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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a broadband switching network for the switching
of asynchronously transferred cells, and to a method of switching
asynchronously transmitted cells of data.
2. Related Art
Broadband switching networks for switching asynchronously transferred cells
are known, in which a predetermined level of bandwidth is allocated to a
transmission channel, connecting a first customer to a second customer. In
some of these known systems, a communications channel is provided over a
significant period of time, effectively of the leased-line type, and
manual measures are implemented in order to set up such a connection or to
modify a connection, in terms of the particular terminations and the level
of traffic being conveyed. Consequently, it is usual for customers to be
charged a fixed rate for such a connection, resulting in payment being
made irrespective as to whether that connection is being used or not.
Alternative systems have been proposed. In particular, it is possible for
connections to be established on a dial-up basis, requiring termination
equipment to be provided with facilities for establishing connections by
issuing signalling commands and responding to similar commands issued by
the network.
The use of permanent circuits to support a private communications network
is widespread. The demand for such circuits is expected to grow to include
broadband rates above 2 Mbit/s, where the traffic may be multiplexed from
sources which are inherently bursty, together with the sources which are
transmitted at constant bit rates that are delay sensitive, such as voice
transmission and constant bit-rate video.
Asynchronous transfer mode (ATM) cells all have a fixed information field
of forty eight octets which can carry customer traffic or customer
originating control information (signalling). These two types of data
transmission are distinguished by setting virtual path (VP) and virtual
circuit (VC) values in the cell headers. Another field provided in the ATM
header is known as cell loss priority, which enables low priority cells to
be distinguished from high priority cells. In the event of congestion, the
10w priority cells will be discarded first.
For private circuits within an ATM based network, the desired route, the
required bandwidth, and the quality of service (QOS) are set up using
network management procedures. The private circuits are known as permanent
virtual circuits because there is no actual physical circuit, only a VP/VC
value or "label" which is associated with information stored in the
switches to determine the route and preserve the bandwidth and QOS
requirements.
A disadvantage of all known permanent circuits is that the bandwidth
remains assigned to the circuit, even when the customer has nothing to
transmit. This means that the customer may have to pay higher charges than
could be obtained if the bandwidth was only made available when needed.
The assumption being made here is that charging is related to reserved
bandwidth, and this is not necessarily correct in terms of the way public
network operators may choose to charge for virtual circuits. However, it
is expected that charging based on reserved bandwidth will become a
significant factor in the future. It has been proposed to see up a
permanent virtual circuit so that it is only available during certain
hours of the day, or during certain days of the week. A difficulty with
this approach is that it does not allow the customer to change the pattern
of usage quickly, and it may only crudely reflect the usage required by
the customer.
A second proposal is to provide the customer with a separate communications
channel to the network management plane, thereby allowing a permanent
virtual circuit to be reconfigured. A difficulty with this approach is
that some time delay will be incurred before the customer can start to use
the virtual circuit.
A third proposal is to introduce equipment at every switching point in the
network that recognises a fast resource management cell, indicating that
bandwidth should now be assigned to the circuit. A difficulty with this
approach is that there is no internationally agreed standard for a
bandwidth-requesting cell that would be recognised by the switching
equipment produced by the various manufacturers.
SUMMARY OF THE INVENTION
The present invention provides a broadband switching network for the
switching of asynchronously transferred cells, the network comprising
switch control means for establishing a connection for the asynchronous
transmission of cells from an input port to an output port; and usage
control means, arranged to downgrade the priority of transmitted cells if
customer usage exceeds a first predetermined level; characterised by
bandwidth control means arranged to detect the rate at which cells are
supplied to the input port and to request the switch control means to
allocate bandwidth to the connection in response to the detection, wherein
the switch control means allocates bandwidth to input cells and increases
the first predetermined level at which said usage control means downgrades
the priority of transmitted cells.
