Abstract:
A method and an apparatus using a system level clocking scheme to remove jitter from multi-media packets distributed over an asynchronous network, in particular an asynchronous network. The present invention overcomes the problems associated with jitter introduced in an asynchronous network by using various time stamps to synchronize a client device clock to a headend clock and to control the data flow in the client device to match the rate that the data is received by a broadband receiver coupled to the headend. A first time stamp is prepended to the transport packets when the packets are received from the headend. A second time stamp is placed in the data frame when the data frame is placed on the network. A third time stamp is placed in the data frame when the data frame is received from the network. The second and third time stamps are used for synchronizing the client clock to the server clock, which is in turn frequency locked to the headend clock. The first time stamp is used for data flow control wherein the client controls the data flow to correspond to the rate the data is received at the server.

Description:
This application claims the benefit under 35 U.S.C. § 365 of International Application PCT/US01/20845, filed Jun. 29, 2001, which was published in accordance with PCT Article 21(2) on Jan. 31, 2002 in English; and which claims benefit of U.S. provisional application Ser. No. 60/219,766 filed Jul. 20, 2000. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an asynchronous communications network, and in particular, to an asynchronous multi-media digital home network having a system level clocking scheme to remove jitter from multi-media data packets. 
   2. Background Information 
   Recent advances in digital communications technology have increased the viability of digital home networks (“DHN”), which can allow various devices in a home to communicate with each other. In particular, the growing availability of broadband access has made it desirable to be able to tie in various devices in a home to a single gateway device that is coupled to the broadband access for centralized access and distribution of multi-media information. In such a digital home network, the gateway device accesses and buffers multi-media content and distributes the content over the network as requested by various client devices coupled to the network. 
   Ethernet is a common option for implementing a LAN. Ethernet is an asynchronous collision detect network. HPNA is also an asynchronous collision detect network and some power line based networks are often asynchronous collision detect networks. However, difficulties may arise when such networks are used to distribute multi-media content. 
     FIG. 1  is a system diagram illustrating a multimedia network. System  100  comprises a plurality of devices  108 - 113  coupled to media server  103  via one of a plurality of communications networks  105 - 107 . In system  100 , media server  103  receives multi-media content via satellite dish  102 , buffers the content, and distributes the content as requested from the various devices coupled to the network. Media server  103  may also be coupled to receive multi-media content via any one of plurality of known digital media access methods, including, cable, and DSL. The transfer of data over the selected network is generally achieved by using large buffers or by allowing a client device to throttle data. 
   In this regard, there are a number of problems associated with taking a live broadcast and distributing it on an asynchronous digital home network. In a live broadcast signal, the data is pushed to the client device, and the client device does not have any control over the incoming data stream. In order to ensure that the networked client device can decode and display all of the audio and video data delivered to it at the correct frame rate, and without repeated or dropped frames, it is in general required that the networked client device be clock synchronized to the network gateway device. The most widely accepted method for clock synchronization in a broadcast A/V network relies on delivering counter samples taken at the broadcast site to the receiver with a fixed delay from the time of broadcast till the time of receipt. The counter at the broadcast site is incremented by the broadcast site&#39;s reference clock. At the instant those clock samples arrive in the receiver, a counter clocked by a voltage controlled oscillator (VCXO) is sampled. The value of the counter and the time stamp received from the broadcast site are compared and if the difference between the samples of the local counter and the clock samples taken at the broadcast site varies over time, the voltage applied to the local VCXO is adjusted to try to frequency lock the local clock to the broadcast site&#39;s reference clock. 
   On an asynchronous collision detect network, such as an Ethernet network, the delay from the time a packet is constructed and placed in a transmit buffer until the time the packet is received by a network connected device is not always constant. In such networks, multiple devices colliding on a network can cause variable packet and unpredictable delays in the reception of a packet. In this environment it is not possible to use the method described above to frequency lock the clock at the networked receiver to a clock at the networked transmitter due to the variable and unpredictable delays. Using such a method to attempt to lock the receiver clock to the transmitter clock may produce significant jitter in the receiver clock. Depending upon the magnitude of the jitter in the receiver&#39;s clock, it may not be possible to produce an NTSC compliant color burst for video display. In addition, this jitter will at a minimum require the audio and video compressed data buffers to be larger than necessary and at worst cause the buffers to underflow or overflow. The side effects would include video freezing, possible audio chirps and the display colors changing dynamically due to the variations in the color burst. 
