Router and method for use in a communication system

A router (200) comprising a router controller (203) coupled to a packet-switched interface (202) and a circuit-switched timedivision multiplexed interface (204) allows framed packet-data to be conveyed through a circuit-switched system, Inbound address information (208) included with inbound framed packet-data (207) is used by the router controller to route inbound packets (209) to associated inbound time slots (211). Conversely, the router controller determines outbound address information (218) for outbound packets (216) based on identifications (214) of associated outbound time slots (213). Such a router can be incorporated into a communication system 700. Sites (701-702) that transceive framed packet-data are coupled to a circuit-switched digital cross-connect switch (705) via the routers. A system controller (706) controls the circuit-switched digital cross-connect switch such that the associated inbound time slots are broadcast to at least the associated outbound time slots.

FIELD OF THE INVENTION 
The present invention relates generally to communication systems and, in 
particulars to a router and method for using the router in a dispatch 
communication system. 
BACKGROUND OF THE INVENTION 
Communication systems comprising packet-switched infrastructures are known 
in the art. FIG. 1 is a block diagram of a typical configuration for a 
wireless dispatch communication system 100 that allows one source of 
information to be concurrently distributed to many destinations and 
incorporates a packet-switched infrastructure in accordance with the prior 
art. 
The wireless dispatch communication system 100 comprises a packet-based 
switch 101, base stations 103-105, a controller 107, and a packet 
duplicator 109. Packet-switched communication links 111-114 are used to 
couple the base stations 103-105 and the packet duplicator 109 to the 
packet-based switch 101. Receiving information from a wireless 
communication unit (such as a mobile or portable radio; not shown), a 
first base station 103 generates inbound framed packet-data. For the 
purposes of illustration, it is assumed that this inbound framed 
packet-data needs to be routed as outbound framed packet-data to two other 
base stations 104-105 in order to complete the communication. To this end, 
the first base station 103 signals the required destinations (i.e., base 
stations) to the controller 107 via a control link 116. Each of the base 
stations 103-105 is linked to the controller, although only one such link 
is shown. 
The inbound framed packet-data is sent to the packet-based switch 101, 
where it is automatically routed to the packet duplicator 109. Responsive 
to the signals received from the first base station 103, the controller 
107 instructs the packet duplicator 109 to generate two identical copies 
of the inbound framed packet-data. Using routing information supplied by 
controller 107, the packet duplicator 109 addresses the new packets (now 
considered outbound packets) with the addresses of the two other base 
station 104-105. The packet duplicator 109 sends the outbound packets to 
the packet-based switch 101, which is turn routes the outbound packets to 
the other base stations 104-105 in accordance with the appended addresses. 
Currently, however, packet-switched systems of the sort described above are 
not capable of communicating with non-packet-based communication systems 
or equipment. For example, there currently is no method for connecting 
circuit-switched time-division multiplexed dispatch equipment to a 
packet-switched system. Conversely, there is currently no way to connect 
packet-based equipment, such as base stations, to a circuit-switched, 
time-division multiplexed infrastructure, e.g., a SMARTZONE infrastructure 
by Motorola, Inc. Therefore, it would be advantageous to provide a system 
that allows the interconnection of packet-switched and circuit-switched 
equipment. In particular, it would be advantageous to provide an apparatus 
and method that allows framed packet-data to be routed and copied for 
concurrent distribution to many destinations using a circuit-switched 
infrastructure. These destinations can be serviced by either 
packet-switched or circuit-switched infrastructure.

DESCRIPTION OF A PREFERRED EMBODIMENT 
The present invention provides a method and apparatus for routing 
addressable, variable-length, framed packet-data, (hereinafter referred to 
as framed packet-data) in addition to non-addressable, fixed length data 
using fixed-length framing (hereinafter referred to as time slots), 
through the use of a unique router and circuit-switched infrastructure. 
The router comprises a router controller coupled to a packet-switched 
interface and a time-division multiplexed interface. Inbound address 
information included with inbound framed packet-data is used by the router 
controller to route inbound packets to associated inbound time slots. 
Additionally, the router controller determines outbound address 
information for outbound packets, received in associated outbound time 
slots, based on identifications (i.e., which physical time slot the 
outbound packets were carried in) of the associated outbound time slots. 