In a preferred embodiment, the first predetermined level is see to zero
when no actual use is being made of the network by the respective
customer, resulting in initial cells being downgraded to low priority.
Preferably, the bandwidth control means adjusts the first predetermined
level set by the usage control means by transmitting a message to the
management control means, which in turn issues a message as to the usage
control means.
The present invention also provides a method of asynchronously transmitting
cells of data through a switchable network, the method comprising the
steps of resetting a usage control means so as to downgrade the priority
of input cells; detecting the presence of input cells requiring a level of
bandwidth through the network; and requesting a switch control to allocate
bandwidth in response to said detection; whereupon bandwidth is allocated
by the switch control; and the usage control means is adjusted by the
switch control so that high priority cells are maintained as such.
In a preferred embodiment, a request may be made to the switch control to
provide as much bandwidth as is available, whereupon the switch control
identifies the available bandwidth; bandwidth is allocated at a level for
which a buffer is provided, and a feedback signal is generated if the
level of data retained in said buffer exceeds a predetermined value.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in greater detail, by way of example,
with reference to the accompanying drawings, in which:
FIG. 1 shows a broadband switching network, configured with sub-switching
networks connected to network termination devices;
FIG. 2 shows a sub-switching network of the type identified in FIG. 1,
including a broadband asynchronous switch, a usage parameter control
device and a dynamic bandwidth controller;
FIG. 3 shows the dynamic bandwidth controller identified in FIG. 2, and
includes a cell monitor; and,
FIG. 4 shows the cell monitor shown in FIG. 3.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
A broadband environment for the transmission of asynchronous data is shown
in FIG. 1. The environment includes a plurality of broadband switching
networks (switches) 15, providing means by which transmission routes may
be established between customers connected to the overall network.
Data is transmitted in the form of asynchronous transfer mode (ATM) cells,
each having an information field of forty-eight octets, in addition to a
header of five octets, which includes information facilitating
transmission through the network itself. Thus, routing is controlled on a
cell-by-cell basis and a plurality of transmission paths and time
multiplexed slots may be employed for any particular link. ATM cells are,
therefore, transmitted via virtual paths and virtual circuits, as defined
by the header information.
The header of each ATM cell also includes a cell loss priority bit, thereby
allowing each cell to be identified as having high priority or low
priority. When congestion occurs, that is to say when elements within the
overall switching network reach capacity, the network is arranged such
that low priority cells are discarded in preference to high priority
cells. Furthermore, in any operational network, the network itself would
be designed such that, under normal operating circumstances, high priority
cells would always be allowed through the network, so that congestion,
resulting in the loss of low priority cells, would only occur during
periods of peak demand. For some forms of data transmission, such as 64
kbit/s voice, cell loss may be acceptable. However, in other
circumstances, it may be highly undesirable for cells to be lost,
particularly when non-redundant data is being transmitted, under which
circumstances a customer would expect traffic controls to ensure that all
data cells were transmitted through the network with very low losses.
The network shown in FIG. 1 is particularly suitable for the transmission
of "bursty" type signals in which the information content and hence data
quantity, varies significantly over time, and may take place in
concentrated intervals of relatively short duration, identified as bursts.
Thus, the network is able to support for the transmission of bursty data;
although, if required by a customer, it may also be configured to
accommodate the transmission of speech, video or any other form of
digitisable communication.
A complete transmission path is identified in FIG. 1 between a first
customer A and a second customer B. However, it should be appreciated that
the overall network would include a large number of switches 15, each
providing access to the network for a plurality of customers.
Customer A has a data communicating device 16, a video communicating device
17, and a voice communicating device 18 connected to the network. Signals
for these devices 16, 17 and 18 are routed to, or are received from, a
first type of network terminating device 19, arranged to supply signals
to, and receive signals from, a second type of network terminating device
20. The second type of network terminating device 20 provides isolation
between data source/data sink equipment and the core network, and
preferably includes electro-optical isolators, resulting in optical
transmission between the terminating device 20 and the switch 15.