   Some networks, such as 1394 networks, provide isochronous capability that can eliminate this problem. Additionally, various methods, such as SRTS, have also been developed for ATM networks to address these timing issues. The SRTS method assumes that the transmitting device and the receiving device both have access to a network clock for the synchronization. However, such methods may not be completely suitable for correcting jitter in an asynchronous collision detect network, such as an Ethernet network. Therefore, what is desired is a system and a method for synchronizing the clocks and controlling the data flow in an asynchronous network, and in particular, in an asynchronous collision detect network to overcome the above-noted problems. 
   SUMMARY OF THE INVENTION 
   The present invention overcomes the jitter problems discussed above, and in particular, provides a system and a method for distributing multi-media content over an asynchronous network. More specifically, the present invention uses time stamps to minimize and reduce the jitter effects caused by software processing and collisions on an asynchronous home network. Time stamps are used to perform synchronization of the clock in the client device to the server clock and also to control data flow in the client device. 
   One set of time stamps is added for flow control. When a packet arrives from a broadband network, a sample of a local counter in the server is latched into a register. The server attaches the sampled counter value to the received transport packets. The local counter is clocked with a system clock that has been frequency locked to the system clock at the headend. Once a packet is time stamped, the server may place the packet in a transmit buffer. This time stamp indicates the moment at which the data was received from the broadband connection and placed into a buffer, and as such, can be used by the client device to remove the data from a receive buffer at the exact same rate as it entered the buffer. 
   Another set of time stamps is used for synchronizing the client device clock with the head end clock. At the moment a frame of data is being placed onto the network, a local counter running on the system clock is sampled. A time stamp based on the sampled clock is placed in the data frame in real-time at the physical layer of the network so that there is a fixed and constant amount of time from the sampling of the local counter and the sampled count value being placed onto the network. If a collision occurs, the time stamp is updated when the data is retransmitted. At the moment the new data frame is received in the physical layer of a client device, a sample of a local counter running on the client&#39;s local clock is latched into a register. This “receive-time” time stamp is then attached to the received frame of data and passed onto successive layers of the client&#39;s network protocol hardware and software. After the client data is passed through the communication protocols, the data is ready to be used by the client. 
   The client device uses the time stamps attached at the physical layer of the server&#39;s network interface, along with the time stamps that were attached when the data was received by the client device to lock the client&#39;s local system clock to the server&#39;s local system clock. Since the server&#39;s local system clock is frequency locked to the headend system clock, the client system clock will also be frequency locked to the headend system clock. Frequent clock samples from the server will allow the client&#39;s system clock to very closely track the server&#39;s system clock. 
   The time stamps attached to the data when the data arrived at the server from the broadband network may then be used to determine the exact time when each data packet should be extracted from the client&#39;s buffer and passed on to the audio/video subsystems in the client. In this way the packet arrival timing at the server is exactly matched at the input to the A/V subsystem in the client device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described with reference to the drawings, wherein: 
       FIG. 1  shows an asynchronous multi-media network including a media server coupled to a plurality of client devices; 
       FIG. 2  is a block diagram illustrating the elements of an asynchronous multi-media network including time stamps placed into the data at various points of the network according to the present invention; 
       FIG. 3  shows format of the data frame including the time stamps utilized in the asynchronous multi-media network according to the present invention; 
       FIG. 4  is a block diagram illustrating the element of a broadband receiver adapted to receive satellite signals; 
       FIG. 5  is a block diagram illustrating the elements of the PCI DBS tuner board illustrated in  FIG. 4 ; 
       FIG. 6  is a block diagram illustrating the elements of the client device illustrated in  FIG. 2 ; 
       FIG. 7  is a block diagram illustrating the elements of the transport formatter board illustrated in  FIG. 6 ; 
       FIG. 8  is a block diagram illustrating the elements of the client video decoder illustrated in  FIG. 6 ; 
       FIG. 9  is a block diagram illustrating the time stamping board used in the asynchronous multi-media network according to the present invention; 
       FIG. 10  is a block diagram illustrating the time stamp controller in the time stamping board shown in  FIG. 9 ; 
       FIG. 11  is a block diagram illustrating the receiver unit in the time stamp controller shown in  FIG. 10 ; and 
       FIG. 12  is a block diagram illustrating the transmitting unit in the time stamp controller shown in  FIG. 10 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2  illustrates a digital home network utilizing the time stamps according to the present invention. The broadband input signal is provided by a headend (not shown) and received by broadband receiver/multi-media server  202 . Server  202  may be configured to receive the input from one of a plurality of broadband sources, such as, satellite, cable or DSL, as indicated by input Sys  1 , Sys  2  . . . Sys n. This input signal may be in the form of transport packets, which include System Clock Recovery (SCRs) information used by receiver  202  to frequency lock its local system clock to the head end system clock. The SCRs are used to mange buffer levels and to derive video timing and color burst signals in an integrated receiver decoder (IRD). It is important to note that the SCR information and packet timing relationships must be maintained throughout system  200 , from entry into the network to the point of final presentation, in order for system  200  to operate properly. 
