Abstract:
Transmissions within a communication channel utilized by devices of a computer network that are outside of a device&#39;s designated time slot are accommodated through the use of a clear channel assessment time. The clear channel assessment time takes into account the device&#39;s designated transmission time slot within the communication channel with respect to those of other network devices. Thus, the clear channel assessment time may be a time period that is the product of a predetermined clear channel waiting time and a numerical representation of the difference between the device&#39;s designated transmission time slot within the communication channel and that of another network device that completed a preceding transmission. The clear channel waiting time may be specified by a network master device as part of a network connection process and the transmissions within the channel outside of a device&#39;s designated time slot may be accommodated after all regularly scheduled transmissions within the channel during a network frame period have been completed.

Description:
RELATED APPLICATION 
     This application is a continuation-in-part of co-pending application Ser. No. 09/151,579, entitled “Method and Apparatus for Accessing a Computer Network Communication Channel”, filed Sep. 11, 1998, by Rajugopal R. Gubbi, Natarajan Ekambaram and Nirmalendu Bikash Patra, and assigned to the Assignee of the present application. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a scheme for communications within a computer network and, in particular, to asynchronous communications as occur between a central server and one or more clients across a wireless communications link. 
     BACKGROUND 
     Modern computer networks allow for inter-communication between a number of nodes such as personal computers, workstations, peripheral units and the like. Network links transport information between these nodes, which may sometimes be separated by large distances. However, to date most computer networks have relied on wired links to transport this information. Where wireless links are used, they have typically been components of a very large network, such as a wide area network, which may employ satellite communication links to interconnect network nodes separated by very large distances. In such cases, the transmission protocols used across the wireless links have generally been established by the service entities carrying the data being transmitted, for example, telephone companies and other service providers. 
     In the home environment, computers have traditionally been used as stand-alone devices. More recently, however, there have been some steps taken to integrate the home computer with other appliances. For example, in so-called “Smart Homes”, computers may be used to turn on and off various appliances and to control their operational settings. In such systems, wired communication links are used to interconnect the computer to the appliances that it will control. Such wired links are expensive to install, especially where they are added after the original construction of the home. 
     In an effort to reduce the difficulties and costs associated with wired communication links, some systems for interconnecting computers with appliances have utilized analog wireless links for transporting information between these units. Such analog wireless links operate at frequencies commonly utilized by wireless telephones. Although easier to install than conventional wired communication links, analog wireless communication links suffer from a number of disadvantages. For example, degraded signals may be expected on such links because of multipath interference. Furthermore, interference from existing appliances, such as televisions, cellular telephones, wireless telephones and the like may be experienced. Thus, analog wireless communication links offer less than optimum performance for a home environment. 
     In the above-referenced co-pending application Ser. No. 09/151,579, which is incorporated herein by reference, a computer network employing a digital, wireless communication link adapted for use in the home environment was described. That architecture included a number of network components arranged in a hierarchical fashion and communicatively coupled to one another through communication links operative at different levels of the hierarchy. At the highest level of the hierarchy, a communication protocol that supports dynamic addition of new network components at any level of the hierarchy according to bandwidth requirements within a communication channel operative at the highest level of the network hierarchy is used. 
     The generalization of this network structure is shown in  FIG. 1. A  subnet  10  includes a server  12 . In this scheme, the term “subnet” is used to describe a cluster of network components that includes a server and several clients associated therewith (e.g., coupled through the wireless communication link). Depending on the context of the discussion however, a subnet may also refer to a network that includes a client and one or more subclients associated therewith. A “client” is a network node linked to the server through the wireless communication link. Examples of clients include audio/video equipment such as televisions, stereo components, personal computers, satellite television receivers, cable television distribution nodes, and other household appliances. 
