Note: Descriptions are shown in the official language in which they were submitted.
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INTEGRATED TELECOMMUNICATION SYSTEM
ARCHITECTURE FOR WIRELESS AND WIRELINE ACCESS
FEATURING PACS RADIO TECHNOLOGY
FIELD OF THE INVENTION
This invention relates to an integrated
telecommunication system that provides both wireless and
wireline access. More particularly, this invention
relates to a system for providing both voice and data
telecommunication which is cost effective, upgradeable,
and capable of use in both wireline and wireless
environments.
BACKGROUND OF THE INVENTION
Various systems have been developed and implemented
to match the explosive demand for high-quality wireless
communication. Moreover, with the increased use of wide
area networks (such as the Internet), there has been a
tremendous demand for systems which support data
communication.
Personal Communications Systems (PCS) are now being
developed to meet these demands. PACS (Personal Access
Communications Systems) is one such PCS that was
developed to support voice, data, and video images for
indoor and microcell use. PACS utilizes digital voice
coding and digital modulation, and is designed to support
low-speed, portable use.
As shown in Fig. 1, PACS architecture comprises four
main components: fixed transceivers 4 or portable
transceivers 2 known as subscriber units (SUs); fixed
base units 6 known as radio ports (RPs); a radio port
control unit (RPCU) 8; and an access manager (AM) 10.
Each fixed RP 6 communicates with a number of SUs 2 and 4
through an interface A (the air interface) in a manner
which permits each SU to simultaneously access that port
on a multiplexed basis.
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In PACS, low power multiplexed radio links provide a
number of separate, fully duplex demand-assigned digital
channels between an RP and each of its associated SUs.
Each RP transmits bit streams on a pre-defined carrier
frequency. In turn, each SU that accesses an RP responds
by transmitting a burst on a common, pre-defined carrier
frequency. With licensed PACS, a large number of radio
frequency (RF) channels are frequency division duplexed
with an 80 MHz separation. A variant of PACS, PACS-UB
was developed within the United States for the unlicensed
PCS band within 1920 and 1930 MHz. PACS-UB utilizes time
division duplexing rather than the frequency division
duplexing utilized in the original PACS standard.
Some of the advantages of PACS arise from its
reliance on relatively small-sized base stations {RPs).
Being both small and relatively inexpensive, RPs can be
widely deployed on utility poles, on buildings, in
tunnels, indoors or outdoors, so as to provide more
comprehensive support for wireless access services. With
its relatively small power needs, an RP can be line or
battery powered.
Both PACS and PACS-UB permit wireline-quality voice
and data communications services at a price and with a
capacity approaching wireline techniques. These
standards are particularly well suited for use in several
environments, including: (1) wireless local loop
environments; (2) low mobility/high density public access
PCS environments ; and (3) in-building (residential or
business) telephony and data environments.
For wireless local loop environments and low
mobility/high density public access PCS environments,
PACS relies on a system architecture which is based on
Advanced Intelligent Network {AIN) and Integrated
Services Digital Network (ISDN) wireline network
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principles. AIN is intended to allow users to have a
single number for both wireless and wireline services and
to permit a more seamless handoff as a subscriber moves
from one location to another. One AIN architecture
consists of three levels: the intelligent level; the
transport level; and the access level. The intelligent
level contains databases for the storage of information
about network users. The transport level handles the
transmission of information. The access level provides
access for each user in the network and contains
databases that update the location of each user in the
network.
ISDN is a complete network framework which utilizes
common channel signaling (CCS), a digital communications
technique that provides simultaneous transmission of user
data, signalling data, and other related traffic
throughout a network. ISDN provides a dedicated
signalling network to complement the public switched
telephone network PSTN. It provides a network for
signalling traffic that can be used to either route voice
traffic on the PSTN or to provide new data services
between network nodes and the end-users.
While useful in the above-noted environments which
include AIN and ISDN capabilities, PACS architecture may
not be suitable for wireless loop or mobility PCS
applications where there is no existing wireline AIN or
ISDN infrastructure. Further, PACS appears to have
extremely limited applicability to in-building wireless
systems, especially in small business settings.
Within the small business environment, the Small
p Computer Systems Architecture (SCSA) may be utilized.