   Data received by server  202  from the broadband network is transport packetized. Some of these transport packets will carry samples from a counter at the head end that is clocked by the head end&#39;s system clock. Using these counter samples and the arrival time of the transport packets that carry these samples, server  202  is able to frequency lock its own local system clock to the head end system clock. 
   In accordance with the present invention, in order to ensure that the exact arrival time of packets at the client device may be reproduced, each packet arriving from the broadband network is stamped with a time stamp, T 1 , when server  202  receives the packet. Time stamp T 1  is a sample of a local counter that is clocked with a local system clock of server  202 . Time stamp T 1  and the packet are stored together in buffer  205  prior to being broadcast together over the DHN. 
   At some later point in time, a group of transport packets, along with header information, is assembled to form a data frame for transmission on the network DHN. The moment the data frame starts to be placed onto the network, local counter in server  202 , running on the local system clock, is sampled to generate a counter sample time stamp T 2 . Time stamp T 2  is prepended onto the beginning of the outgoing data frame such that the time from sampling the counter value to the time the counter value enters the network is fixed and constant. It is important that this time is fixed and constant, and for this reason the counter sampling and time stamp placement is performed in the physical layer hardware of the network interface. Since the propagation delay from server  202  to client device  204  over the network is fixed and constant, the overall propagation delay from sampling T 2  until the packet containing time stamp T 2  arrives at client device  204  is also fixed and constant. Additionally, in a network where the symbol times may change, the present invention is able to provide synchronization as long as the propagation times remain constant. 
   At the moment the data frame arrives at client device  204 , a local counter of client device  204 , running on the client system clock, is sampled to generate a time stamp T 3 . This counter value is compared to time stamp T 2  in the arriving data frame to ensure that the difference between the client counter and the server counter is not changing. If this difference is changing over time, the client device will modify the voltage on its local VCXO to move its local system clock into frequency lock with the system clock of server  202 . Thus, time stamps T 2  and T 3  are used to frequency lock the system clocks of server  202  and client device  204  together. Time stamps T 2  and T 3  may be discarded after they are used in client device  204  for synchronization. At this point, data frames that encapsulate transport packets and their associated time stamps T 1  may be stored in buffer  203  in client device  204 . 
   The next step is to remove data from the buffer of client device  204  at a rate that exactly matches the rate at which the packets arrived at the input of server  202 . When client device  204  has accumulated enough data in a receive buffer to absorb a reasonable amount of network jitter client device  204  starts extracting data from the receive buffer, and making that data available to the decoders within client device  204 . A local counter, which is based on the client clock is initialized with the count value stored with the first transport packet to be extracted from memory. The counter is clocked with the client&#39;s system clock, which is locked to the server clock, so the subsequent packets may be extracted from memory when their stored time stamp value matches the counter value. In this manner, the rate of removal from the receive buffer of client device  204  matches the rate at which the data was received at server  202 . The format of the data packet and placement of the time stamps T 1 , T 2  and T 3  within the data frame are shown in  FIG. 3 , wherein the time stamps T 1  are generated as the packetized data is received and time stamps T 2  are placed into the data frames as the frames are placed on the network DHN and time stamp T 3  is generated when the data frame is received from the network DHN. 
   The format of the data packets and the placement of the times stamps into the data frames in an Ethernet environment are further described below. The Ethernet packets in the present embodiment use the UDP/IP protocols, but it is to be understood that other suitable protocols, such as TCP, or “raw” Ethernet may be used to implement the time stamping features of the present invention. In accordance with the present invention, the IP header utilizes an IP option identifying the data frames that need to have a time stamp placed therein. The time stamps are placed in the data frames following the UDP header. There is a unique location in the extended UDP header for the outgoing and incoming packets. 
   The clock synchronization time stamps T 2  and T 3  are placed after the UDP header. Time stamp T 2  is a 32-bit time stamp and is applied when the IP packet has an option  25  set. The time stamp placed into the data frame at the physical network layer. Although the present embodiment utilizes option  25  to indicate the presence of time stamps in the frame, it is to be understood that other suitable method for indicating time stamps, such as designating specific IP address and port number may be utilized. 
   Time stamp T 3  is also a 32-bit incoming time stamp and is applied when an IP packet with option  25  is detected. This time stamp is also placed into the data frame at the physical network layer. Table 1 below shows the placement of time stamps T 2  and T 3  in the UDP data frame. 
                                 TABLE 1               UDP data frame                                    0   15   16       31           16-bit source port number   16-bit destination port               number           16-bit UDP length   16-bit UDP checksum           32-bit time stamp T2           32-bit time stamp T3           Data (if any)                        
Using the time stamps at the physical layer, the present invention can develop an algorithm to synchronize the two independent nodes, in this case server  202  and client device  204 .
 