     Server  12  may be a separate computer that controls the communication link, however, in other cases server  12  may be embodied as an add-on card or other component attached to a host computer (e.g., a personal computer)  13 . Server  12  has an associated radio  14 , which is used to couple server  12  wirelessly to the other nodes of subnet  10 . The wireless link generally supports both high and low bandwidth data channels and a command channel. Here a channel is defined as the combination of a transmission frequency (more properly a transmission frequency band) and a pseudo-random (PN) code used in a spread spectrum communication scheme. In general, a number of available frequencies and PN codes may provide a number of available channels within subnet  10 . As is described in the co-pending application cited above, servers and clients are capable of searching through the available channels to find a desirable channel over which to communicate with one another. 
     Also included in subnet  10  are a number of clients  16 , some of which have shadow clients  18  associated therewith. A shadow client  18  is defined as a client which receives the same data input as its associated client  16  (either from server  12  or another client  16 ), but which exchanges commands with server  12  independently of its associated client  16 . Each client  16  has an associated radio  14 , which is used to communicate with server  12 , and some clients  16  may have associated subclients  20 . Subclients  20  may include keyboards, joysticks, remote control devices, multi-dimensional input devices, cursor control devices, display units and/or other input and/or output devices associated with a particular client  16 . A client  16  and its associated subclients  20  may communicate with one another via communication links  21 , which may be wireless (e.g., infra-red, ultrasonic, spread spectrum, etc.) communication links. 
     Each subnet  10  is arranged in a hierarchical fashion with various levels of the hierarchy corresponding to levels at which intra-network component communication occurs. At a highest level of the hierarchy exists the server  12  (and/or its associated host  13 ), which communicates with various clients  16  via the wireless radio channel. At other, lower levels of the hierarchy the clients  16  communicate with their various subclients  20  using, for example, wired communication links or wireless communication links such as infrared links. 
     Where half-duplex radio communication is used on the wireless link between server  12  and clients  16 , a communication protocol based on a slotted link structure with dynamic slot assignment is employed. Such a structure supports point-to-point connections within subnet  10  and slot sizes may be re-negotiated within a session. Thus a data link layer that supports the wireless communication can accommodate data packet handling, time management for packet transmission and slot synchronization, error correction coding (ECC), channel parameter measurement and channel switching. A higher level transport layer provides all necessary connection related services, policing for bandwidth utilization, low bandwidth data handling, data broadcast and, optionally, data encryption. The transport layer also allocates bandwidth to each client  16 , continuously polices any under or over utilization of that bandwidth, and also accommodates any bandwidth renegotiations, as may be required whenever a new client  16  comes on-line or when one of the clients  16  (or an associated subclient  20 ) requires greater bandwidth. 
     The slotted link structure of the wireless communication protocol for the transmission of real time, multimedia data (e.g., as frames) within a subnet  10  is shown in FIG.  2 . At the highest level within a channel, forward (F) and backward or reverse (B) slots of fixed (but negotiable) time duration are provided within each frame transmission period. During forward time slots F, server  12  may transmit video and/or audio data and/or commands to clients  16 , which are placed in a listening mode. During reverse time slots B, server  12  listens to transmissions from the clients  16 . Such transmissions may include audio, video or other data and/or commands from a client  16  or an associated subclient  20 . At the second level of the hierarchy, each transmission slot (forward or reverse) is made up of one or more radio data frames  40  of variable length. Finally, at the lowest level of the hierarchy, each radio data frame  40  is comprised of server/client data packets  42 , which may be of variable length. 
     Each radio data frame  40  is made up of one server/client data packet  42  and its associated error correction coding (ECC) bits. Variable length framing is preferred over constant length framing in order to allow smaller frame lengths during severe channel conditions and vice-versa. This adds to channel robustness and bandwidth savings. Although variable length frames may be used, however, the ECC block lengths are preferably fixed. Hence, whenever the data packet length is less than the ECC block length, the ECC block may be truncated (e.g., using conventional virtual zero techniques). Similar procedures may be adopted for the last block of ECC bits when the data packet is larger. 