SCSA is an open industry specification for computer-based
telephony systems. The SCSA architecture consists of 32-
card nodes with local, non-blocking time slot interchange
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SCbus backplanes that are hierarchically connected in up
to 16-node systems via a SCxbus. A non-blocking SCbus
preassigns transmit slots at system configuration, thus
limiting the dynamic configurability of the 16.384 Mbps
(4048 octet/frame) SCbus.
In addition to having limited dynamic
configurability, node to node data traffic in an SCSA
system may require routing on three busses: the two
SCbuses in the two nodes and the interconnecting SCxbus.
Control messages are routed on a separate, multi-master
contention bus. Thus, a relatively high degree of
switching is required to provide hardware connectivity.
Apart from SCSA systems, other conventional
architectures often utilized in the small business
environment includes various key system and PBX
architecture commercially available from
telecommunications equipment manufacturers. Key systems
typically serve less than 125 lines; small PBXs typically
serve 125 to 1000 lines, medium PBXs 1000 to 10,000
lines, and large PBXs greater than 10,000 lines.
Frequently, different system architectures are applied to
products in each of these groups. Thus, it is extremely
difficult to modify an existing system to provide
additional lines as a number of user increases. As a
result, scalability is relatively limited with such
systems.
In view of the foregoing, there is a need for an
architecture which can furnish the advantages of PACS and
PACS-UB wireless access technology in a "village
telephony" environment (that is, one characterized by a
high density of low mobility users) (for PACS) and in an
in-building telephony and data environment (for PACS-UB),
particularly in a low-cost, modular way. To avoid making
any detailed wireline infrastructure assumptions, there
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is a related need for an architecture that is capable of
providing a "standalone" PACS capability, that is, one
that can exist without preexisting AIN or ISDN
architecture.
SUMMARY OF THE INVENTTON
In addressing these needs and others, we have
developed a design which recognizes the importance of
modularity and of integrated support for a large range of
telecommunications services. Modularity, both in terms
of system cost and in the amount of system hardware, is
an important attribute because a village telephony system
or an in-building voice and data system could span three
orders of magnitude in the number of supported terminals.
Further, in view of the explosive growth in demand for
data connectivity (fueled largely by Internet access), it
is desirable that the system be capable of supporting a
range of telecommunication services. Integrated support
for wireline access as well as wireless access is
extremely desirable, whether to provide for wireline
voice terminals in a business communication setting or to
achieve significantly higher data communication rates
than is feasible with PACS wireless technology.
It is therefore an object of our invention to
provide a telecommunication system that is both highly
cost effective for small scale applications (for example,
those having less than 80 lines), yet field upgrade-
expandable to applications having a significant number of
additional lines (for example, 30,000 lines). It is a
further object of the invention to provide an integrated
voice/data telecommunication system that is flexible
enough to handle low bandwidth (for example 64 kbps mu-
w law) speech as well as high bandwidth multimedia data
switching. Further, it is an object of the invention to
provide a low cost, standalone PACS system for "village
telephony" or "PACS-on-POTS" applications, as an
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alternative to requirements for PACS infrastructure when
appropriate wireless network facilities are unavailable.
As described in greater detail below, our invention
provides a significantly higher bus bandwidth (1.0486
Gbps) that is dynamically allocated, thus allowing the
system to take advantage of usage statistics. Further,
with our invention, all data and control traffic use a
common 32-bit wide backplane. A small system may be
implemented in a single card cage. Larger systems use
multiple card cages that are interconnected in a ring
arrangement via single, high-bandwidth, serial fiber
links. No switching is necessary in the hardware
providing inter-cage connectivity.
In contrast to prior art key systems, the invention
gracefully scales from applications requiring very few
lines (for example, less than ten) up to systems with
30,000 lines. Finally, the system backplane has
sufficiently large bandwidth to support high-speed
wireline connectivity to desktop computing stations. In
addition to voiceband-rate voice and data connectivity to
desktop and wireless voice and data terminal equipment.
Other advantages of the invention will be apparent
to those skilled in the art in view of the description
set forth below.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
Fig. 1 is block diagram of conventional PACS
architecture.
Fig. 2 is a block diagram of a telecommunications
system in accordance with an embodiment of the invention.
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Fig. 3 is a diagram of backplane frame structure in
accordance with the invention.
Fig. 4 is a diagram of a control channel in
accordance with the invention.
Fig. 5 is a diagram of address word bit assignments
in accordance with the invention.