   The data flow control time stamps T 1  are placed into the data stream when the data is received from a broadband network. Time stamps T 1  are generated using a local counter in server  202  that is incremented by a clock that is frequency locked to the system clock at the headend. By placing a time stamp T 1  on each transport packet as the packet arrives at server  202 , client device  204  can reproduce the same bit rate as the rate received at server  202 . Data flow control time stamps T 1  are placed in the UDP payload. Table 2 shows the data format used in the exemplary embodiment of the present invention. This data is placed in the data portion of the UDP data frame shown in table 1. 
   
     
       
             
           
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
               Data including transport packets and time stamps 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               0   15   16   31 
             
             
                 
               Transport Packet Marker 
             
             
                 
               16-bit MSBs of time stamp T1 (0) 
             
             
                 
               16-bit LSBs of time stamp T1 (0) 
             
             
                 
               Transport Packet 0 
             
             
                 
               Transport Packet Marker 
             
             
                 
               16-bit MSBs of time stamp T1 (1) 
             
             
                 
               16-bit LSBs of time stamp T1 (1) 
             
             
                 
               Transport Packet 1 
             
             
                 
               . 
             
             
                 
               . 
             
             
                 
               . 
             
             
                 
               Transport Packet Marker 
             
             
                 
               16-bit MSBs of time stamp T1 (n) 
             
             
                 
               16-bit LSBs of time stamp T1 (n) 
             
             
                 
               Transport Packet n 
             
             
                 
                 
             
           
        
       
     
   
   Once the data frame has been time stamped, the UDP checksum is checked. If the checksum is not zero, the old checksum is replaced with a newly created checksum. After resolving the UDP checksum, a new CRC is calculated and replaces the old CRC value in the MAC layer. If at any point of the packet interrogation it is determined that the data frame is not a valid time stamp frame, the frame is passed through with no alterations to the data. 
   The time stamp value is generated from an external system clock. The external clock drives a 32-bit counter. A snapshot of the counter is taken when a data frame is placed onto the network. On an incoming packet, a snapshot of a counter in the client device is taken. If it is a valid time stamped packet, the snapshot of the counter is placed into the payload portion of the data frame. The 32-bit counter is a free running rollover counter synchronous to the external system clock. 
   The following describes the search sequence to determine whether a particular data frame needs a time stamp, and if so, where the time stamp data is placed. The first test is to validate the MAC frame. Validating the MAC frame requires the CRC to be calculated and compared. If the CRC test fails, the frame is allowed to pass through without any modifications to ensure that a valid CRC is not added to a data frame that was received with an invalid CRC. If the CRC test is passed, a second test is conducted to find an Internet Protocol (IP) in the MAC frame. Twenty octets into the MAC frame, from the 802.3 specification section 3.2.6, Length/Type, is the Length/Type field. From ITEF RFC 1700 EtherTypes table, an Internet IP (Ipv4) is 0x800 (Hexadecimal). If the value is not 0x800, the packet is passed through. If the Length/Type is 0x800, another test is conducted to determine the size of the IP header and whether it contains an UDP packet. The contents of a MAC frame are specified in IEEE 802.3, section 3.1.1. 
   If the MAC frame contains an IP packet, the next test determines the size and the option settings. First, in order to determine the size, the second nibble of the IP packet, which is the Header Length, is examined. The Header Length is the number of 32-bit words that make up the IP header. Without options, the IP header is normally 20 bytes, so the Header Length is 5. The IP Header options have a 20 decimal offset from the start of the IP Header. 
   The steps to determine whether the packet is a valid time stamp packet is now described. If any of the steps fail, the packet is passed through without any modifications. First, the header length field is examined to determine whether it is greater than 5. If so, the 1 st  IP option located 20 bytes from the beginning of the IP Header is checked against 0x99040000. The format of the option is shown in Table 3 below: 
                                         TABLE 3                   IP Option 25                Class           Parm       Copy   (2   Number   Length   (16       (1 bit)   bits)   (5 bits)   (8 bits)   bits)               1   0   25   4   0                    
Table 4 shows an IP header with IP option  25  included.
 