The network may operate in several ways, although two preferred modes of
operation will be identified. One mode is to provide permanent connections
between customers, of the leased-line type. In this mode of operation, a
communications channel between customer A and customer B is established,
and thereafter this channel remains open until the relationship is
terminated. In a second mode of operation, the network operates in a
switched-like way, in that signalling commands are issued by a customer.
These signalling commands are, in turn, interpreted by the switch 15,
resulting in the connection being established. However, in some data
networking applications, it is envisaged that, once the connection has
been established, it would remain in place for a significant period,
possibly days. Therefore, such a link differs significantly from
connections made via the public switch telephone network (PSTN), for
example, where connection time is usually measured in minutes. Thus, in
this second mode, the network provides a service similar to that provided
by leased lines; and, in its first mode of operation which provides the
basis for the preferred embodiment described herein, customer's equipment
behaves as if a leased line were provided between communicating sites.
Under this preferred mode of operation, it is unnecessary for customer's
equipment to include any mechanisms for generating signalling, allowing
customer A immediately to initiate communication with customer B.
Thus, it can be appreciated that, given cells are switched on a
cell-by-cell basis and each cell is individually routed in response to its
own header information, the ability to direct a specific number of cells
to a particular destination is equivalent to allocating a level of
bandwidth to that particular channel.
Thus, signals generated by customer A will be supplied to the switching
environment from the network termination device 20, and will be directed
to customer B, via a network terminating device 21 and a network
terminating device 22. At the location of customer B, customer equipment
23 is provided for the transmission and reception of traffic. Therefore,
transmission between customer A and customer S is only in the form of
data, with other communication channels being be required for customer A
to effect communication of voice and video signals via the network.
The switching network 15 illustrated in FIG. 1 is shown in more detail in
FIG. 2. At the heart of the switching network 15, a broadband switch 21 is
provided, the broadband switch having a plurality of input ports and a
plurality of output ports. Thus, output signals from customer A are
supplied to an input port 22 of the broadband switch 21, and the switch is
arranged to supply cells received at that port to an output port 23, which
in turn directs said cells, via similar switching devices, to customer B.
Similarly, output cells from customer B are ultimately supplied to the
broadband switch 21 at an input port 24, allowing the switch 21 to direct
said cells to an output port 25.
The establishment of logical paths within the broadband switch 21 is
effected by a network management computer 26, which is also responsible
for overseeing the operation of a plurality or similar broadband switches.
Communication between the network management computer 26 and the broadband
switch 21 is made via a respective element manager 27, which is
switch-specific and provides an interface between the switch and the
network management computer. Thus, proprietary broadband switches from a
plurality of manufacturers may be configured within an overall network,
interfaced via respective element managers 27.
Traffic control of the broadband switch 21 is effected via a call
controller 28. Thus, it is necessary for the call controller 28 to receive
information defining the bandwidth requirement for any particular logical
connection, thereby ensuring that suitable operation of the broadband
switch 21 is made under the control of the call controller, so as to
minimise cell loss. As previously stated, the arrangement should be
configured such that cells identified as being of a high priority are
never lost, and low priority cells are only lost when the broadband switch
21 is overloaded when congested.
The network includes usage parameter control devices 29, which conform to
the operations defined within Recommendation I 371. Thus, a usage
parameter control device 29 is provided for each customer, and is
programmed in accordance with the required level of service required by
the customer. Thus, a customer's usage contract will specify a first
traffic threshold, below which, all cells identified as being of high
priority will remain at this priority, thereby ensuring their successful
transmission through the network. However, once traffic exceeds the first
threshold, some cells will have their priority downgraded to low priority,
such that it cannot be guaranteed that transmission will successfully take
place through the network. Thus, a user will have a guaranteed bandwidth,
beyond which traffic may be accepted, but transmission through the network
cannot be guaranteed.
