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CROSS REFERENCE TO RELATED APPLICATIONS
This application is related to copending U.S. patent applications filed in
the names of M. Jurkevich and S. Bernstein on even date herewith, and
assigned to the same assignee as the instant application, as follows:
"Frame Compression in Integrated Services Networks", Ser. No. 07/676,535;
"Composite Frame Reconfiguration in Integrated Services Networks", Ser. No.
07/676,537;
"Bandwidth Seizing in Integrated Services Networks", Ser. No. 07/676,539,
now U.S. Pat. No. 5,164,938;
"Adaptive VCP Control in Integrated Services Networks", Ser. No.
07/676,540;
"Prioritizing Attributes in Integrated Services Networks", Ser. No.
07/676,515; and
"Fixed Interval Composite Framing in Integrated Services Networks", Ser.
No. 07/676,536.
BACKGROUND OF THE INVENTION
The present invention relates generally to packet switched digital
telecommunication networks, and more particularly to improvements in fully
integrated voice, data, and video (multimedia) communication services
through the shared use of transmission and switching facilities in an
integrated services network, including but not limited to networks such as
those defined by the CCITT ISDN (Integrated Services Digital Network) and
Broadband ISDN (B-ISDN) standards. The present invention provides for the
coexistence and integration of 1.2 kilobits per second (kbps) to 2.045
megabits per second (mbps) applications with B-ISDN (>2.048 mbps)
applications in a true multimedia network.
In recent years, the International Telegraph and Telephone Consultative
Committee (CCITT), a telecommunications industry international
standards-setting group, established Study Group 18 to undertake
cooperative planning of B-ISDNs. A principal aspect of B-ISDN is the
support it would offer to multimedia traffic applications, in which a
multiplicity of traffic component types including voice, data, and video
are to be communicated through the network. Each traffic component type
exhibits significantly different characteristics or attributes from the
others, and may have different characteristics among the members of its
own type or class. For example, pure data traffic components may be
interactive data, local file transfer data, facsimile data, and so forth,
which have different burst sizes, or "burstiness". Such different
attributes create differences in the requirements imposed on the network
and local equipment for efficient and effective handling of the traffic
component types in the communication between sources and destinations of
the traffic. For instance, isolated loss of voice packets may be tolerated
in telephone communications because the listener can comprehend the
overall tenor of the conversation despite these slight gaps. Although
quality suffers, the "human ear" is quite forgiving in these
circumstances. Delays between different voice packets, i.e., a change in
the sequence of the packets from source to destination, however, is
unacceptable. In contrast, transmission of data such as X.25 packets may
not be adversely affected by delay among packets in transmission, but the
loss of individual packets can prevent restoration of an entire message.
In 1988, CCITT Study Group 18 approved recommendation I.121 which
identified Asynchronous Transfer Mode (ATM) as the target solution for
implementing B-ISDNs. ATM is an asynchronous time division multiplexing
technique employing fast packet switching which communicates user
information through the network in fixed length packets (called "cells" in
the AT jargon) of 53 bytes each. One mission of the Study Group and its
Working Party 8 has been to standardize B-ISDN user network interfaces,
including one at 155 mbps and another at 600 mbps. The present focus of
the industry, however, is on fast packet (broadband) switching products at
1.54 to 45 mbps. For multimedia networks, the ATM scheme advanced by Study
Group 18 uses fixed size cells each of which is assigned to a single user
or traffic component type. Depending on user requirements at a given time,
considerable bandwidth may be unused because partially empty channels are
being transmitted.
In U.S. Pat. No. 4,980,886 titled "Communication System Utilizing
Dynamically Slotted Information" (the "'886 Patent"), assigned to the same
assignee as the present application, S. Bernstein discloses a multimedia
system in which each packet or frame has the same payload size, with a
fixed number of slots assigned to users, and in which the slot assignments
may be changed periodically to improve communication performance. These
are composite frames, packing several users/traffic component types into
each frame, rather than only one user per frame.