                                                                                   TABLE 4                   IP Header with IP Option 25                15   16           0   4-bit   8-bit type       4-bit   header   of service   31       version   length   (TOS)   16-bit total length (in bytes)                    16-bit           3-bit   13-bit fragment offset       identification           flag            8-bit time to   8-bit       16-bit header checksum       live (TTL)   protocol            32-bit source IP address       32-bit destination IP address       32-bit option 25       data                    
If the IP options include the designated time stamping option described above, the present system places a time stamp into the data frame. The time stamps are placed in the UDP payload portion that follows the IP header, and in particular, at the beginning of the payload portion. The UDP header is 8 bytes long. The first 32 bit word of the UDP payload is reserved for time stamp T 2 , the outgoing time stamp, and the second 32 bit word of the UDP payload is reserved for T 3 , the incoming time stamp. Table 1 shows the general format of the UDP data frame while table 2 illustrates the specific format chosen for the exemplary embodiment of the present invention.
 
   A significant aspect of the present invention is that the time stamps and the CRC are generated and placed into the data frames at the physical layer. As such, the time stamps and CRC are placed into the data frames as the data frames are actually being placed onto the network or are received from the network. This allows the time stamps to accurately reflect the time the data enters the network and the time the data is received from the network. 
   The elements of a system for implementing the time stamping described above is now described in further detail with respect to a DBS receiver. However, it is to be understood that similar time stamping may be performed for multi-media data received on other types of broadband networks, such as DSL or cable. 
   As shown in  FIG. 4 , server  202  consists of three functional blocks connected by a bus. The satellite signal is received by server  202  through PCI DBS tuner  402 . Host controller  406  reads the transport packets from the satellite signal out of PCI DBS tuner  402  via PCI bus interface  405 . Host controller  406  then processes the data and sends the processed data out to time stamp board  404  via well-known protocols, including UDP. Also, PCI DBS tuner  402  and time stamp board  404  have a common clock between them. The clock is generated on PCI DBS tuner  402 , and used as the time reference for time stamp board  404 . 
   In operation, host controller  406  reads the transport packets out of the FIFO memory of PCI DBS tuner  402 . The packets are then processed according to the network protocol software and the network frames are then written into the Ethernet MAC of time stamp board  404 . The MAC waits for the Ethernet Network to become available, and then sends the data through a time stamp controller to an Ethernet PHY. When a video network packet is detected by the time stamp controller, a time stamp is added to that packet. The time stamp T 2  is a sample of a counter in the time stamp controller that is clocked by a recovered clock source. In server  202 , the clock source for the time stamp controller is a VCXO in PCI DBS tuner  402 . 
     FIG. 5  shows a block diagram of PCI DBS tuner  402 . PCI DBS tuner  402  performs three major functions. First, PCI DBS tuner  402  ensures that the correct satellite transponder is selected, tuned, and demodulated. This is accomplished by satellite tuner  502  and demodulator  504 . These devices perform the well-known conversion of a satellite signal into a digital transport stream. 
   Second, PCI DBS tuner  402  filters the packets of the transport stream. Tuner controller  506  contains logic to filter out only the transport packets requested by host controller  406 . The transport packets are then stored in FIFO buffer  512 . Host controller  506  can then read the packets out of FIFO buffer  512  via PCI bus interface  405 . In order to facilitate the PCI bus interface, a PCI bridge chip  508  is used. 
   Third, PCI DBS tuner  402  performs clock recovery. Tuner controller  506  executes software instructions from Flash and SRAM  514 . A software control loop is created to adjust the frequency of Voltage Controlled Crystal Oscillator (“VCXO”) A counter running from VCXO  510  clock is sampled each time a packet containing a time stamp from the service provider&#39;s head end. Tuner controller  506  compares the normalized error in VCXO  510  clock to the headend clock. This way, the local clock can be adjusted to be the same frequency as the headend clock. 