     As shown in the illustration, each radio data frame  40  includes a preamble  44 , which is used to synchronize pseudo-random (PN) generators of the transmitter and the receiver. Link ID  46  is a field of fixed length (e.g., 16 bits long for one embodiment), and is unique to the link, thus identifying a particular subnet  10 . Data from the server  12 /client  16  is of variable length as indicated by a length field  48 . Cyclic redundancy check (CRC) bits  50  may be used for error detection/correction in the conventional fashion. 
     For the illustrated embodiment then, each frame  52  is divided into a forward slot F, a backward slot B, a quiet slot Q and a number of radio turn around slots T. Slot F is meant for server  12 -to-clients  16  communication. Slot B is time shared among a number of mini-slots B 1 , B 2 , etc., which are assigned by server  12  to the individual clients  16  for their respective transmissions to the server  12 . Each mini-slot B 1 , B 2 , etc. includes a time for transmitting audio, video, voice, lossy data (i.e., data that may be encoded/decoded using lossy techniques or that can tolerate the loss of some packets during transmission/reception), lossless data (i.e., data that is encoded/decoded using lossless techniques or that cannot tolerate the loss of any packets during transmission/reception), low bandwidth data and/or command (Cmd.) packets. Slot Q is left quiet so that a new client may insert a request packet when the new client seeks to log-in to the subnet  10 . Slots T appear between any change from transmit to receive and vice-versa, and are meant to accommodate individual radios&#39; turn around time (i.e., the time when a half-duplex radio  14  switches from transmit to receive operation or vice-versa). The time duration of each of these slots and mini-slots may be dynamically altered through renegotiations between the server  12  and the clients  16  so as to achieve the best possible bandwidth utilization for the channel. Note that where full duplex radios are employed, each directional slot (i.e., F and B) may be full-time in one direction, with no radio turn around slots required. 
     Forward and backward bandwidth allocation depends on the data handled by the clients  16 . If a client  16  is a video consumer, for example a television, then a large forward bandwidth is allocated for that client. Similarly if a client  16  is a video generator, for example a video camcorder, then a large reverse bandwidth is allocated to that particular client. The server  12  maintains a dynamic table (e.g., in memory at server  12  or host  13 ), which includes forward and backward bandwidth requirements of all on-line clients  16 . This information may be used when determining whether a new connection may be granted to a new client. For example, if a new client  16  requires more than the available bandwidth in either direction, server  12  may reject the connection request. The bandwidth requirement (or allocation) information may also be used in deciding how many radio packets a particular client  16  needs to wait before starting to transmit its packets to the server  12 . Additionally, whenever the channel conditions change, it is possible to increase/reduce the number of ECC bits to cope with the new channel conditions. Hence, depending on whether the information rate at the source is altered, it may require a dynamic change to the forward and backward bandwidth allocation. 
     The use of the slotted link communication architecture described above can present challenges for accommodating asynchronous data transmissions within a subnet  10 . For example, consider a file transfer process between a network master (e.g., server  12 ) and a client  16  that takes place in an operating environment under the control of the Windows™ operating system (or one of its variants) produced by Microsoft corporation of Redmond, Wash. In such a transfer, the requesting entity (i.e., the network resource, say a personal computer, requesting the transfer of data) initiates the transfer by sending a Server Message Block (SMB) protocol command to open the file on the target platform (e.g., a server or another personal computer storing the requested material). The target device responds with another SMB command and supplies the requesting device with a pointer to the requested file. The requesting device, using this pointer, then reads a block of N-bytes from the designated file, specifying the block size using an offset from the pointer provided by the target device. In response, the target device reads N-bytes worth of data from the subject file and delivers this information to a transmission control protocol/internet protocol (TCP/IP) (assuming this is the transfer protocol being used) layer that handles communication between the devices. The TCP/IP layer fragments the N-bytes of data into smaller TCP/IP packets and begins transmitting the packets to the requesting device across the communication channel. As the requesting device receives the packets, it transmits acknowledgements back to the target device. After the last packet for the N-byte transfer has been received and acknowledged, the requesting device transmits another SMB request for the next block of M-bytes from the subject file. M and N may be the same or different, and this process continues until the file transfer is complete. 