With such a router, a circuit-switched infrastructure can be used to copy 
and route packet-data to multiple destinations. Sites that transceive 
framed packet-data are coupled to a circuit-switched digital cross-connect 
switch via the above-described routers. Under the control of a system 
controller, the circuit-switched digital cross-connect switch broadcasts 
(i.e., copies) the associated inbound time slots, an octet at a time, to 
at least the associated outbound time slots. In this manner, communication 
is provided between at least a pair of packet-based sites using a 
circuit-switched infrastructure. 
The present invention can be more fully described with reference to FIGS. 
2-8. FIG. 2 is a block diagram of a router 200 in accordance with the 
present invention. The router 200 comprises a packet-switched interface 
202, a router controller 203, and a circuit-switched time-division 
multiplexed interface 204. The router 200 would typically be implemented 
using a customized, microprocessor-based platform, as known in the art. In 
practice, the packet-switched interface 202, the router controller 203, 
and the circuit-switched time-division multiplexed interface 204 could be 
most efficiently implemented as software algorithms carried out using 
memory and processors (not shown), as known in the art. 
The packet-switched interface 202 receives inbound framed packet-data 207 
and transmits outbound framed packet-data 217. The term "inbound" 
hereinafter describes data, in any form, that is generally being sent from 
a site to a switch, whereas the term "outbound" hereinafter describes 
data, in any form, that is generally being sent from a switch to a site. 
Both the inbound and outbound framed packet-data 207, 217 conform to any 
of a number of well-defined packet data protocols, such as high-level data 
link control protocol (HDLC) or frame relay In the preferred embodiment, 
the packet-switched interface 202 comprises an framed packet-data 
interface. 
The circuit-switched time-division multiplexed interface 204 transmits 
associated inbound time slots 211 and receives associated outbound time 
slots 213. The term "associated" is used to describe a one-to-one 
correspondence between packets coming from/going to a given source and 
time slots within a circuit-switched time-division multiplexed 
infrastructure. The circuit-switched time-division multiplexed 
infrastructure relies on fixed periodic framing structures to organize the 
information it carries. Time slots are allocated 8 bits (one octet) every 
framing period. The time slots can be considered a physical address, 
assigned in a predetermined, fixed fashion. The circuit-switched 
infrastructure does not append address information to the packets being 
routed through the circuit-switched infrastructure. 
The time slot octets are transferred through the circuit-switched 
infrastructure in a fixed, periodic framing format. In this manner, the 
circuit-switched interfaces can determine the individual time slots based 
on the fixed time relationship of the time slots to the fixed, periodic 
framing structure. In the preferred embodiment, the circuit-switched 
time-division multiplexed interface 204 comprises a non-addressable serial 
data interface that uses fixed framing length and fixed data length 
protocols, similar to the North American DS-1 standard or the CEPT 
standard E-1. 
The router controller 203 divides the inbound packets 209 into inbound 
sub-packets 210, which are allocated to the associated time slot. The 
allocation of the sub-packet (one octet of the entire inbound packet) is 
based on inbound address information 208 as determined by the 
packet-switched interface 202. In the opposite direction, the router 
controller 203 routes outbound sub-packets 215 as outbound packets 216 
based on identifications of the associated outbound time slots 214. 
Additionally, the router controller 203 determines outbound address 
information 218 based on the identifications of the associated outbound 
time slots 214. In the preferred embodiment, the router controller 203 
performs the routing functions on both inbound and outbound data using a 
routing table 205. 
The routing table 205, which is typically stored in memory (not shown), is 
unique to each site supported by the router 200 and describes the 
one-to-one correspondence between framed packet-data addresses and their 
associated time slots. In this manner, the router controller 203 can 
determine an identification for an associated inbound time slot 211 based 
on the inbound address information 208 and/or can determine outbound 
address information for outbound framed packet-data 217 based on the 
identification of an associated outbound time slot 214. For example, at 
any given router, the routing table could map inbound packets received by 
the router with packet address #2 to time slot #2, and in the opposite 
direction, outbound time slot #4 to packet address #4. In the preferred 
embodiment, the inbound address information 208 comprises the framed 
packet-data addresses or the data link connection identifier (DLCI) 
appended to each inbound framed packet-data. Likewise, the identifications 
of the associated outbound time slots 214 comprise framing information 
used in the time-division multiplexed scheme implemented by the 
circuit-switched time-division multiplexed interface 204. 