The invention disclosed in the '886 Patent departs from prior burst
switching technology by distributing user payloads among the available
slots in a multimedia frame based on the specified bandwidth requirements
of each user. The slots, which constitute portions of the available
bandwidth for each frame, are not necessarily occupied by respective users
from start to finish of a transmission. Instead, each user is guaranteed a
certain minimum amount of bandwidth and all users contend for any unused
bandwidth in each frame, according to their individual needs. The sending
side packet switch allocates bandwidth on a frame-by-frame basis, so that
users may be moved from one slot to another or to several slots in
midtransmission (i.e., on a "per burst" basis).
In the invention of the '886 Patent, unused bandwidth is not locked out; if
a particular user has nothing to send or is not using its minimum
guaranteed bandwidth (total slot or slots), the respective slot or portion
thereof is allocated to a user having need for it. As the circumstances
change, the allocations change. The receiving side packet switch monitors
the slots in each incoming frame to keep track of the arriving information
(data, voice, video, and so forth) and its sources, and to dispatch the
information to its proper destination. Thus, the invention of the '886
Patent provides an entirely controllable bandwidth in which users are
assigned priority rights to particular slots, but, depending on each
user's particular need for bandwidth, bursts or blocks of information are
temporarily allocated to unused slots or unused space in slots on a
frame-by-frame basis.
SUMMARY OF THE INVENTION
The present invention also utilizes a composite frame approach for fast
packet multimedia or integrated services networks, but instead of users
contending for bandwidth in each frame as in the invention of the '886
Patent, bandwidth is conserved and efficiently utilized in a different
way--namely, through techniques of frame compression and bandwidth
seizing. The concepts of bandwidth contention within a frame as disclosed
in the '886 Patent, and frame compression as disclosed in this application
and its related applications, are based in part on the relatively recent
concept of packet switching using fixed sizes. For example, older packet
switching techniques such as X.25 use variable size packets. The ATM
scheme employs fixed size cells (with its disadvantages), but is of only
recent vintage. The present invention utilizes variable size packets or
frames having fixed size channels, and a scheme by which frames may be
compressed to conserve bandwidth rather than employing techniques of
contention for the available bandwidth.
The terminology "composite data frame" or "composite frame" as used herein
refers to frames or packets which are composed of multimedia information
components, that is, different traffic component types assembled into a
single frame for transmission between subscribers through the network, and
which may utilize techniques of frame compression and bandwidth seizing
according to the invention. Within that terminology it will be understood
that the term "data" is used in a broad sense, encompassing all traffic
component types rather than being restricted to pure data only, although
in other instances herein the terminology "data" will be used in the
narrower sense.
It is a principal object of the present invention to provide an improved
method for multimedia frame configuration and transmission in integrated
services networks (ISNs), including those of the ISDN type.
It is another broad object of the present invention to provide improved
techniques for configuring the payload and control information of a
multimedia composite frame for communication between subscribers in an
integrated services network.
According to an important aspect of the present invention, all of the
various traffic component types in the data streams from multiple
subscribers are assembled into composite frames configured for
transmission to other subscribers through the integrated services network
in such a way as to provide optimum network utilization with minimum cost,
and at the same time to satisfy the individual performance requirements of
each of the particular traffic component types. The various subscriber
data streams are combined by traffic component type at the entry point to
the network, if destined for the same exit point. At the exit point, the
individual traffic component types are dispersed in separate directions
according to their predetermined destinations.
Each traffic component type, whether voice, video, low speed data, high
speed data or otherwise, possesses different characteristics or
attributes, such as length of burst, ability to tolerate delay, and so
forth. The network itself also has different characteristics or
attributes, such as the inherent tendency to introduce transmission delay,
which impacts on the attribute of each of the various traffic components'
capacity to tolerate delay. Another inherent or intrinsic network
attribute is the tendency to cause data loss depending on the nature of
the traffic in the network. The extent of data loss that a traffic
component can suffer and still allow the network to provide adequate
service also varies from traffic component to traffic component. The
phenomenon that different components of traffic in an integrated services
network are affected differently by transmission characteristics of the
network is, in and of itself, well known. Proposals in the prior art to
solve this problem, however, have proved inadequate.