Clearly, an operator must be given some indication of maximum demand,
otherwise it would be impossible to design suitable switching devices. The
usage parameter control device, therefore, defines a second traffic
threshold, beyond which cells will be removed from the data stream
altogether, thereby ensuring that the bandwidth requirement made of the
switch never exceeds, for that particular customer, the traffic level
defined by the second threshold.
Thus, in normal operation, a customer would aim to stay below the first
threshold, thereby ensuring that all cells are transmitted through the
network. However, if (possibly through some unexpected occurrence) traffic
demand increases, the network may have capacity to accommodate this
increase in traffic, thereby maintaining data integrity for the customer,
while at the same time being provided with a mechanism for removing
offending cells, thereby ensuring that cells to which bandwidth has been
guaranteed are transmitted through the network.
Thus, within each broadband switch 21 and under the control of the call
controller 28, low priority cells may be rejected when congestion occurs.
However, as previously stated, the overall network would be configured
such that high priority cells would be guaranteed transmission through the
network.
In the present preferred embodiment, a customer may remain permanently
physically connected to the network, and may be under the impression that
the network is permanently configured so as to make bandwidth available,
even when the customer does not have a requirement to make use of this
available bandwidth. However, the customer is only charged when an actual
use of the network is made, preferably by charging the customer on the
basis of the number of cells transmitted over the logical link. Thus,
prior to transmission being invoked by, say customer A, the broadband
switch 21 may be actually allocating no bandwidth whatsoever for
transmission between the input port 22 and the output port 23.
Furthermore, it is not necessary for any special activity to be made by
customer A in order for bandwidth to be allocated. A level of bandwidth
will be assigned to customer A when required.
After transmitted cells have passed the usage parameter control device 29,
they are directed to a dynamic bandwidth controller 30. It may take a
short interval for the call controller 28 to find the required level of
bandwidth required by the bandwidth controller 30 so that, during the
interval required by call control to process the bandwidth request, all
transmitted cells are downgraded to low priority.
In addition to customer A generating signals, which, as previously stated,
are supplied to the input port 22, signals are also transmitted back to
customer A via the output port 25. These output signals are also supplied
to the dynamic bandwidth controller 30, allowing said controller to
transmit signals back to customer A. Thus, in particular, the dynamic
bandwidth controller 30 may issue a signal back to customer A, instructing
customer A to reduce its data output to avoid the risk of cells being
lost.
The dynamic bandwidth controller 30 is shown in more detail in FIG. 3, and
includes a cell monitor 31, a processor 32, a feed back controller 33 and
an attribute data store 34.
Data is transmitted to the port 22 and similar data is received from the
port 25 in the form of discrete cells, having forty-eight octets of user
information plus a five octet header.
The five octet header includes a virtual path identifier (VPI) and a
virtual channel identifier (VCI) which effectively define the connection
such that cells forming part of a common message will be transmitted over
the same connection. In addition, the last 8-bit octet of the header is a
header error check field which, as its name suggests, provides a degree of
redundancy, by which error checking may be performed on the header
information. Thus, the principal reason for providing the header error
check field is to ensure that the header information is correct, thereby
ensuring that cells are not transmitted to erroneous addresses.
Within the cell monitor 31, the header error check field is used to
identify the start of a cell. Thus, the cell monitor 31 is provided with
logic for implementing the algorithm associated with the header error
check field in order to identify the position of said field. Thus, the
logic provided within the cell monitor 31 is not concerned with the
identification of errors given that, on start-up, it is not aware of the
position of the header error check field. It, therefore, assumes that a
particular octet is the error check field and performs the error checking
algorithm on what, given the assumed position of this field, would be the
remainder of the field upon which an error checking operation is to be
performed. If the algorithm results in a to determination to the effect
that an error is present, it is assumed that the device has selected the
wrong position for the header error checking field and a modification is
made. Thus, modifications of field position will continue to be made until
the algorithm produces a result to the effect that no error is present.