FIG. 5 is a flowchart illustrating operation of the router 200 with respect 
to inbound framed packet-data. At step 501, the router receives inbound 
framed packet-data. As described above, the inbound framed packet-data is 
received by the packet-switched interface 202. This is further illustrated 
in FIG. 3. In particular, a complete inbound frame of packet-data 301 is 
shown. The additional parts that typically make up a frame, such as the 
start and stop flags, headers, payload field and error checking codes, are 
not shown. A previous frame of packet-data 305 and subsequent frame of 
packet-data 306 are also shown. As depicted in FIG. 3, the inbound packet 
301 comprises a plurality of inbound sub-packets 308 In practice, each 
inbound packet can comprise up to thousands of inbound sub-packets, each 
inbound sub-packet comprising an octet (i.e., a group of eight bits) of 
information. 
At step 502, the inbound packets from the framed packet-data are provided 
by the packet-switched interface to the router controller independent of 
the outbound address information The outbound address information from the 
framed packet-data is separately provided to the router controller. 
At step 503, the router controller divides the inbound packets into 
sub-packets and routes the inbound sub-packets to associated inbound time 
slots based on the inbound address information provided at step 502. As 
described above, the inbound address information is used to access a 
routing table which provides information regarding the appropriate 
associated inbound time slot. This is illustrated in FIG. 3, where, for 
example, a first inbound sub-packet is routed to a second time slot (TS 2) 
in a first frame of time slots 302. Each frame of time slots 302-303 
comprises N time slots demarcated by framing information 310-311. The 
router controller performs this by providing the sub-packet to the 
circuit-switched time-division multiplexed interface 204 at times 
corresponding to the particular associated inbound time slot. The router 
controller has knowledge of when each associated time slot occurs based on 
the framing information 310-311 for each frame of time slots, as provided 
by the circuit-switched time-division multiplexed interface. 
At step 504, the circuit-switched time-division multiplexed interface 
transmits the inbound sub-packet, received at step 503, in the associated 
inbound time slot. Thus, the circuit-switched time-division multiplexed 
interface performs the actual physical layer implementation of interfacing 
the router to the circuit-switched infrastructure. For each inbound 
packet, steps 503 and 504 are repeated until all inbound sub-packets have 
been transmitted. This is illustrated in FIG. 3 where the last inbound 
sub-packet of the inbound packet is routed to the second time slot in an 
m'th frame of time slots 303. Because there is a unique one-to-one 
correspondence between inbound framed packet-data addresses and time 
slots, the sub-packets retain their identity as they are conveyed through 
the circuit-switched infrastructure without the inbound addresses. That 
is, packets can be reconstructed (as outbound packets) from the 
sub-packets and later routed based on the identification of the time slot 
from which the sub-packets were taken, as described below. 
FIG. 6 is a flowchart illustrating operation of the router 200 with respect 
to outbound framed packet-data. At step 601, the router receives, via the 
circuit-switched time-division multiplexed interface 204, associated 
outbound time slots. Each outbound time slot comprises (conveys) an 
outbound sub-packet. 
Each outbound sub-packet comprises the same amount of information as each 
inbound sub-packet described above (i.e., an octet). This is further 
illustrated in FIG. 4. Frames of time slots 402-403 are received. Each 
frame of time slots is demarcated by framing information 410-411. At step 
602, the circuit-switched time-division multiplexed interface provides the 
outbound sub-packets, received from the outbound time slots, to the router 
controller. Additionally, the identifications of the associated outbound 
time slots are provided to the router controller using the framing 
information 410-411. At step 603, the router controller constructs 
outbound packets from the outbound sub-packets. To properly reconstruct 
the outbound packet, the sub-packets included in similarly identified time 
slots (i.e., all sub-packets from the first time slot in each frame of 
time slots) are used. Step 603 is repeated until a complete packet is 
formed. This is shown in FIG. 4 where the outbound sub-packet from the 
first time slot (TS 1) of the first frame of time slots 402 is used as the 
first outbound sub-packet of an outbound packet 401. The outbound packet 
401 is complete when the outbound sub-packet from the first time slot (TS 
1) of the m'th frame of time slots 403 is used as the last outbound 
sub-packet of the outbound packet 401. 