     The above transfer process is serialized under the control of the SMB requests and subsequent responses. This serialization is necessary because the SMB layer on the requesting platform waits for the arrival of the last packet in the previously requested portion of the file before sending the next request. Because of this serialization, the wireless communication channel may be idle during the response times between the SMB and TCP/IP layers on both sides of the transaction. Furthermore, because the underlying wireless communication channel is a time division multiple access (TDMA)-based architecture, any delay in these responses can cause the transmitting device to miss its allocated slot time and, hence, result in increased latency. Thus, it would be desirable to have a mechanism for avoiding such latencies in a wireless communication scheme for a computer network. 
     SUMMARY OF THE INVENTION 
     In one embodiment, transmissions within a communication channel utilized by devices of a computer network that are outside of a device&#39;s designated time slot are accommodated through the use of a clear channel assessment time. The clear channel assessment time takes into account the device&#39;s designated transmission time slot within the communication channel with respect to those of other network devices. Thus, the clear channel assessment time may be a time period that is the product of a predetermined clear channel waiting time and a numerical representation of the difference between the device&#39;s designated transmission time slot within the communication channel and that of another network device that completed a preceding transmission. The clear channel waiting time may be specified by a network master device as part of a network connection process and the transmissions within the channel outside of a device&#39;s designated time slot may be accommodated after all regularly scheduled transmissions within the channel during a network frame period have been completed. 
     In another embodiment a clear channel assessment that takes into account a first device&#39;s designated transmission time slot within a communication channel with respect to those of other network devices in order to determine idle times that exist after completion of regularly scheduled transmissions within the communication channel is provided. The first device may transmit within the common communication channel upon an indication that the channel is available for transmission. Such indication is preferably made upon the expiration of a time period that is the product of a predetermined clear channel waiting time and a numerical representation of the difference between the first device&#39;s designated transmission time slot within the communication channel with respect to that of another network device. This predetermined clear channel waiting time may be designated by a network master device upon a connection thereto by the first device. 
     In still another embodiment, a network client having a clear channel assessment indicator and being configured to transmit within a communication channel of a computer network at a time determined in part by a notification from the clear channel assessment indicator and in part by transmission characteristics of other devices transmitting within the channel is provided. These transmission characteristics may include a numerical difference between a designated transmission slot for the network client and that of at least one of the other devices. In some cases, the channel is a time division multiplexed wireless communication channel. 
     In a further embodiment, a scheme for negotiating a transmission time in a time division multiplexed communication channel according to a heed to transmit asynchronous data within idle times of a transmission frame period is provided. According to such a scheme, transmissions of asynchronous data within the idle times are scheduled by devices utilizing the communication channel according to a clear channel assessment time and transmission characteristics, for example designated transmission time slots within the transmission frame period, of other devices transmitting within the channel. 
     These and other features and advantages of the present invention will be apparent from a review of the detailed description and its accompanying drawings that follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example, and not limitation, in the figures of the accompanying drawings in which: 
         FIG. 1  illustrates a generalized network structure that is supported by a wireless protocol that is one embodiment of the present invention; 
         FIG. 2  illustrates a hierarchical arrangement for the transmission of data within a subnet according to one embodiment of the present invention; and 
         FIG. 3  illustrates a network client device configured with a collection of timers and counters, some of which may be used in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Described herein is a scheme for avoiding latencies in asynchronous communications within a wireless communication channel of a computer network. The present scheme is generally applicable to a variety of network environments, but finds especially useful application in a wireless computer network which is located in a home environment. Thus, the present scheme will be discussed with reference to the particular aspects of a home environment. However, this discussion should in no way be seen to limit the applicability or use of the present invention in and to other network environments and the broader spirit and scope of the present invention is recited in the claims which follow this discussion. 