Fig. 6 is a diagram of an address data word in
accordance with the invention.
Fig. 7 is a block diagram illustrating a multi-cage
system in accordance with the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following is a description of preferred examples
of the invention. First, a description of a single-cage
embodiment of the invention is described. As explained
below, this system is particularly well suited for use in
a wireless local loop environment, a "village telephony"
environment, and/or an in-building environment. A multi-
cage example is also discussed which illustrates the
scalability provided by the invention.
Fig. 2 is a diagram showing a single-cage example in
accordance with the invention. A telecommunications
system 50 includes a unit 75 which provides voice and
data access for various communication devices. As
explained below, this system provides wireless and
wireline voice and data communication among various types
0 of terminals arranged in different networks. In this
example, the system provides access among "stand-alone"
terminals and among; terminals in the PSTN; terminals in
a PACS-based wireless network; terminals in a wide area
network (WAN); and terminals in a local area network
(LAN).
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As shown, trunk lines 66 lead to a central office
(CO) switch which forms a part of the PSTN. Serial
interfaces 64 provide access to a PPP server 60, which
may connect to a LAN 62. This architecture similarly may
support a router (such as an Internet Protocol (IP)
router) that links with a WAN (such as the global
Internet).
One or more standalone terminals, such as a desktop
personal computer 58 may also access the system 50 for
data (or voice} transfer. Similarly, one or more voice
terminals, such as wired station 56, provides wireline
voice access.
As illustrated, the system 50 supports PACS or PACS-
UB architecture. One or more RPs 54, each of which may
service a plurality of terminals, such as portable
terminal 52, is connected to the PSTN and the other
networks illustrated through the unit 75. This
architecture is particularly suitable for a relatively
dense distribution of wireless low-mobility users.
The card cage 75 through which these
interconnections are made principally contains a
backplane bus 68 that connects a control processor card
72 and several peripheral cards 70, 74, 76, 78, 80,
82, and 84. The backplane bus 68 provides high-speed
communication among the various peripheral devices and
networks connected to the unit 75. Using the addressing
scheme described in greater detail below, the backplane
bus 68 provides a message stream or an infonaation stream
communication path between any two system entities under
control of the control processor card 72, hereinafter
referred to as the control unit (CUj.
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In this example, the peripheral cards comprise a
PSTN interface card 70, a plurality of RPCU cards 1 to N
(represented by cards 74 and 80), a wired station control
unit card (WSCU) 76, a hub controller card 78, a feature
card 82 and a data interworking peripheral card 84. A
general description of each of these peripheral cards is
now set forth.
The PSTN interface card 70 serves as the primary
network interface peripheral to support telephony
services. The trunks 66, which may either be analog or
digital, provide a line interface from a local exchange
central office. In addition to either analog POTS or
ISDN interfacing, the PSTN interface card is responsible
for transcoding speech between 32 kb/s ADPCM (used over
the air and on the backplane) and either analog waveforms
or 64 kb/s PCM. Several wire drops may interface with
the PSTN interface 70. However, if only two-wire drops
are provided, and the two-to-four wire hybrids are
located out in the plant or in the central office, then
this peripheral may also be required to implement echo
control measures.
With a single card-cage system (or where a chain of
card-cage control units are utilized in accordance with
the embodiment described below), the initial PSTN
interface card terminates analog POTS lines and make them
available as external line appearances for voice
terminals. Dialing information is communicated from the
system control unit card 72 to the PSTN interface card 70
via the backplane virtual control channel. Call progress
tones are digitized and passed in band via one of the
assignable backplane time slots back to the client voice
terminal.
The RPCU cards 74 and 80 provide a centralized
architecture to support the radio-specific functions
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described generally in reference to Fig. 1. In
accordance with the architectural philosophy of PACS and
PACS-UB, each RPCU services a plurality of RPs 54 which
in turn provide wireless access to several SUs 52. As
known in the art, the RPs 54 have limited functionality,
in order to allow high density coverage of a service area
at minimum cost. The RPs 54 provide a high performance
modem capability, translating downlink (RPCU to SU)
information streams from baseband to RF, and conversely,
translating uplink (SU to RPCU) information streams from
RF to baseband with error detection. As shown, the RPs
54 are interfaced to the RPCU peripherals (cards 79 and
80 in this example) via standard twisted pair
distribution wiring.