Under these conditions, the correct header error check field will have
been selected, and will continue to be selected on a cell-by-cell basis.
By making use of the header error check field in this way, it is possible
for the cell monitor 31 to identify the start of an asynchronously
transmitted cell.
In addition to customer originating data being transmitted in the form of
cells, it is also possible for the processor 32 to communicate with
customer equipment and sub-switching networks within the overall switching
environment by generating control information cells. These cells would be
given unique VPI and VCI values to route them within the broadband switch
21 to, for example, the controller 26. Similarly, the processor 32 may
generate cells which are supplied to customer equipment, via the feed back
controller 33.
As previously stated, the usage parameter controller 29, shown in FIG. 2,
is capable of modifying the priority of transmitted cells. It will be
appreciated that information defining priority thresholds would be held by
the network manager 26, and so the usage parameter controller 29 may be
adjusted in response to signals received in the form of cells, from the
network manager 26.
In previous systems, signals would only be supplied to the usage parameter
controller 29 when a customer's contract as modified, reflecting a change
in the customer's requirement for bandwidth. However, in the present
embodiment, the allocation of bandwidth to customers is dynamically
controlled so that, in its non-active state, a transmission path having
zero bandwidth capacity is effectively allocated for a customer. Thus, in
order to reflect this state, an instruction is issued by the network
manager 26 to the usage parameter controller 29 setting the first
threshold level to zero. Under these conditions, all cells received by the
usage parameter controller will be modified to cells of low priority, so
that it cannot be guaranteed that the cells will be transmitted through
the network. However, during these non-active periods, it is assumed that
no demand is being made of the network and that switching capacity may be
allocated to other users.
In the preferred embodiment, it is not necessary for a customer to perform
any signalling functions in order to initiate a call. The data itself is
identified by the dynamic bandwidth controller 29, and triggers the
generation of bandwidth requests to the network management or call
control. At the initiation of a transmission, all cells are changed to low
priority and this state remains true until bandwidth has been assigned to
the connection.
Referring to FIG. 3, low priority cells, during the initiation of
transmission, are supplied to the cell monitor 31 which writes these cells
to a first-in-first-out (FIFO) shift register for re-transmission, with
minimal delay, to the port 22. The shift register includes taps which
supply values to combinational logic which, as previously described, is
capable of identifying cell phase, with reference to the header error
check field.
On identifying The presence of cells within its shift register, the cell
monitor 31 generates a transmission active signal, which is supplied to
the processor 32 along with an indication of the VCI and the VPI
associated with the cells being transmitted. In response to this
information, the processor 32 will interrogate the attribute data store 34
so as to identify the bandwidth allocation associated with said
identifiers. In response to this enquiry, data is supplied back to the
processor 32, identifying the bandwidth provision for the particular
communication identified by the call monitor 31.
The processor 32 is arranged to configure control cells for transmission to
other elements in the network, and in response to the "transmission
active" signal generated by the cell monitor 31 and the data read from the
attribute data store 34, a message is assembled for transmission to the
network manager 26, requesting the establishment of bandwidth within the
broadband switch 21, facilitating communication between the input port 22
and the output port 23. In response to this request, the network manager
26 will determine whether this amount of bandwidth is available and, if
so, bandwidth is assigned has been the connection.
In addition, the network manager 26 also transmits a message to the port
25, for the usage parameter controller 29 which, in response to this
message, adjusts its threshold values such that cells are no-longer marked
as low priority provided that the raze is less than, or equal to, the
bandwidth now granted to the connection.
In some situations, a customer may wish to transmit important data where a
guarantee is required that the data will be received at its destination.
