  connect to one another through switch fabric  130 . Each of PFEs  120  may include one or more packet processors  210  and pairs of redundant input/output (I/O) interfaces  220 A- 220 D. Although  FIG. 2  shows two pairs of redundant I/Os  220 A and  220 B and I/Os  220 C and  220 D connected to each of packet processors  210  and three packet processors  210  connected to switch fabric  130 , in other embodiments consistent with principles of the invention there can be more or fewer I/Os  220  and packet processors  210 . 
         [0030]    Each of packet processors  210  performs routing functions and handles packet transfers to and from I/Os  220 A- 220 D and switch fabric  130 . For each packet it handles, packet processor  210  performs the previously-discussed route lookup function and may perform other processing-related functions. 
         [0031]    I/Os  220 A- 220 D may transmit data between a physical link and packet processor  210 . In one implementation, each of I/Os  220 A- 220 D may be a line card. Different pairs of I/Os may be designed to handle different types of network links. For example, one pair of I/Os may be an interface for an optical link while another pair of I/Os may be an interface for an Ethernet link, implementing any of a number of well-known protocols. Each pair of I/Os provides redundancy in the event of failure of one of the I/Os in the pair. That is, if I/O  220 A fails, for example, packet processor  210  may transmit data to the link via I/O  220 B. The channel connecting a pair of I/Os and packet processor  210  will now be described with respect to  FIG. 3 . 
         [0032]      FIG. 3  is a block diagram illustrating an exemplary configuration of a channel  310  connecting a pair of I/Os (e.g., I/Os  220 A and  220 B) to packet processor  210  in an implementation consistent with the principles of the invention. While the foregoing description focuses on transmitting signals from I/Os  220 A and  220 B to packet processor  210 , it will be appreciated that the techniques described herein are equally applicable to the transmission of signals from packet processor  210  to I/Os  220 A and  220 B. Moreover, the links connecting I/Os  220 A and  220 B to packet processor  210  may be bi-directional. As such, I/Os  220 A and  220 B and packet processor  210  may be configured to send and receive signals. 
         [0033]    As illustrated, each of I/Os  220 A and  220 B may include a driver  320  and  330 , respectively, for transmitting signals to a receiver  340  of packet processor  210 . Each driver  320  and  330  may include a digital finite response filter (FIR) that compensates for intersymbol interference (ISI) jitter and reflections in the transmission line connecting I/Os  220 A and  220 B to receiver  340 . 
         [0034]    In one implementation, channel  310  may be a high speed (e.g., 1 gigabit per second or greater), source synchronous channel. Channel  310  may include a switch  350  that acts to selectively transfer signals from one of drivers  320  and  330  based on a control signal received at switch  350 . In one implementation, switch  350  may include two re-channel field effect transistors (FETs)  352  and  356 . In an alternative configuration, switch  350  may include other electronic, mechanical, and/or optical switch configurations and types. The output of driver  320  connects, via a transmission line, to the drain of FET  352 , while the output of driver  330  connects to the drain of FET  356  via a different transmission line. The source of each of FETs  352  and  356  connects to receiver  340  via a single transmission line. 
         [0035]    The gates of FETs  352  and  356  may be coupled to control signals  354  and  358 , respectively. In one implementation, a single control signal may be provided to the gates of FETs  352  and  356 . The single control signal may be inverted for one of the gates, or alternatively, the FET pair can be configured to operate at different bias levels (e.g., one FET operating at a high level and the other FET operating at a low level). In operation, switch  350  selectively transfers signals from one of drivers  320  and  330  to receiving device  340  based on the control signal applied to the gates of FETs  352  and  356 . In this way, if the primary driver (e.g., driver  320 ) fails, switch  350  can switch transmissions from failed driver  320  to backup driver  330 . 