A twisted pair interface to the remote radio port
electronics supplies both a full duplex digital link and
DC power. In a large PACS-UB system, the remote port
electronics may be remote a significant distance from the
system controller. In order to increase the reliability
of the link, it is desirable to minimize the signaling
rate between the ends. For example, since the air
interface rate of 384 kb/s is shared in a time division
fashion, each half-duplex direction could use a FIFO
buffer to rate adapt the line interface to 192 kb/s.
The RPCU peripherals 74 and 80 terminate many of the
radio-specific PACS protocols. Each handles SU requests
for air interface resources, and makes requests for bus
and other peripheral resources, such as the network
interface via elements 64, 60 and 62. In addition,
because the RPCU maintains information about connection
status for all timeslots on the RPs that it serves, it
can provide high level information and instruction to the
RPs in order for the RPs to comply with spectrum use
regulations. In this example, a single RPCU peripheral
card, such as card 74 or 80, is capable of serving two
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single-carrier RPs or one dual carrier RP for a total of
eight full duplex voice quality (32 kb/s) channels.
In addition to including twisted pair and backplane
interfaces, the RPCU peripheral preferably contains a
dedicated microcontroller running a small real time
kernel. The processor provides the peripheral with the
intelligence necessary to communicate with the control
unit 72, to manage and communicate with the served RPs
and to terminate the higher layer protocols used in link
maintenance and call control algorithms.
One or more WSCU cards 76 support the use of wired
stations, illustrated by a single wired station 56.
While only one such wired station is shown in Fig. 2,
each WSCU card 76 can support up to eight stations in a
manner which is analogous to the eight full duplex 32
kb/s channels that an RPCU peripheral card can support.
In this example, the wired stations interface with
the WSCU card 76 via a single twisted pair that carries
phantom power for the wired terminal and time division
duplexed TDD digital data for both an in-band fast
channel (for example, 64 kb/s mu law PCM) and an out-of-
band slow, control channel (key presses to call
processing). The WSCU card 76 may, for example, use the
PACS layer 3 protocol message (type INFO) to communicate
keygress and hook status messages (for the uplink
direction) and to control the station display and request
user signalling or keypad input (for the downlink
direction). The peripheral could also use the same
implementation as the RPCU cards 74 and 80 for converting
keypress control channel messages into audio channel
DTMF, for example, for post-origination dialing
applications like voicemail system interaction.
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The data interworking peripheral 84 is also
consistent with previous architecture definitions for
data services in PACs. Functionally, the data
interworking peripheral 84 can be viewed as another
network interface peripheral, analogous to the line
interface peripheral 70, but used for nonvoice services
instead of voice services. For example, when an SU
signals to its serving RPCU to set up a data call, the
RPCU requests backplane resources to request service from
and communicate information streams to and from the data
interworking peripheral 84. The data interworking
peripheral then communicates via known data-specific
protocols to the data interworking function (IWF). The
IWF then handles the specific network interface protocols
required by the service. Preferably, the IWF should
support IP interworking, for access to both local IP-
based enterprise data networks and the global Internet.
CONTROLLER CARD 78:
The proliferation of powerful desktop computers and
the need to connect them has created significant demand
for computer networking hardware. In contemporary
business communication systems there are increasing
efforts to integrate computing and telephony hardware,
with a primary emphasis on new functionality (e. g.,
computer/telephony integration). For small businesses in
particular, it would be advantageous to provide both
voice and basic high-speed data connectivity in the same
system architecture, instead of requiring the use of
physically separate network hardware for each. Since the
system backplane has such significant capacity, it would
be feasible to dedicate a number of time slots to the
support of high-speed, shared-media data connectivity,
and employ a backplane peripheral to arbitrate the use of
this resource among connected desktop computers. This
peripheral would function very much like a standalone
ethernet lab controller, hence the label in Figure 2.
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FEATURE CARD 82:
Added feature functionality could be provided either
in system software or in hardware as "feature cards."
For example, a set of conferencing bridges could be
implemented on a peripheral card with a backplane
interface to a low three-way and other multiparty calls
to be established.
A fully loaded card cage will contain between 10 and
16 cards. For example, each card may support either
eight wired terminals or two PACS-UB RPs. This gives an
approximate capacity of eighty (simultaneous) lines per
cage, assuming a few of the physical card slots will be
dedicated to network interface functions.