     One important term used throughout the following discussion is “channel”. As indicated above, a channel is defined as the combination of a transmission frequency (more properly a transmission frequency band) and a pseudo-random (PN) code used in a spread spectrum communication scheme. In general, a number of available frequencies and PN codes may provide a number of available channels within a subnet. Network masters and clients are capable of searching through the available channels to find a desirable channel over which to communicate with one another. Table 1 below illustrates an exemplary channel plan according to this scheme. 
                                               TABLE 1                   Available   Available PN Codes            Frequency Bands   PN Code 1   PN Code 2   . . .   PN Code n               Frequency Band 1   Channel 11   Channel 12   . . .   Channel 1n       Frequency Band 2   Channel 21   Channel 22   . . .   Channel 2n       . . .   . . .   . . .   . . .   . . .       Frequency Band N   Channel N1   Channel N2   . . .   Channel Nn                    
In one embodiment, a channel plan using two frequency bands is adopted and details of channel selection within such a scheme is discussed in greater in the above-cited co-pending application.
 
     As explained in greater detail in the above-cited co-pending application, when a client device  16  joins a subnet, the client receives a Connection Agreements (CAG) package from the network master device (e.g., server  12 ). This package includes, among other things, information regarding the forward and backward bandwidth (e.g., the slots of the channel) to which the new client  16  is entitled. In addition, the maximum number of bytes the new client  16  can send/expect in each data packet is set for each type of packet (e.g., video data, audio data, etc.). The Connection Agreements package may also contain information regarding the total number of data frames that the new client  16  needs to wait (i.e., before transmitting its traffic) from the start of server&#39;s transmission and the identification of the preceding client (i.e., the client that owns the preceding reverse transmission slot). The client is also assigned a unique session identifier (CS-ID). 
     In addition to receiving its unique CS-ID, each client is also provided (either as part of the Connections Agreement package or in another client-master exchange) with the CS-ID of the last device to transmit within a network frame  52  (i.e., the unique identifier of the last device that has an allocated time slot for transmission in the TDMA architecture of the channel). This allows each client to identify any idle times at the end of a network frame. By knowing when these idle periods commence, clients are able to take advantage of available, but otherwise unused, channel bandwidth for asynchronous data transmissions. Such idle times may appear for a variety of reasons, for example, some devices in the subnet  10  may not always make full use of their available transmission times. Also, the entire network frame period may be more than is needed to accommodate existing devices within the subnet  10 , thus providing excess time at the end of such a period. 
     After receiving the Connection Agreements packet, the client  16  configures itself to transmit its data in its assigned time slot (e.g., B 1 , B 2 , etc.) and waits for that slot to come around. At the designated time slot, the client  16  may initiate normal communications with the server  12  and transmit any data or commands it may have. In order to help maintain proper time slot synchronization, part of this configuration process may involve programming an accurate slot timer (AST). 
       FIG. 3  illustrates a client  16  configured with an AST  54 . The AST  54  may be a conventional timer (e.g., a register or memory location, which is incremented or decremented on a regular basis according to an appropriate clock signal maintained by the client device) that is programmed using information provided by the network master as part of the connection process. For example, the network master (e.g., server  12 ) may provide the client  16  with an indication of where it lies in within the slotted link structure of the communication protocol (i.e., at what time it should begin its transmissions). By then monitoring this time using AST  54  (which may be reset upon each master transmission or each transmission of the associated client), the client  16  can accurately predict when it should begin its transmission. Of course, the expiration of an AST-monitored time should not allow transmission in the event a client detects that a preceding client has not yet completed its own transmissions. Otherwise, the transmissions may overlap and cause confusion within the network. 