   Additionally, time stamps T 1  can be added to each transport packet using the recovered system clock from VCXO  510 . Time stamp T 1  is based on a sample of the local counter at the moment the packet is received at tuner  402 . 
   The elements of network client  204  are shown in  FIG. 6 . Data arrives at the client&#39;s time stamp board  602 , and time stamp T 3  is added to the data frame at the moment of arrival. The packets are processed by client controller  608 , which is connected to time stamp board  602  via PCI bus  605 . The transport packets are extracted from the network frames, and are then written to transport formatter board  604 . The output of transport formatter  604  is a serial transport stream that may be used by decoder  606 . The output of decoder  606  is connected to a television or other display device.  FIG. 8  shows the elements of decoder  606 . 
   Client controller  608  is responsible for executing a clock recovery algorithm. The data frame departure times T 2  from server  202  are compared to the arrival times T 3  at client  204 . In this manner, client controller  608  can determine whether decoder VCXO  812  is faster or slower than server VCXO  510 . Client controller  608  sends commands to decoder controller  810  to speed up or slow down decoder VCXO  812  through a low speed serial (RS-232) connection. 
   The time stamp board of the client device is similar to the time stamp board of the server device except that the recovered clock source in the client device is derived from the clock of decoder  606  rather than VCXO  510  of PCI DBS tuner  402 . 
   The elements of transport formatter  604  are shown in  FIG. 7 . Transport formatter  604  is connected to client controller  608  via PCI bus  702 . A PCI bus interface converts the PCI bus to a standard local bus, which is connected to transport formatter controller  704 . Transport formatter controller  704  outputs a serial transport stream at TTL logic levels. High speed serial converter  706  circuit converts those signals to low voltage differential signals, which are passed to decoder  606 . 
   Transport formatter controller  704  has two modes of operation. The first is flow control mode in which transport packets are forwarded to decoder  606  according to the optional time stamps added by tuner controller  506  of PCI DBS tuner  402 . In this manner, the relative transport packet departure times from transport formatter  604  match the transport packet arrival times at tuner controller  506 . This has the effect of making the packet timing at the input of decoder  606  appear to be connected directly to tuner  502  and demodulator  504  of PCI DBS tuner  402 . 
   The second mode of operation allows the data to flow immediately from transport formatter controller  704  to decoder  606 . In this mode, decoder  606  is required to buffer the data internally. This requires decoder  606  to contain more memory than the first mode. 
   Data arrives from transport formatter  604  into high speed serial interface  802  of decoder  606 . Transport processor  804  sorts the incoming transport packets and puts them into the correct buffer (video, audio, program guide, other data, etc.). The video and audio data are decompressed in the video/audio decoder  806 . After that, decompressed digital data is converted to analog NTSC signals via NTSC encoder/audio DAC  808 . 
   The time stamping board used in implementing the present time stamping method is now further described. Although, described with reference to the time stamping board associated with server  202 , as noted above, the time stamping board associated with client  204  is similar. Time stamping board  404  is a PCI Ethernet NIC that can support both 10 Mb/s and 100 Mb/s data rates. It is capable of resetting the UDP check sum and placing 32-bit time stamps in the data frames, as well as recalculating the CRC for the designated data frames. The external interfaces allow board  404  to be integrated into PC PCI or embedded PCI architectures. An external clock interface is provided to generate time stamp values. 