At step 604, the router controller determines the outbound address 
information for the outbound packets formed at step 603. As described 
above, the outbound address information is determined from the routing 
table based on the identification of the associated outbound time slots 
used to provided the outbound sub-packets. For example, an outbound packet 
constructed entirely from outbound sub-packets received in a first time 
slot of each frame of time slots, the associated outbound address 
information could correspond to framed packet-data address #1. The 
resulting outbound packets 216 and outbound address information 218 are 
forwarded to the packet-switched interface 202. 
At step 605, the packet-switched interface appends outbound address 
information from step 604 to the outbound packets formed at step 603. The 
packet-switched interface transmits the outbound framed packet-data in 
accordance with the packet-switched protocol Because there is a unique 
one-to-one correspondence between outbound framed packet-data addresses 
and time slots, the reconstructed outbound packets accurately represent 
the packets originally sent as inbound packets, as discussed below. 
Using the router described above, a one-to-many dispatch communication 
system having a circuit-switched infrastructure can be used compatibly 
with framed packet-data. FIG. 7 is a block diagram of a communication 
system 700 that incorporates routers as described above. The communication 
system 700 comprises at least two routers 703-704 in communication, via 
packet switched links 709-710, with an associated one of at least two 
wireless communication sites 701-702. The routers are in communication, 
via non-addressable serial data links 712-713 that use fixed framing 
length and fixed data length protocols (such as T-1 or E-1 links, as known 
in the art), with a circuit-switched digital cross-connect switch 705. The 
circuit-switched digital cross-connect switch 705 operates on a 
time-division multiplexed basis using time slot interchange technology and 
is capable of supporting 960 time slots. A suitable circuit-switched 
digital cross-connect switch 705 is an EMBASSY switch manufactured by 
Motorola, Inc. 
The circuit-switched digital cross-connect switch 705 is also in 
communication with a system controller 706 and an operator position 707. A 
suitable system controller 706 is a SMARTZONE zone controller manufactured 
by Motorola, Inc. A suitable operator position 707 is a Centracom Gold 
Series operator position manufactured by Motorola, Inc. In practice, the 
operator position 707 requires audio input in either a waveform encoded or 
compressed format. As such, a transcoder 708 may be required to convert 
the information received as sub1 packets from the circuit-switched digital 
cross-connect 705 into the appropriated format required by the operator 
position 707. Such transcoders are known in the art, and can be readily 
adapted for use in a variety of systems. From the vantage of the 
circuit-switched digital cross-connect switch 705, the operator position 
707 is no different than a site 701-702 as far as sending and receiving 
data. The system controller 706 manages the setup and breakdown of 
communication within and between the sites 701-702 using control links 
729-730. In accordance with the present invention, the system controller 
706 also includes a static and dynamic table (not shown) comprising the 
routing tables from the inbound and outbound time slots on communication 
links 712-713 from/to each of the routers 703-704. In this manner, the 
system controller 706 can control the passage of associated inbound and 
outbound time slots, as described above, through the circuit-switched 
digital cross-connect switch 705. 
Each wireless communication site 701-702 includes base stations 715-716, 
722-723 in wireless communication with communication units 717-718, 
724-725 via one or more wireless communication resources 719-720, 726-727. 
A suitable base station is an Intellirepeater manufactured by Motorola, 
Inc. A suitable wireless communication unit is an ASTRO SABER radio 
manufactured by Motorola, Inc. 
The base stations 715-716, 722-723, when transceiving a wireless 
communication that needs to be routed through the circuit-switched 
infrastructure to another site, produce inbound framed packet-data. The 
inbound address information described above corresponds to a unique 
inbound address used by each base station 715-716, for site 701 and unique 
inbound address used by each base station 722-723 for site 702 to form 
inbound framed packet-data (not shown) that is carried on packet-switched 
data links 709, 710 which is sent to router 703, 704, respectively. The 
routers 703-704 use this inbound address information to route the inbound 
packet-data to the correct time slot. Because each router is independent 
of the other routers and the inbound address is not sent through the 
circuit-switched infrastructure, the inbound address information for the 
inbound framed packet-data and outbound address information for the 
outbound framed packet-data can be reused between each router 703, 704 and 
its respective base stations 715-716, 722-723 and their respective sites 
701,702. 