The present invention, in part, is effective to decouple the traffic
component attributes and the network attributes and provide priorities for
individual network attributes on a traffic component basis. The principles
employed, in which all network attributes are controllable entities on a
per-traffic component basis, are to be contrasted with specialized network
approaches employed in present day telecommunications systems, in which a
single priority level scheme applies for all network attributes. The
latter are truly effective where there is only one traffic component and
only one or relatively few network attributes which apply to that
component, such as in an X.25 data network or a pure voice network. The
present invention includes assigning of priorities so that, for example,
voice traffic may be allowed to suffer data loss but no delays, while data
packets such as X.25 are permitted to suffer delay but no data loss. Such
conflicting requirements are resolved in one aspect by assigning traffic
component types to separate frames according to their respective
sensitivities and tolerances, while satisfying the need for rapid
transmission and increased throughput performance in the network.
It is therefore another object of the present invention to provide systems
and methods in an integrated services network by which the transmission
and throughput performance of various traffic component types is enhanced
by prioritizing them on the basis of their respective attributes in the
environment of the ISN, so that priority of transmission can be given to
those composite data frames containing the traffic component types
assigned the higher priorities during periods of traffic congestion or
when traffic flow otherwise requires control.
According to a feature of the present invention, the multimedia
communication method and system utilizes a composite data frame configured
with a multi-slotted payload, each slot being a channel which is allocated
to a subscriber having requirements for transmission of a particular type
of traffic component. The payload of the composite frame is divided into
multiple channels and the channels are grouped according to traffic
component type, with each grouping of plural channels in the frame
referred to herein a traffic component slot, or simply, T-slot. The frames
are composed with a particular configuration of channel assignments and
inclusions on a per call connection basis, dedicated for the duration of
the call connection, and may be reconfigured on request by subscriber
according to established priorities or based on traffic conditions such as
link congestion on the network.
Present day schemes provide static allocation of channels, and contention
for channels by active connections. In contrast, the present invention
allocates channels dynamically upon request at connection activation time
(and deallocates on call termination); and there is no contention for
channels--rather, the channels are dedicated to one connection for the
entire duration of that connection. The multimedia information (voice,
data, video and/or other traffic component type) to be transmitted from
multiple subscribers located at a network entry point is assembled from
the subscriber data streams into fixed size packets for consolidation in
the same size channels allocated to the subscribers in the payload of a
composite frame, provided that the various traffic components are all
destined for the same network exit point. That is, assignment of the
various subscriber data streams (of like or varying T-slot types) to the
payload of a composite frame for transmission through the network is
limited to those traffic components which share the same source node and
same destination node in the network.
Hence, another object of the invention is to provide a composite data frame
of variable size which is configured as a vehicle to convey through the
network data streams emanating from subscribers at a source endpoint node
of the network, in the form of a plurality of traffic component types, in
channels grouped and of fixed size according to traffic component type,
provided that the traffic components assembled within any given composite
frame are destined for the same endpoint node.
According to another feature of the invention, the composite frames are
assembled by fixed interval framing and transmitted through the network by
synchronous frame launching. To that end, each packet is shipped at a
predefined fixed interval of time relative to the timing of shipment of
the immediately preceding packet, without regard to whether or not each
channel in the packet is completely filled at that point in time. The
synchronous frame launching is used to build composite frames with fixed
channel sizes, which permits elimination of overhead control information
including specification of channel size, amount of information to be
received, and maximum amount of information to be transmitted on the
connection, typically associated with other existing composite frame
schemes. This reduces the amount of bandwidth required for transmission of
the frames.
Another object of the invention, therefore, is to provide a fast packet
switched integrated services network in which composite frames are
assembled and launched onto the network at fixed intervals of time, in
which the fixed interval is consistent throughout the network.
Decomposition information is transmitted to the exit point for the
composite frames in the network by specifying the number of channels being
allocated and the traffic component type for each, in a separate control
frame carried outside the composite data frames. The control frame is
built by the local endpoint node and sent to the remote endpoint node,
when a network subscriber requests a connection or termination of a
connection. Each control frame is built to contain only the delta change
from the prior frame format to the current frame format, identifying the
channels being added or released in the composite frame to the network
remote endpoint. When a channel or channels are added, the control frame
must specify the traffic component type of each such channel.