         [0036]    It will be appreciated that FET switches, such as switch  350 , can cause problems when operated at very high speeds (e.g., speeds on the order of 1 gigabit per second) because their associated electrical parasitics (R, L, C) represent impedance discontinuities along the channel and produce reflections on the transmission lines. For example, switch  350  may be associated with a parasitic capacitance that can result in reflection noise in channel  310 . While channel  310  is shown to include a single switch  350 , it will be appreciated that channel  310  may include other devices, such as connectors, vias, etc., that may also produce signal distortions (e.g., reflections) in channel  310 . Moreover, due to the length of the transmission lines in channel  310  and the speed at which signals are transmitted across channel  310 , different symbols (e.g., 1&#39;s and 0&#39;s) may be present at any one time along a transmission line. As the reflections are generated by the switch (and/or other devices) on the transmission line, the symbols interact with each other to produce timing uncertainties, which are commonly known as ISI jitter. 
         [0037]    As will be described in additional detail below, the impedance discontinuities in channel  310  can be characterized in the time domain using conventional techniques. In one implementation, a time domain reflectometer  360  may be used to characterize channel  310  in the time domain. A vector network analyzer  370  may also be used to characterize channel  310  in the frequency domain. This characterization illustrates the effects of the ISI jitter on channel  310 . 
         [0038]    To reduce the effects of ISI jitter and reflections, drivers  320  and  330  may include fractional or symbol-spaced FIR filters. In an alternative implementation, drivers  320  and  330  may include infinite impulse response (IIR) filters. The FIR filters provide pre-emphasis that acts to suppress pre-cursor and/or post-cursor symbols in channel  310 .  FIG. 4  is a diagram illustrating an exemplary configuration of a driver  400 , which may be driver  320  or  330 , with two FIR filter taps in an implementation consistent with the principles of the invention. 
         [0039]    As illustrated, a main symbol portion of driver  400  may include a pair of re-channel FETs  410  and  420  that is connected in parallel to a voltage source VDD via resistors R. In one implementation, each resistor R may be 50Ω. The gate of FET  410  connects to a positively biased input (IN(+)) via a delay device  460  and the gate of FET  420  connects to a negatively biased input (IN(−)) via delay device  460 . Delay devices  460  may cause a one symbol bit delay or fractional symbol bit delay. A current source I  430  for the main symbol may connect to the drain of FETs  410  and  420 . 
         [0040]    The post-cursor FIR filter tap portion of driver  400  includes an n-channel FET pair  440 . The source of a first FET  442  of FET pair  440  connects to the negative output of the main symbol portion of driver  400 . The source of a second FET  444  of FET pair  440  connects to the positive output of the main symbol portion. The drains of FETs  442  and  444  connect to a current source  450 . The current from current source  450  is equivalent to the current in main symbol current source  430  multiplied by a FIR filter coefficient (FIR 1 ). As will be described in additional detail below, the filter coefficient may have a coefficient that corresponds to an equalization in the frequency domain that is the inverse of the channel so as to reduce the effects of ISI jitter in channel  310 . The gate of FET  442  connects to a positively biased input (IN(+)) via delay devices  460  and the gate of FET  444  connects to a negatively biased input (IN(−)) via delay devices  460 . Delay devices  460  may cause a one symbol bit delay (or half symbol bit delay) for the post-cursor FIR filter tap. 
         [0041]    The pre-cursor FIR filter tap portion of driver  400  includes an n-channel FET pair  470 . The source of a first FET  472  of FET pair  470  connects to the negative output of the main symbol portion of driver  400 . The source of a second FET  474  of FET pair  470  connects to the positive output of the main symbol portion. The drains of FETs  472  and  474  connect to a current source  480 . The current from current source  480  is equivalent to the current in main symbol current source  430  multiplied by a FIR filter coefficient (FIR 0 ). The gate of FET  472  connects to a positively biased input (IN(+)) and the gate of FET  474  connects to a negatively biased input (IN(−)). 