The peripherals serving the terminals are supported
by the aforementioned high speed backplane bus 68 and a
fixed CU 72 that is provided in each unit 75. The CU 72
establishes voice and data circuit switch connections on
the high speed digital backplane bus 68 that employs time
slot interchange for data exchange. In this particular
example, up to thirty-one slave peripheral cards may be
plugged into a card cage having a fixed CU and high speed
backplane. This architecture provides a low cost system
which are particularly useful for relatively small
enterprises (such as one utilizing less than 80 lines).
At the same time, this architecture permits an elegant
growth migration path to much larger systems utilizing
greater than 20,000 lines.
As shown in Fig. 3, the backplane is thirty-two bits
wide and has 4096 time slots per frame. Each thirty-two
bit time slot is divided into four eight bit octets, each
defining four physical channels 0, 1, 2, and 3 (for bits
0-7, 8-15, 16-23, and 24-31, respectively). The frame
repeats every 125 ws, a rate corresponding to an 8 kHz
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voice sampling rate. At 1.0486 Gbps, the backplane
provides 16,384 {16k) octet slots per frame for data and
voice communication, with each octet in the frame for
providing a 64 kbps unidirectional channel.
In the last 256 time slots of the frame, all four
octets (channels 0-3) are dedicated to system control
data {reference 104); in the first 256 time slots
{reference 102), each lower octet (channel 0) is
dedicated to system control data. Thus, more than 15,000
assignable octets remain available for circuit switched
data. This would support, for example, 7,500
simultaneous simple full duplex voice conversations and,
in turn, could support 30,000 voice terminals, assuming
an activity factor of less than 25~.
Fig. 4 illustrates the control channel provided in
the last 256 time slots of each frame (designated by
reference numeral 104 in Fig. 3). Each of the last 256
time slots in frame N is paired with one of the first 256
time slots in frame N+2 and is dedicated to one specific
card cage. The upper octets 106 (data bits 24-31 and
data bits 16-23) are defined as address bytes that select
a specific register on a specific card. The arrangement
of the address bytes is illustrated in Fig. 5.
The next octet in each time slot (data bits 8-15)
comprises a data byte 108 written from the CU to the
slave card. The final octet 110 (data bits 0-7) is
reserved for unsolicited service requests from a slave
cards to the controller CU. In the first 256 time slots
of the frame (reference numeral 102 in Fig. 3), only the
lowest octet (data bits 0-7) is dedicated to the control
channel. It contains reply data bytes (112 in Fig. 4)
from slave cards to the system CU. All told, there are
five octets dedicated to each cage for bidirectional
communication with the CU. For example, physical channel
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0 (the first octet) of time slot 0 and physical channels
0-3 (all four octets) of time slot 3840 (slot 0 of the
last 356 slot block) are dedicated to cage 0. Likewise,
physical channels 1 and 3841 are dedicated to cage 1 and
so on.
As shown in Fig. 4, there is up to one frame of
delay from the output of the final cage N (hexadecimal
FF) to the input of cage 0 (hexadecimal 00). This delay
is included to compensate for the unpredictable aggregate
delay from the N parallel-to-serial-to-parallel
conversations (one conversion set per cascade) which take
place. Thus, the maximally configured system would have
255 card cages.
i5
To address a specific register on a specific card in
a specific cage, a combination of control channel slot
location and 16 bit address is used. For example, for
the first cage (cage 0) in a system, physical channels 2
and 3 of time slot 3840 are concatenated to provide 16
bits of addressing for data message communications. Bit
15 (the most significant bit) is the Read/Write bit, and
bits 10-14 are used to address one of the possible
thirty-two card locations in the cage. The remaining ten
bits (0-9) are available for peripheral card register
addressing (see Fig. 5).
Each cage has one of the last 256 time slots
dedicated as a control channel from the slave cards in
that cage to the system CU. The lower octet 110 in that
time slot is reserved for unsolicited service requests
from a slave card to the call processing CU. This octet
is a shared resource among the cards in the cage. A
wired-AND control line is provided for self-arbitration.