     In conjunction with the use of an AST  54 , a client  16  may also incorporate an early trigger timer (ETT)  55  to ensure that packets are ready for transmission within the subnet when the client&#39;s transmission slot arrives. That is, an ETT  55  may be used to advance the internal construction of a packet or packets for transmission, with the goal being to have those packets assembled and ready for transmission when the client&#39;s transmission slot becomes available. Thus the ETT  55  (which may be implemented as a conventional timer) triggers the process of formation of packets and their error protection bits, etc., using a packet construction engine  56  (which in some cases may be a part of client  16  or in other cases may be a part of radio  14 ) to keep a few packets, at least, prepared before the actual start of transmission. For example, in one embodiment packets could be assembled and stored for transmission one network frame in advance of their actual transmission. Such preassembly is helpful in avoiding the idle times on the channel when the first few packets for a transmission sequence are being formed. Although collecting data for one network frame in advance of transmission may penalize the system with a one network frame latency at the first transmission slot, it is expected that this period can be reduced to a few milliseconds. 
     As indicated above, each client  16  may be required to keep track of the present client occupying the channel, thereby trying to detect its immediately preceding client in line. If the channel is quiet, the current client waits for a predetermined length of time before starting its own transmission. In one embodiment, the length of this waiting time depends on the quiet time threshold allowed between two clients (termed the clear channel assessment or CCA time) and the number of clients yet to transmit before the current client. For example, the waiting time may be the product of the CCA threshold and the number of devices yet to transmit. This waiting time calculation thus makes use of the order of transmission that is established during the connection setup. The only exception to the quiet time is the Q slot, when all on-line clients  16  should refrain from transmitting. 
     As indicated, a CCA transmission (i.e., a client&#39;s transmission based on a waiting period timeout) is essentially the result of a decision made by the client that the channel is free and hence is suitable for transmission. Each client device keeps track of how many devices have completed the transmission from the time the network master completed its transmission. This can be done using a counter that is incremented each time a client transmission is detected and the implementation of such counters is well known in the art. Then, by knowing the index of its own position in the network frame (a value received from the network master at the time a connection is established), each client device can determine when to initiate a CCA transmission. This is best illustrated with an example. 
     Each client device may program an associated CCA timer  57  (see  FIG. 3 ) to a predetermined value. The network master may specify this value at the time a master-client connection is established and it generally represents a period of time that must expire before a network client is permitted to assume that another client is not using the channel. Now, suppose a client device is 5 th  in line for transmission after the master device and suppose it detects a clear channel (e.g., because its CCA timer  57  times out) after the device that is second in line has completed its transmission. Because the device is 5 th  in line, it cannot immediately begin its own transmissions (after all, there are still two client devices in line ahead of it). However, the 5 th  device may increment an associated CCA counter  58  at this time. If then both the 3 rd  and 4 th  devices are silent (i.e., if the 5 th  device&#39;s CCA timer  57  times out two more times in succession), then the 5 th  device will have again twice incremented its CCA counter  58  and may immediately start its own transmission on the 3 rd  CCA detection. 
     Note that in the above example, the 3 rd  device in line will also have detected the absence of the second device&#39;s transmission and may therefore immediately start its own transmission if indeed it has traffic to send. In other words, the 3 rd  device&#39;s waiting period is only one CCA timeout from the absence of the 2 nd  device&#39;s transmissions, while that of the 5 th  device is three CCA timeout periods. Similarly, the 4 th  device in line is two CCA timeout periods away from the second device. This use of the client slot assignments in determining when CCA transmissions may be initiated allows the client devices to maintain their relative sequence with one another within the slotted link structure of the communication channel. 
     The CCA timeout period may also be used to detect idle times at the end of a network frame  52 . For example, by knowing the CS-ID of the last device to transmit within the TDMA architecture, all clients  16  can determine when the idle time commences by monitoring the transmission of the client that is last in line. All clients  16  in the subnet  10  can then estimate the duration of the idle time after the last-in-line client has completed its transmission (by knowing the total available time for a network frame, which as indicated above, has a fixed duration). By sharing this available idle time amongst themselves, the clients  16  of the subnet  10  can provide for asynchronous data transmissions in the subnet  10 . Of course, other methods of detecting idle times may be used. 