         [0042]    The configuration illustrated in  FIG. 4  is provided for explanatory purposes only. One skilled in the art will appreciate that other numbers of pre-cursor and post-cursor taps may be used based on channel characteristics. For example, in a 7 tap implementation, driver  400  may include 1 pre-cursor tap, a main symbol, and 5 post-cursor taps. Each post-cursor tap may be associated with a different post-cursor value (e.g., I*FIR 1 , I*FIR 2 , . . . , I*FIR 5 ). 
       Exemplary Processing 
       [0043]    To provide hardware redundancy for high speed links, such as source synchronous buses, designers often implement arrays of switches, such as switch  350 , to switch between the primary and backup links. As described above, these switches  350  produce signal reflection noise along the transmission lines connecting to switches  350 . The reflection noise creates voltage overshooting, undershooting, and ringing on fast edge rate signals, which are necessary for high speed links. The magnitude of the reflection noise may be significant to produce large timing uncertainties. This resultant timing uncertainty, when added to other timing uncertainties that may be present in the high speed link, such as from intersymbol interference and other noise sources, prohibits high speed operation, thus limiting the maximum speed at which the redundant high speed links can function. 
         [0044]    Implementations consistent with the principles of the invention use digital signal processing techniques to provide equalization to cancel out reflections that are created from transistor switches and reduce ISI jitter in high speed links. Equalization may be provided at the driver. As described above, the driver may be a digital output driver with a current source n-tapped FIR filter that performs the necessary equalization. The FIR filter taps may be spaced at either integer or fractional symbol bit times and can have one or more pre-cursors, which may be either integer or fractional symbol spaced. 
         [0045]      FIG. 5  illustrates an exemplary process for providing equalization in a redundant system in an implementation consistent with the principles of the invention. Processing may begin by characterizing the channel, such as channel  310 , in the time domain (act  510 ). In one implementation, time domain reflectometer  360  may be used to characterize channel  310  in the time domain. Each FET  352  and  356  represents a capacitive discontinuity over the transmission line connected thereto. Therefore, in the time domain, each FET  352  and  356  is associated with a certain impedance change (Δz) and time duration (Δd). Drivers  320  and  330  use digital signal processing techniques to compensate for these discontinuities. 
         [0046]      FIG. 6  is an exemplary graph of impedance (z) versus time for channel  310  in an implementation consistent with the principles of the invention. In  FIG. 6 , a solid line  610  represents the measured (actual) characterization of channel  310  and a dotted line  620  represents the ideal channel characterization. The ideal channel characterization is a flat line (i.e., the impedance does not vary over time). FET  352 , for example, may be associated with a capacitive discontinuity  630  that may be determined, as set forth above, via the use of time domain reflectometer  360 . The change in impedance (Δz) and time duration (Δd) of capacitive discontinuity  630  may be measured. 
         [0047]    Channel  310  may be characterized in the frequency domain (act  520 ). In one implementation, vector network analyzer  370  may be used to determine the loss or attenuation of the signal over different frequencies. Intersymbol interference is caused when different amounts of attenuation for different frequencies are present in the signal. Vector network analyzer  370  is able to measure ISI jitter in a channel, such as channel  310 . 
         [0048]      FIG. 7  illustrates an exemplary graph of the scattering (s) parameter S 21  versus frequency for channel  310  in an implementation consistent with the principles of the invention. In  FIG. 7 , a solid line  710  represents the measured (or actual) characterization for channel  310  in the frequency domain and a dotted line  720  represents the ideal channel characterization in the frequency domain. Flat line  720  represents no signal loss due to the channel. As shown in  FIG. 7 , the s parameter S 21  of channel  310  decreases at higher frequencies. For example, at 1 gigahertz (GHz), channel  310  experiences a loss of approximately 5 decibels (dB). At 5 GHz, channel  310  experiences a loss of approximately 20 dB. To compensate for this loss and make channel  310  closer to ideal  720  in the frequency domain, the inverse of actual channel characterization  710  may be determined. This inverse is depicted in  FIG. 7  as line  730 . It will be appreciated that the sum of actual channel characterization  710  and inverse channel characterization  730  produces ideal channel characterization  720 . 