Each card is keyed to a particular physical slot. for
example, a card in physical cage slot number 3 will know
it is in slot 3 by examining five hard wired address
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lines passed to it via the backplane connector. If a
card in slot 1 desires to access the service request
octet, it must pull a service request arbitration control
line low sometime within the first sixty-four time slots
of the frame. If a card in slot 2 desires the service
request octet, it first examines the control line to
determine if card 1 has seized control of the octet,
then, if not, it will pull the control low sometime
within the second sixty-four slots of the frame. This
IO continues for the first thirty-two time slot groups so
that by the time the control channel arrives only one
card is granted access to the service request octet.
With this architecture, the data path from a slave
card register to the CU is provided by physical channel 0
of time slot 0. Because of the one-frame time delay
inherent in reverse communications from cages 1 to 254 to
cage 0, cards residing in cage 0 are required to delay
reply data for data reads for one frame, as mentioned
above in reference to Fig. 4. Thus, cage 1 would use
time slots 1 and 3841, cage 2 time slots 2 and 3841, and
so on.
The broadcast channel utilized in the invention is
critical to the functionality of the higher layer PACS
protocols. PACS-UB specifies common layer 2 and layer 3
protocols with PACS, to enhance interoperability between
licensed and unlicensed systems. At various times, the
fixed system infrastructure must stream various
information out to the portables over the air. This
information includes the system information channel, with
such items as port ID, system ID and access rights,
registration area ID, encryption modules, or messages to
change portable parameters; and the alert channel, on
which alert or "ringing" message are sent to inform
registered, inactive portables that an incoming call has
been received for them. Many of these items could be
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downloaded once from the system controller to the RPCU
peripherals at system power-up, and the formatting of
this information into the proper messages at the proper
times could be done by the RPCUs. However, the
controller must intervene in real time in order to
process incoming call requests and create alert messages,
that are then broadcast over the alerting area (which is
the entire system in this case). As explained above, our
method to implement a broadcast capability for the system
controller uses a control channel timeslot (slot number
255) for all broadcast messages (see Fig. 4). This
reduces the maximum number of cages supportable in a
large system by only one cage, to 255, but allows a
single message to reach all peripheral cards in all the
cages of a system. The implication for the peripheral
backplane interfaces is that any given peripheral in cage
254 must be capable of reading two consecutive time slots
on the backplane, since the broadcast timeslot and the
control timeslot for that cage are adjacent.
It is a feature of the invention that the system is
not limited to a single cage architecture. For example,
in the embodiment shown in Fig. 7, the system can support
up to 255 cages that are cascaded with a serial, high
speed, fiber link 150. This provides for greater than
20,000 lines in a maximally configured system, while at
the same time allowing a minimal system configuration
that could support up to eighty lines before requiring a
second cage and the cascading hardware. As the system
capacity requirements grow, additional cages may be
cascaded via a high speed (1.0486 Gb/s) serial link.
Each additional cage is connected sequentially in a ring.
In a preferred embodiment of the multi-cage system,
the cage controller card resides in card slot address 0
of each cage. It is responsible for providing the
backplane 32,768 MHz clock and a separate Frame Start
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pulse so that the slave cards can synchronize to the
backplane timing. The Frame Start line is high during
slot 0 of the frame and low otherwise. During slot 0 the
cage controller must place the Cage Number in channel 1
so that slave cards know which cage they are plugged into
and which control channel to monitor. A synchronization
bit pattern is placed in channels 2 and 3 of time slot 0
so that cascade cards may recover frame timing. The cage
controller card is either the system CU in cage 0 or a
Cascade Card.
The physical address of the card is hardware encoded
by 5 backplane lines that are tied to the appropriate
level for each card slot. In this way, a card can be
plugged in "hot" and within two frame periods (250 ~,s) it
will know what cage it is in and what physical slot it is
plugged into. In this way, the card knows which control
channel time slot to monitor and which address range to
respond to. Thus, a card can be plugged into an
operating system, and can automatically determine its
address in the system, and send a Service Request to the
main Control Unit for configuration.
Simplex time/channel assignments are communicated
with fourteen bits contained in two data octets. Two
MSB's define the physical channel and the 12 MSBs define
the time slot (see Fig. 4). Two hundred fifty-five cages
containing sixteen cards each with eight lines per card
capacity provides 32,640 line capacity. Four thousand
ninety-six time slots with four physical channels less
the 256 time slots by five channels of control results in
14,104 simplex channels or 7,552 duplex calls. As noted
above, assuming a 25~ occupancy this allows for 30,208
lines.