According to an important aspect of the invention, if a subscriber is not
fully active, in the sense that the information stream generated by that
subscriber to be transmitted to the remote endpoint within the composite
data frame being assembled at the local endpoint is inadequate to fill the
channel allocated to that subscriber, that channel is eliminated from the
frame. In this way, any unused bandwidth is compressed out of the
composite frame payload before the frame is launched into the ISN.
A further object of the invention, then, is to provide bandwidth
conservation in an integrated services network in which information is
conveyed in the form of composite data frames containing a plurality of
traffic component types, by a technique of compressing out of each frame
any unused bandwidth.
Frame compression is one of three interrelated aspects of the invention
which, however, may be employed independently in ISN FPS networks. The
other two of this triumvirate are reconfiguration of the composite frames,
and bandwidth seizing. As has been observed herein, the composite data
frame is configured with the traffic component types assigned to
respective separate groups of adjacent channels for each traffic component
type, so that each group is limited to channels transporting traffic
components of the same type, with each channel in a group assigned
entirely to a selected subscriber associated with the traffic component
type for that group. According to the invention, a composite frame is
reconfigured to modify the channel assignments when necessary to
accommodate priorities for traffic flow among the subscribers on a network
path (virtual circuit path) between entry and exit points (the two
endpoint nodes or fast packet switches of the virtual circuit path) of the
ISN. Bandwidth seizing is implemented when, because of priority
assignments among the various traffic component types relating to concepts
of guaranteed bandwidth, and traffic congestion on the network or more
specifically on links or trunks of the virtual circuit path of interest,
bandwidth allocation is taken at least in part from one or more traffic
component types and redistributed to another or other traffic component
types.
Traffic flow control is initiated at a node along the network path of
interest when a link on the path associated with that node exceeds a
predetermined link utilization threshold level indicative of traffic
congestion. Such flow control may be undertaken either when a request for
additional bandwidth (i.e., the making available of a channel) is made by
any traffic component type (or more specifically, a subscriber of that
traffic component type) which is below its minimum guaranteed bandwidth,
or when an unusually large number of subscribers at an endpoint node are
simultaneously seeking to transmit information for assembly into composite
data frames. The flow control affects those traffic component types which
are exceeding their minimum guaranteed bandwidth, starting with those of
lowest priority. For each composite data frame in the receive queue on the
congested link of the affected transit node along the network path the
node modifies a field in the header of the composite data frame to
indicate that flow control is being exercised.
A reconfiguration request control frame is issued at the endpoint node of
the subscriber needing additional bandwidth and meeting the necessary
predetermined criteria. This request for additional bandwidth for the
justified traffic component type will ultimately result in the seizure of
bandwidth from any traffic component type which is exceeding its
respective minimum guaranteed bandwidth in the composite data frames. At
the endpoint node launching the composite frames to which the request
applies, frame compression is implemented to unlock bandwidth by seizing
it from the traffic component type(s) targeted by the reconfiguration
request control packet. A less frequent posting of cells comprising
portions of the information streams from the affected subscribers, for
assembly into the composite frames, results in frame compression by
eliminating some or all channels of the traffic component types associated
with the excessive bandwidth usage in at least some of the composite data
frames. The freed bandwidth is thereby reallocated or redistributed and
the reconfiguration request control frame is dispatched to the next
transit node along the network path when the traffic profile indicates
that the associated link is no longer congested. The reconfiguration
request control frame is a packet analogous to a call setup packet, and is
transported along the same virtual circuit path as the composite data
frames, but acts as a control element to change the format of the
composite frames so long as the request is not blocked (rejected) by a
node along the path. The existing format of the composite frames is
contained in a template stored at each of the nodes along the path, and
another stored template indicates the amount of change of bandwidth which
is permitted for a particular traffic component type.
Therefore, yet another object of the invention is to provide a method and
system for selectively reconfiguring composite data frames in an
integrated services network as necessary for optimum bandwidth
utilization, traffic flow and throughput performance.
Still another object is to provide a scheme for selectively seizing
bandwidth from one or more traffic component types and redistributing the
seized bandwidth to one or more other traffic component types having a
greater priority for the bandwidth in an integrated services network.