     The idle time “sharing” plan between the clients makes use of the CCA in that each client waits a time T idle =T CCA *C (microseconds) before transmitting a packet in the idle time. T CCA  may be the standard CCA time for a regular transmission and C may be determined as the difference between the transmission slot number of the current client and that of the client from which the immediately preceeding packet was received (or monitored). If the master device has any asynchronous data to transmit, it may use C=1 and begin its transmission from the time of the end of the transmission of the last-in-line client. 
     In order to provide a somewhat fair allocation scheme, in one embodiment each client is permitted to transmit only one packet in the idle time at the end of a network frame  52 . This allows the devices in subnet  10  to take turns transmitting asynchronous (e.g., low priority) data over the channel. This protocol may be abandoned and a transmission commenced if a packet from a previous client is detected in the idle time and there is sufficient time in the idle period for a packet transmission. However, before any transmission in the idle time occurs, a device wishing to send data should allow sufficient time for the Q slot before the commencement of the next network frame. 
     Within the present scheme, there are some idle-time transmission situations that warrant further discussion. For example, consider the situation of a short idle time. Because the time at which a device may transmit in the idle time is determined by transmission slot number (C), it is possible that only a subset of the total number of network devices get the opportunity to make use of the idle time in each network frame period. To make idle time allocation more fair, the present scheme allows devices in a subnet  10  to request reallocation of its transmission slot by sending such a request to the network master. Thus, if a client determines that it has a significant amount of asynchronous traffic to send but that it is unable to do so because of its transmission slot number, that client can request a new transmission slot, earlier in a network frame period, so as to have a better chance of making use of the idle time at the end of the frame. 
     If a device wishes to transmit multiple packets within the idle time of a single network frame, a two-step process may be invoked. First, the device transmits an initial packet in its regular space in the idle time, determined according to the above protocol. Then, the device may reprogram its T idle  time such that T idle =T CCA *N, where N is the total number of devices (including the master) in the subnet  10 . The next idle time transmission for the subject device can then occur at this new T idle  time, provided sufficient time remains before the Q slot. If a packet from another device is received before this new T idle  time expires, the CCA may be reprogrammed to the time between the device&#39;s transmission and the reception of the newly received packet. This helps assure an equal opportunity for all devices in the subnet  10  to use the idle time for transmission of asynchronous data. 
     When a subnet  10  is sharing a channel with one or more overlapping subnets, the start and end of each subnet&#39;s transmission periods should be strictly limited to a time duration within each network frame  52 . If there are idle times within such a duration, the devices in the subnets may make use of the time in the manner described above, however, such transmissions should not exceed the time duration of their respective subnet&#39;s transmission window. 
     During channel change situations (described in detail in the above-cited co-pending application), client devices are prohibited from transmitting until permitted to do so by the master device or certain time outs. This protocol should be strictly observed and idle time transmissions halted during such periods so that there is no loss of data within the subnet  10 . 
     Packet size will play a role in determining how many devices are afforded the opportunity to transmit within an idle time. That is, even if a device&#39;s time for transmission in an idle time has arrived, that device should not transmit if it detects an ongoing transmission of another device. Also, devices sending smaller packets during an idle time may be benefited to a lesser extent than those transmitting larger packets, as a device is only (usually) able to transmit one packet per idle time. To balance this equation, and reduce the potential for collisions within an idle time, one embodiment of the present scheme restricts devices to a predetermined time duration for any such transmissions. In other words, only packets up to a predetermined size are eligible for transmission in an idle time. 
     Thus, a scheme for synchronizing communications within a computer network communication channel has been described. Although discussed with reference to certain illustrated embodiments, the present invention should not be limited thereby. Instead, the present invention should only be measured in terms of the claims that follow.