If the terminal transmits a sequence of trial cells all marked as high
priority then, under the scenario described above, it can be appreciated
that initial cells will have their priority level downgraded by the usage
parameter controller 29 until a state has been reached such that bandwidth
allocation is guaranteed through the broadband switch 21. Thus, at the
receiver, a succession of low priority trial cells may be received prior
to the reception of high priority cells, which are only transmitted after
bandwidth has been granted.
Thus, where a customer requires a guaranteed communication to be
established, it is possible for a receiver to examine the status of
received cells, and then to issue a signal back to the originating
customer when cells of high priority are being received. The receipt of
high priority cells will indicate that high priority cells will be
maintained and, under this condition, the originating customer may then
transmit the high priority data, secure in the knowledge that the high
priority cells will remain as such throughout the transmission through the
network; it only being the initial start up period where down grading
occurs. Thus, the network automatically responds to a request for
bandwidth provision without undergoing an initiation or signalling
routine. The trade-off for this functionality is that the priority level
of initial cells will be downgraded, and the duration over which this
downgrading occurs will depend upon the capacity of the network to
establish the bandwidth requirement and thereby issue modifying signals to
the relevant usage parameter control devices; thereafter, bandwidth
provision is guaranteed.
The cell monitor 31 is also capable of detecting the absence of cells
passing therethrough. As previously described, provision is made for the
identification of cell transmission, and the cell monitor 31 includes a
time-out circuit which will ensure that a periodic investigation is made
as to whether cells are continuing to be transmitted. When transmission
ceases, the cell monitor 31 will effectively time-out and supply a signal
to the processor 32 to the effect that transmission has ceased. In
response to this signal, the processor 32 will configure a message and
transmit this cell to the network manager 26 via the cell monitor 31. In
response to this message, the network manager 26 will allow re-allocation
of the bandwidth within the broadband switch 21, and issue a message to
the usage parameter controller 29, again re-setting the threshold level to
zero, such that, on the next iteration, initial cells will have their
priority level reset to low priority.
It will be appreciated that many logical connections may be achieved via
common physical links. Thus, at a particular input to the overall network,
an input port, such as the port 22, may receive cells relating to a
plurality of Logical connections; and, similarly, the dynamic bandwidth
controller 30 will control bandwidth allocation for each of these logical
connections.
The allocation of bandwidth has been described for the broadband switch 21
shown in FIG. 2, which is the first broadband switch encountered on entry
to the overall network of the type shown in FIG. 1. The bandwidth request
made by the dynamic bandwidth controller 30 will also result in
interrogations being made of each switch through which the connection
passes in order to establish the availability of bandwidth. Thus,
provision may have to be made at a plurality of broadband switches, before
a message is returned to the dynamic bandwidth controller 30, confirming
that bandwidth has been allocated, thereby ensuring that high priority
cells are transmitted through the overall network.
As previously stated, the header information is identified by the processor
32 and, in response to this information, attribute data is read from the
attribute data store 34, identifying the bandwidth allocated to that
particular channel. Thus, for example, the attribute data store 34 may
identify the channel as a 1 Mbit/s channel, a 5 Mbit/s channel or a 10
Mbit/s channel etc, depending upon the level of service contracted to the
customer.
A further feature provided by the dynamic bandwidth controller 30 is that
of allowing a customer access to whatever bandwidth is available at the
particular time when a request is made. Thus, when cells arrive from a
customer to whom this provision has been made available, an indication is
supplied to the processor 32, from the attribute data store 34, to the
effect that the cells are to be given whatever bandwidth is available for
providing a channel between the two communicating stations.
The processor 32 will construct a message and transmit this cell,
identifying the customers request, to the network manager 26. In response
to this request, the network manager 26 is required to determine the level
of available bandwidth from the communicating source to the communication
destination. The available bandwidth over the logical channel will be
restricted by whichever physical link has the minimum available bandwidth.