         [0049]    Once channel  310  has been characterized in the time and frequency domains, digital signal processing techniques can be used to compensate for the loss (impedance discontinuities and ISI jitter) in channel  310  (act  530 ). In one implementation, current source n-tapped FIR filters are used in the drivers (i.e., drivers  320  and  330 ) to compensate for these channel effects. The FIR filter taps may be spaced at either integer or fractional symbol bit times and can have one or more pre-cursors, which can be either integer or fractional symbol spaced. 
         [0050]    The FIR filter coefficients may be determined based on the characteristics of channel  310 . The FIR filter coefficients have an equalization in the frequency domain that is the inverse of the channel (line  730  in  FIG. 7 ). Therefore, when multiplied in the frequency domain, ideal channel characterization  720  is obtained. Moreover, when the convolution of the FIR filter coefficients is taken in the time domain, ideal channel characterization  620  is obtained. 
         [0051]      FIG. 8  is an exemplary channel response in an implementation consistent with the principles of the invention. Line  810  in  FIG. 8  represents the channel response for channel  310 , which may be obtained via an Inverse Fast Fourier Transform operation. Line  820  represents the ideal channel response (i.e., the channel response if channel  310  was lossless). Channel response  810  includes a main symbol portion (represented by a “1”), and post-cursor and pre-cursor portions which contain residue from the main symbol. The symbols in the post-cursor and pre-cursor portions are represented by a “0” in  FIG. 8 . The FIR filter taps may be integer symbol spaced (e.g., spaced 1 nanosecond apart) or fractional symbol spaced to suppress the energy in those adjacent symbols following the main symbol (post-cursor) and those preceding the main symbol (pre-cursor). 
         [0052]    For power and chip area and electrical parasitics reasons, large numbers of FIR filter taps are undesirable to implement. To prevent the need for excessively large numbers of FIR filter taps, two techniques may be implemented. In a first technique, the transmission line lengths between drivers  320  and  330  and switch  350  and between switch  350  and receiver  340  are constrained so that the channel characteristics are bounded and deterministic. For example, in the exemplary configuration illustrated in  FIG. 3 , the transmission line lengths between each driver  320  and  330  and switch  350  and the transmission line lengths between switch  350  and receiver  340  may be constrained and controlled to make channel  310  deterministic. Knowing the channel characteristics enables an optimal design of the digital FIR filter, and its corresponding number of taps and tap coefficients. 
         [0053]    In a second technique, the FIR filter taps may be moved simultaneously by integer multiples of the symbol bit delay times. By positioning the grouped FIR filter taps at the bit time impulse response time aberrations based on the known channel characteristics, the number of optimal FIR filter taps can be dramatically reduced (e.g., by one quarter). 
       CONCLUSION 
       [0054]    Systems and methods consistent with the principles of the invention provide equalization to compensate for reflections and frequency dependent dispersions (i.e., ISI jitter) in redundant, high speed links. Exemplary implementations perform pre-emphasis to greatly reduce signal distortions in the high speed links caused by the presence of one or more FET switches in the high speed links. 
         [0055]    The foregoing description of preferred embodiments of the present invention provides illustration and description, but is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. For example, while the above description described the high speed links as being source synchronous, implementations consistent with the principles of the invention are equally applicable to non-source synchronous links (e.g., asynchronous links). 
         [0056]    Moreover, while a series of acts was described with respect to  FIG. 5 , the order of the acts may differ in other implementations consistent with the principles of the invention. Also, non-dependent acts may be performed in parallel. 
         [0057]    No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items. Where only one item is intended, the term “one” or similar language is used. 
         [0058]    The scope of the invention is defined by the claims and their equivalents.