According to still another aspect and feature of the invention, logical
connections are established between subscribers at endpoint nodes of the
ISN at the time of call setup, in the form of virtual circuits (VCs), and
between pairs of endpoint nodes to accommodate a multiplicity of VCs, in
the form of virtual circuit paths (VCPs), and the establishment, location
and relocation of VCP anchors at endpoint nodes within the ISN are
adaptively controlled according to the needs of the network and its
subscribers. Each endpoint node, or more precisely the point of
multiplexing within the node, may anchor more than one VCP. Each VCP not
only constitutes a logical connection between a pair of endpoint nodes,
but has a one-to-one coupling with the composite data frame transported on
it.
Information concerning each VCP anchored at a particular endpoint node (a
fast packet switch) is stored at that node. In some instances a VCP is
anchored at the trunk side of the switch fabric, and in other instances a
VCP is anchored at the subscriber side of the switch fabric. The decision
on where to anchor the VCP in these instances is based on the traffic
patterns between the source and destination endpoint nodes, and includes
such factors as whether the VCs to be multiplexed terminate on the VCP
anchor node, whether all trunk line subsystems (TLSs) and subscriber line
subsystems (SLSs) at the endpoint node have the capability of anchoring a
VCP, and whether the subscriber data stream will pass through the switch
fabric not more than once (except in the case of local switching).
The choices of whether to have multiple parallel VCPs between endpoint
nodes and of where to locate the VCP anchor(s) within a particular
endpoint node, are determined by the opportunity to multiplex VCs onto the
VCP. Periodic reevaluation is performed within the ISN for optimal VCP
anchor locations and VC loading (i.e., number of VCs multiplexed). As
network traffic conditions change over time, the invention implements
adaptive relocation of the VCP anchor to the optimal location for those
conditions. Each endpoint node is made capable of rerouting VCPs,
relocating VCP anchors, consolidating VCP anchors, and even subdividing a
VCP. As the VC load increases between a pair of endpoint nodes, multiple
SLS-anchored VCPs are consolidated into a single TLS-anchored VCP which
uses the network-wide frame launch period. A TLS-anchored VCP may be
converted to an SLS-anchored VCP when the VCP traffic load drops to a
level in which the payload/header ratio of the composite data frames is
unacceptably small. An existing VCP may be rerouted/reconnected if the
existing route is not optimal for the network topology or traffic
conditions.
According to this aspect of the invention, anchor relocation is triggered
by either (1) relocation on demand, or (2) periodic relocation. In
relocation on demand, anchor location is reevaluated during each VC call
request from a subscriber. In periodic relocation, the relocation occurs
at a fixed time or time interval. Periodic relocation is somewhat less
likely to result in thrashing between anchor locations, than relocation on
demand.
Accordingly, it is another object of the invention to provide methods and
systems for adaptive control of VCPs in an integrated services network
designed to transmit a multiplicity of traffic component types between
endpoint nodes of the network within configurable composite data frames
via VCPs established as logical connections between pairs of the endpoint
nodes.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and still further objects, features, aspects and attendant
advantages of the present invention will become apparent from a
consideration of the following detailed description of a presently
preferred method and embodiment of the invention, taken in conjunction
with the accompanying drawings, in which:
FIG. 1 is a simplified diagram useful for explaining some of the basic
concepts of the integrated services network environment in which systems
and methods of the present invention may be used;
FIG. 2 is a simplified block diagram of the basic structure of a packet
switch useful for implementing certain concepts of the invention;
FIG. 3 is a block diagram of a pair of endpoint fast packet switches
establishing a call connection, useful to explain source and destination
designations on a VC or VCP;
FIG. 4 is a block diagram illustrating the relationship of VCs to VCPs;
FIG. 5 is a representation of an exemplary composite data frame according
to the preferred embodiment and method of the invention, with a fixed
payload size and composition accommodating a plurality of traffic
component types;
FIG. 6 is a simplified comparison of three different packet types, the
composite data frame according to the preferred embodiment and method of
the present invention being shown in part (a), and the ATM cell and LAPD
frame of the prior art being shown in parts (b) and (c), respectively;
FIG. 7 is a simplified block diagrammatic representation of a VCP with
synchronous frame launching according to the invention;