FIG. 8 is a flowchart illustrating operation of the communication system 
700. At step 801, a first site 701 transmits inbound framed packet-data to 
a (uniquely associated) first router 703. This occurs when a communication 
unit 717-718 in the first site 701 initiates a communication requiring the 
participation the operator position 707 or another communication unit 
724-725 at a second site 702. For the purposes of illustration, it will be 
hereinafter assumed that a first unit 717 has transmitted a communication 
request and is attempting to communicate with the operator position 707 
and a second communication unit 725 in the second site 702. Assuming that 
the first unit 717 wirelessly communicates with a first base station 715, 
the inbound address information, forming part of the inbound framed 
packet-data, will correspond to the first base station 715. 
At step 802, the first router 703, in accordance with its operation 
discussed above relative to FIGS. 3 and 5, transfers the inbound 
sub-packets included in the inbound packets to associated inbound time 
slots, and then transmits the associated inbound time slots to the 
circuit-switched digital cross-connect switch 705. As described 
previously, the associated inbound time slots uniquely correspond with the 
address assigned for use by the first base station. Furthermore, because 
the unaddressed serial data links 712-713 are uniquely associated with 
each router 712-713, each associated inbound time slot coming into the 
circuit-switched digital cross-connect switch 705 is unique within the 
system to the extent that each base station in the system is uniquely 
identified. 
Either prior to or simultaneous with step 802, the system controller 706, 
responsive to the communication request made by the first unit 717, 
instructs the circuit-switched digital cross-connect switch 705 to 
broadcast the associated inbound time slots to at least one set of 
associated outbound time slots at step 803. Because the system controller 
706 includes the master table, it can determine, based on the requested 
targets for the communication included in the communication request, which 
associated outbound time slots are required to complete the communication. 
Following the previous example, the system controller would instruct the 
circuit-switched digital cross-connect switch 705 to broadcast (meaning 
copy to one or more time slots) the contents of the associated inbound 
time slots to associated outbound time slots corresponding to the operator 
position 707 and a second base station 723 corresponding to the second 
unit 725. At step 804, the circuit-switched digital cross-connect switch 
705 broadcasts the associated inbound time slots, as they are received, in 
accordance with the instructions received from the system controller at 
step 803. As soon as the inbound sub-packets in each associated inbound 
time slot is copied into and associated outbound time slot, it effectively 
becomes an outbound sub-packet to the extent that the circuit-switched 
digital cross-connect switch 705 will send it to a site or operator 
position. 
At step 805, the second router 704 receives the associated outbound time 
slots that included the now outbound sub-packets. At step 806, the second 
router 704, in accordance with its operation discussed above relative to 
FIGS. 4 and 6, constructs outbound packets from the outbound sub-packets 
included in the associated outbound time slots, and then appends outbound 
address information to the outbound packets to produce outbound framed 
packet-data. In the present example, the outbound address information, as 
determined by the identification of the associated outbound time slots, 
will correspond to the second base station 723. At step 807, the second 
router 704 transmits the outbound framed packet-data, via the 
packet-switched link 710, to the second site 702. Upon receiving the 
outbound framed packet-data, the second base station 723 wirelessly 
transmits the information contained therein to the second unit 725, 
thereby completing the call. Although the present example considers only 
one-way communication, the present invention can be equally applied to a 
two way communication by performing the same operation described above in 
the opposite direction. Furthermore, more than one destination site could 
be specified in accordance with the present invention and is required for 
use by a dispatch-type two-way communication system. 
The present invention provides a method and apparatus for routing framed 
packet-data in a circuit switched infrastructure through the use of a 
unique router. By uniquely associating inbound and outbound packet 
addresses with corresponding inbound and outbound time slots within the 
circuit-switched infrastructure, the router allows a circuit-switched 
infrastructure to copy and route packet-data to multiple destinations. In 
this manner, communications between two or more packet-based sites using a 
circuit-switched infrastructure can be achieved. This is an improvement 
over prior art techniques that did not allow circuit-switched 
infrastructures to convey framed packet-data.