Thus, if a communications channel is constructed over three serial
physical links, a first of which has 10 Mbit/s available, a second of
which also has ten megabits available while a third only has 2 Mbit/s
available, the bandwidth available over the combination is only 2 Mbit/s
and the remaining 8 Mbit/s at the first and second stages of the link
cannot be employed because a bottleneck exists at the station which only
has 2 Mbit/s available. Thus, in response to the request made by the
processor 32, the network manager 26 will determine the total bandwidth
available, and return this information back to the dynamic bandwidth
controller 30.
A complication arises when provision is made for allocating whatever
bandwidth is available in response to a customer request. At the time the
request is made, the requesting customer is not aware of what actual level
of bandwidth will be available. Furthermore, the network is not aware of
the extent to which the requesting customer will require bandwidth. Thus,
the network will provide whatever bandwidth is available; and, under some
circumstances, this may be insufficient for the customer's requirements.
Under these conditions, it is necessary for the network to issue
instructions back to the customer, in the form of suitably constructed
cells, so as to inform the customer that congestion is imminent, and that
action must be taken to reduce the level of traffic supplied to the
network.
Cells of this type, instructing a transmitting customer's terminal to
reduce its output traffic, are generated by the processor 32 and supplied
to the originating terminal via the feedback controller 33. In order to
determine whether such a feedback instruction requires to be generated,
the cell monitor 31 includes buffers arranged to buffer incoming data of
this type, and to generate overflow signals when it is detected that data
is being written to the buffers at a faster rate than it is being read
from said buffers.
The cell monitor 31 of FIG. 3 is shown in more detail in FIG. 4. It should
be appreciated that, by its very nature of being capable of transmitting
cells in accordance with the available bandwidth, cells of this type will
be transmitted at different rates, depending on the bandwidth available
when the particular request is made, and that separate buffering devices
must be provided for each particular rate.
In theory, the level of available bandwidth would be continuously variable
and, in response to such a request, any number (within the definition of
the system) could be returned to the requesting processor. Under such
circumstances, in order to make full advantage of the available bandwidth,
it would be necessary to provide buffering devices for every possible
transmission rate, or to provide variable rate buffers for each logical
channel being transmitted through the system. Each of these alternatives
is undesirable.
As shown in FIG. 4, four physical buffers 41, 42, 43 and 44 are provided.
The first buffer 41 is arranged to buffer cells being transmitted at 1
Mbit/s, the second buffer 42 is arranged to buffer cells being transmitted
at 2 Mbit/s, the third buffer 43 is arranged to buffer cells transmitted
at 5 Mbit/s, and the fourth buffer 41 is arranged to buffer cells
transmitted at 10 Mbit/s. In practice, the actual number of buffers
provided will depend upon operating requirements, as will the actual data
rates for which they accommodate cells.
Cells are transmitted from customer equipment and supplied to the port 22
as a serial stream. However, in the cell monitor 31, the serial stream is
converted into parallel bit octets by a serial-to-parallel converter 45,
which in turn supplies these octets to a parallel shift register 46. The
shift register 46 is tapped, allowing a plurality of octets to be read
simultaneously by a logic circuit 47 which, as previously described, is
arranged to identify the presence of data transmission, the position of
headers within cells, and to supply header information to the processor
32. The processor 32 will, in turn, identify the stored bandwidth
allocation for the particular cell being transmitted and; in response to
this determination, will supply control signals to a switch 48.
If a cell is identified as belonging to a logical communications channel
for which the required bandwidth has been specified, the switch 48 is
arranged, in response to signals received from the processor 32, to supply
cells directly to a multiplexer 49. However, if the cells detected are of
the type which will result in a request for "whatever bandwidth is
available" to be made available for their transmission, cells supplied to
the switch 48 will be directed to one of the buffers 41 to 44, rather than
being supplied directly to the multiplexer 49.
After a request has been made by the processor 32 to provide "whatever
bandwidth is available", the network manager 26 will supply information
back, in the form of an appropriately addressed message, to the processor
32 identifying the level | | |