     FIG. 9  illustrates a block diagram of time stamping board  404 . The time stamping board comprises an input jack  904 , for example RJ45, coupled to Ethernet network  902 . Input jack  904  is coupled to 10/100 Base-TX Phy transformer  906 . Transformer  906  is coupled to physical layer device  908 , which is a full feature physical layer device with integrated PMD sublayers to support both 10BASE-T and 100BASE-X Ethernet protocols. Physical layer device  908  is coupled to bus buffer  912 , which translates the voltage levels between physical layer device  908  and time stamp controller  916 . Bus buffer  912  is a low voltage CMOS octal bus buffer. It is ideal for low power and high-speed 3.3V applications and can be interfaced to 5V-signal environment for both inputs and outputs. Bus buffer  912  is coupled to time stamp controller  916 , which controls and performs much of the time stamping functions, including, resetting the UDP check sum, placing the time stamps and recalculating the new CRC for the valid data frames in both the receive and transmit directions. 
   Time stamp controller  916  is coupled to configuration device  918 , which stores and loads the program to operate time stamp controller  916 . Time stamp controller  916  is also coupled to Ethernet MAC unit  914 , which is a LAN controller for both 10 Mb/s and 100 Mb/s data rates. Ethernet MAC  914  provides a direct interface to the peripheral component interconnect (PCI) local bus or the CardBus. 
   Time Stamp controller  916  provides the Mll interfaces to the PHY and the MAC. As shown in  FIG. 10 , controller  916  consists of receiver unit  1002  and transmitter unit  1004 . The two units are implemented separately, but share the same external system clock and the reset signal. 
   As shown in  FIG. 11 , receiver unit  1002  comprises receiver controller  1102 , time stamp generator  1114 , shift register  1112 , verify CRC module  1110 , calculate new CRC module  1106 , 3:1 MUX  1108  and 4:1 MUX  1104 . Receiver controller  1102  detects the status of data stream to enable the 3:1 MUX and the 4:1 MUX for appropriate out put data (receive data, reset UDP check sum, time stamp, new CRC). 
   Calculate new CRC module  1106  calculates the CRC check sum for the modified data stream (Ethernet packet which has time stamp and reset UDP check sum), this new CRC will be placed into the modified data stream before it arrives at the MAC. The difference of receiver unit  1002  to transmitter unit  1004  is that receiver unit  1002  included verify CRC module  1110 . It is necessary to verify the CRC on packets that are received, due to the fact that this data may have been corrupted on the medium. If the CRC is not correct, this indicates that the data has indeed become corrupted. Instead of adding a new, correct CRC to the data, the data is allowed to pass through to the MAC with an incorrect CRC. 
   Controller  1102  synchronizes and controls the operations of all modules and components. Two clocks are used: a receive clock and an external system clock. The time stamp is generated by the external clock and latched into the receive clock domain at the start of every packet. Data is received from the PHY and the appropriate data (receive data, reset UDP check sum, time stamp, new CRC) is transmitted to the MAC in accordance with IEEE 802.3. When the data is received, the time stamp T 3  is placed in the appropriate area and a new CRC is calculated and replaces the old CRC value in the MAC layer. Again, a significant aspect of the present invention is that the time stamps and the CRC are generated and placed in the data frames at the physical layer as the data frame is received from the network. As such, the time stamps and the CRC, which reflect the actual time the data frames are received from the network, are quickly and efficiently placed in the data frames as they move to the MAC layer. If at any point of the packet interrogation it is determined that the data frame is not a valid time stamp frame or there are errors while receiving, the data frame is allowed to pass through with no alterations to the data. 
   As shown in  FIG. 12 , transmitter unit  1202  comprises transmitter controller  1202 , time stamp generator  1210 , shift register  1208 , calculate new CRC module  1206 , 3:1 MUX  1212  and 4:1 MUX  1204 . Transmitter controller  1202  controls the operations of all modules and components for each appropriate process. Two clocks are used: a transmit clock and an external system clock. The time stamps are generated by the external clock and latched into the receive clock domain at the start of every packet. Here again, the time stamps are generated and placed into the data frames at the physical layer. 
   While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. For example, the time stamping features may be implemented in other asynchronous networks, including, but not limited to, HPNA, PLC, and certain wireless networks, for example IEEE 802.11b. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.