FIG. 8 is a representation of a composite data frame which provides an
illustrative example of payload size for a plurality of highly active
subscribers;
FIG. 9 is a set of exemplary charts illustrating the disposition of
bandwidth allocation requests (FRRs) under various traffic conditions,
i.e., BW grant/reject scenarios;
FIG. 10 is a simplified diagram of a VCP anchor EFPS illustrating the
launching of composite data frames utilizing the preferred frame
compression method of the invention;
FIGS. 11(a)-(d) are sequences of frame processing diagrams illustrative of
the initiation of flow control through bandwidth seizing according to the
invention;
FIG. 12 is a block diagram illustrating the technique for anchoring a VCP
in an EFPS;
FIG. 13 is a block diagram useful for explaining a local switching example
in VCP anchoring;
FIG. 14 is a graph illustrating a hypothetical case of the VCP anchoring
process in real time;
FIGS. 15 and 16 are flow charts indicative of the processing required for
adaptive anchoring of VCPs with relocation on request for a channel and
release of a channel, respectively;
FIG. 17 is a flow chart illustrating the A bit set-up procedure for
bandwidth seizing;
FIG. 18 is a table indicating an exemplary link/T-slot profile for A bit
set-up conditions in conjunction with bandwidth seizing;
FIGS. 19(a) and (b) are flow charts illustrating the B and C bits set-up
procedure for frame composition at the source node, and the PFC field and
payload analysis for frame decomposition at the destination node; and
FIG. 20 is a simplified block diagram illustrating the retrieval and
delivery of data from the received composite data frames by the
destination node.
DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENT AND METHOD
Referring to FIG. 1, a fast packet switch (FPS) network serving as an
integrated services network (ISN) 10 of a type in which the present
invention is employed transports multimedia information in data frames or
packets, each possibly containing a plurality of traffic component types.
The frames are transported at fast packet speeds between a pair of
subscribers at endpoints of the network, such as endpoints A and B.
Network 10 typically has a multiplicity of endpoints A, B, C, D, etc.,
each serving a plurality of subscribers, such as 11-1, 11-2, . . . , 11-n
at endpoint A and 12-1, 12-2, . . . , 12-n at endpoint B. The actual
number of subscribers served at the various endpoints of the network may
differ from endpoint to endpoint.
According to an aspect of the invention, an endpoint fast packet node or
switch (EFPS) is located at each endpoint, and a transit fast packet
switch (TFPS, or sometimes referred to herein simply as a transit switch)
is located at each of a multiplicity of intermediates nodes such as 13, 14
and 15, of network 10. Each transit switch accommodates a plurality of
transmission links or trunks within network 10. Thus, a packet launched
from endpoint A to endpoint B, for example, may travel through trunks 16,
17 and 18 across transit switches 13 and 14, or, depending upon the
traffic conditions, through trunks 16, 19, 20 and 18 across transit
switches 13, 15 and 14. Each EFPS and TFPS of the network is a packet
switch in the form of a communication processor, but the EFPS and TFPS
differ from one another in implementation or the algorithms they
implement, as will be explained presently.
A logical connection established between two subscribers of the integrated
services network through ordinary call set-up procedures is referred to
herein as a virtual circuit (VC). For example, a VC is established between
subscribers 11-1 and 12-3 for a call (communication session) between the
two, and remains in place for the duration of that call. To reduce
individual call processing, a plurality of VCs which share a single
source-destination EFPS pair may be routed (actually, multiplexed) by
defining an end-to-end network path for them. Each such network path
constitutes a single physical link referred to herein as a virtual circuit
path (VCP). Thus, each VCP defines a logical connection between a
particular pair of EFPSs such as the EFPS at endpoint A and the EFPS at
endpoint B, or more specifically, between the points of VC multiplexing
within the two EFPSs, in contrast to the logical connection between two
subscribers defined by a VC.
A simplified block diagram of the basic switch or switching node structure
22 usable for each EFPS or TFPS is shown in FIG. 2. The different
functionalities of the switch 22 are accommodated by the manner in which
connections are made in the Switching Fabric Subsystem (SFS) 24, as will
be described presently. SFS 24, Subscriber Line Subsystem(s) (SLS) 25 and
Trunk Line Subsystem(s) (TLS) 26 provide the major infrastructure of the
switch. SLS 25 includes one or more Universal Control Units (UCU) 27 each
of which is associated with one or more Subscriber Processing Units (SPU)
28, and if desired, a Port Multiplexer/Controller (PMC) (not shown). The
SPU(s) 28 and associated UCU 27 communicate via a system peripheral bus
30. The PMC may be used to provide extended multiplexed access and control
to SFS 24.