In a single cage system, slot assignments are all
made by call processing so only one device is permitted
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to write access a given time/channel octet. In multi-
cage systems, however, a contention exists between the
cascading card that is linked to the previous cage and
the other slave cards that reside in the same cage.
Time/channel octets are still assigned by one CU within
the cage, but the cascading card blindly repeats the data
found on the backplane of the previous cage with no
knowledge of call processing.
Further, a system with N cages results in a variable
delay line being inserted between the output of cage N-1
and the input to cage 0, which amounts to exactly one
frame of delay feeding back into cage 0 where the full
number of cages are used. This maintains the frame slot
structure when the loop is closed. This also inserts one
frame of delay for the cards in cages 1 through N-1 for
reply data to data read commands. Accordingly, cards in
cage 0 must recognize their location and insert the one
frame of delay themselves to be in an alignment with the
rest of the system.
A frame sync word located in the upper two octets of
the first time slot of the frame provides for frame and
time slot synchronization. Each cage has an M time slot
delay referenced to the timing of the previous cage,
caused by the parallel-to-serial-to-parallel conversion
required in the use of the 1.0486 Gb/s serial link.
Since the cage number is displayed in the second octet of
the first time slot in each cage, for card address
decoding, this will allow the cards in cage N-1 to
recognize the (N-1) M time slot delay in their local cage
timing relative to the system's cage 0 frame timing.
This relative timing information will also be used by the
RPCU cards to subtract integer slot times from the frame
start timing to allow for system-wide superframe and
hyperframe synchronization for all radio ports, as
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required by the PACS-UB protocol. The resulting time
accuracy will be well within 1 microsecond.
The frame sync word is sent non-inverted in the
first frame of a sixteen frame superframe. The
superframe structure allows for bit scrambling based on a
pn sequence sixteen frames in length. This prevents the
consistent appearances of the frame sync word that may be
embedded in user data.
In cages 1 through N-1, the high speed serial
receiver generates the bus clock signals and the frame
sync pulls and copies the frame data from the previous
cage. This receiver is fairly simple in that it is not
required to maintain knowledge of the call processing
state. For example, four wired-AND control lines may be
used to indicate to the receiver card whether it should
write to a specific octet in the frame or tri-state and
allow one of the lcfcal cards to fill the slot. In the
time slot prior to the one in which a slave card will
write to the backplane, the slave card must pull the
appropriate control line low indicating to the receiver
card that it should tri-state that octet in the next time
slot. This resolves the above-noted potential conflict
between a cascading card and other slave cards in the
same cage.
The above described CU functionality may be
implemented with a call control software running, for
example, on an Intel x86 family processor under a
commercially available kernel. Of course other
configurations may be used in accordance with the
invention.
Since the backplane switching fabric exchanges
groups of one or more 64 kbps data streams, the system is
very flexible with respect to the nature of the cards
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plugged into the cages. For example, data interworking
cards may be used to support wireless data as per the
PACs data architecture. Additional cards for voice path
bridging (e. g. for conference or three-way calling),
voice messaging, etc. can also be utilized.
Access Management cards may be added to large
systems to free up the main CU from tasks such as access
authentication and key management for link encryption.
In this regard, the PACS specification describes AM
functionality that provides a number of services to the
radio system. The prototype PACS-UB system implements a
portion of those functions as an integral component of
the call control software. These functions include:
establishment, maintenance and clearance of SU
registration records, and assignment of related radio
system; authentication and validation of SU registration
requests, possibly including decipherment of SU
credentials; initiation of SU alerting associated with
call delivery, in response to an incoming call service
request from a line interface peripheral; and regulation
of SU call origination attempts via registration records.
The PACS specification call for RPCU-to-AM communication
over ISDN channels, with the use of standard National
ISDN-1 messages. In a private access telephony/data
system, it may be desirable to provide a local AM
function to administer the private user group.
Other modifications to the system are also possible.
For example, it is conceivable that different RPCU
peripherals for different air interface protocols could
be specified, given the relative generality of the
backplane structure and the call processing software
functionality.
Detailed descriptions of preferred embodiments of
the invention have now been described in fulfillment of
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the above objects of the invention. It should be
understood that this description is merely illustrative.
Many additional variations and modifications which are
within the spirit and scope of the invention will be
apparent to those skilled in the art.