Each SLS 25 supports system protocols, provides access to network
subscribers (which, for example, may be individual telephone, T1 trunk,
PBX signal, computer and/or other devices, lines or signals) on lines such
as 31, 32, 33 and 34 at the endpoint where switch 22 is located (if the
switch is used in the EFPS mode or functionality), and provides the
interface to the SFS 24. The SPU 28 is implemented to provide access,
support and control for the designated category of each of the subscriber
lines, maintain intelligent interface to the associated UCU to provide
flow control and network management functions bidirectionally on the
peripheral bus, and perform all necessary native protocol emulation.
The UCU 27 is implemented to provide FPS internal protocol support in
either of two modes, a tandem mode or a stand-alone mode. In the tandem
mode, two UCUs share responsibility for configurable frame formatting and
dispatching. Toward that end, the UCU in the SLS 25 sends subscriber data
streams to an associated UCU in the TLS 26 for composition of the frame
payload. In the stand-alone mode, the UCU in the SLS handles the entire
process. In a sense, the UCU acts as a concentrator, receiving data from
the various subscribers via the SPUs, concentrating the data, providing
the necessary levels of functionality, and presents the data to the
switching fabric (SFS) for routing to a TLS and subsequent transmission to
the external world.
TLS 26 also has UCU(s) 36, which provides the functionality described above
for the SLS/UCU(s), and Trunk Processing Unit(s) (TPU) 37, which provides
access, support and control for the FPS trunk lines such as 38 and 39, and
a physical interface to the associated UCU for frame transmission, error
detection and correction, and synchronization. For example, the data from
the SLS 25 is received at the TLS 26 after traversing the switching
fabric, is collected by the UCU 36, composed in the frame payload and
presented to the TPU 37 for transmission to the next node.
Several different connection scenarios--SLS to SLS, or SLS to TLS, or TLS
to TLS--in the switching fabric are available (shown in dotted lines in
FIG. 2) according to the desired use of the switch. The connection of TLS
to TL provides transit switch (TFPS) functionality. An SLS to TLS
connection provides endpoint node (EFPS) functionality from the subscriber
to the trunk; and SLS to SLS connection provides functionality internal to
the node from one subscriber to another subscriber.
In the exemplary embodiment each SLS 25 and TLS 26 supports T1/T3
interfaces because this BW range is more suited to effective
implementation of the composite frame, but other interfaces are not
precluded. At T1/T3, the data stream at the SFS should be .ltoreq.1.544
mbps (2.048 mbps in European standard).
It is desirable at times to refer to "source" and "destination" or to use
other, but analogous, terms to identify the two sides of a logical
connection--whether in reference to subscriber connections (VCs) or EFPS
connections (VCPs). The two sides of a connection will also be referred to
sometimes herein as the local side and the remote side. At times, the
remote side may be the destination side; and at other times, the remote
side may be the source side. In the architecture for VCPs according to the
present invention, however, the source side of the VCP connection is
determined (i.e., designated) at the time that the particular VCP is
created.
For example, referring to FIG. 3, a trunk line subsystem (TLS) 40
associated with EFPS 41 is implemented and organized to recognize the need
to build a VCP upon receipt of a number of subscriber connection (VC)
requests destined for the same endpoint EFPS 43, from subscriber line
subsystems (SLSs) 44. At that point, TLS 40 initiates a VCP call request
(CR) and sends it to the "destination" TLS 45 associated with EFPS 43. If
TLS 45 responds to the CR with a call accept (CA), which will depend upon
customary considerations for establishing a call, a VCP is established
between the two endpoint EFPSs. Because the CR originated from the EFPS 41
side of the connection, that side is thereafter referred to as the
"source" side of the VCP, and the other side--the EFPS 43 side--is termed
the "destination" side, of this particular VCP.
The concept of source and destination sides of the connection is useful for
a variety of reasons. For example, if the connection of interest were to
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