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Claims  |
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We claim:
1. A mobile-communications connection-and-routing method comprising the
steps of:
detecting an attempted mobile communication in the vicinity of a first base
station in a network comprising a plurality of switches, communication
links and base stations, each base station corresponding to a geographical
cell;
setting up a connection tree within the network, the connection tree
comprising a plurality of communication routes, each route extending from
a first switch within the network through at least one other switch and at
least two communication links to a base station, each base station
situated within a predetermined distance of the first base station, so
that a route is defined from each of the base stations within the
predetermined distance to the first switch;
setting up a fixed route within the network which extends from the first
switch to a location within the network and outside of the connection
tree;
providing the first switch with first routing information from the first
switch to the connection tree and with second routing information from the
first switch to the location within the network and outside of the
connection tree;
admitting the communication to the connection tree; and
routing the communication through a first route between the first base
station and the first switch, the first route being one of the plurality
of communication routes within the connection tree, and between the first
switch and the location outside of the connection tree through the fixed
route, whereby the mobile user can communicate back and forth along a
complete communication route with the location outside of the connection
tree;
further comprising, at the first switch and upon a communication from the
connection tree to the location within the network and outside of the
connection tree, the steps of:
storing a virtual channel identifier,
identifying a current base station,
determining whether the identified current base station differs from a
previously identified base station, and, in case of difference,
replacing the first routing information with new first routing information
from the first switch to the connection tree.
2. A mobile-communications connection-and-routing method comprising the
steps of:
detecting an attempted mobile communication in the vicinity of a first base
station in a network comprising a plurality of switches, communication
links and base stations, each base station corresponding to a geographical
cell;
setting up a connection tree within the network, the connection tree
comprising a plurality of communication routers, each route extending from
a first switch within the network through at least one other switch and at
least two communication links to a base station, each base station
situated within a predetermined distance of the first base station, so
that a route is defined from each of the base stations within the
predetermined distance to the first switch;
providing the first switch with first routing information from the first
switch to the connection tree and with second routing information from the
first switch to the location within the network and outside of the
connection tree;
admitting the communication to the connection tree; and
routing the communication through a first route between the first base
station and the first switch, the first route being one of the plurality
of communication routes within the Connection tree, and between the first
switch and a second base station within the predetermined distance of the
first base station through a second route, the second route being one of
the plurality of communication routes within the connection tree, whereby
the mobile user can communicate back and forth along a complete
communication route with the second base station within the connection
tree;
further comprising, at the first switch and upon a communication from the
connection tree to the location within the network and outside of the
connection tree, the steps of:
storing a virtual channel identifier,
identifying a current base station,
determining whether the identified current base station differs from a
previously identified base station, and, in case of difference,
replacing the first routing information with new first routing information
from the first switch to the connection tree.
3. A connection-and-routing method for mobile communications in a network
comprising base stations, switching nodes, and communication links, each
one of the base stations corresponding to a geographical cell, the method
comprising:
determining a communication route within the network, the connection route
connecting a first one of the base stations to a first one of the
switching nodes by a first portion of the communication route, and the
connection route connecting the first one of the switching nodes further
by a second portion of the communication route, the first one of the base
stations being within communication range of a mobile user,
determining a connection tree within the network, the connection tree
comprising the first portion of the connection route and at least one
potential first portion of the communication route within the network, the
at least one potential first portion connecting a second one of the base
stations to the first one of the switching nodes,
providing the first switch with first routing information from the first
switch to the connection tree and with second routing information from the
first switch to the location within the network and outside of the
connection tree;
admitting the communication to the connection tree; and
routing the communication through a first route between the first base
station and the first switch, the first route being one of the plurality
of communication routes within the connection tree, and between the first
switch and the location outside of the connection tree through the fixed
route, whereby the mobile user can communicate back and forth along a
complete communication route with the location outside of the connection
tree;
further comprising, at the first switch and upon realization of a
predetermined condition depending on movement of the mobile user, the
steps of:
storing a virtual channel identifier,
identifying a current base station,
determining whether the identified current base station differ is from a
previously identified base station, and, in case of difference,
replacing the first routing information with new first routing information
from the first switch to the connection tree.
4. A system for mobile-communications connection and routing comprising:
a network comprising a plurality of switches, communication links and base
stations capable of supporting asynchronous transfer mode communications;
means for detecting an attempted mobile communication in the vicinity of a
first base station in the network;
means for setting up a connection tree within the network, the connection
tree comprising a plurality of communication routes, each route extending
from a fixed point of the network, the fixed point being a switch, through
at least one other switch and at least three communication links to all
base stations within a predetermined distance of the first base station so
that a route is defined from each of the base stations within the
predetermined distance to the fixed point switch;
means for setting up a fixed route within the network which extends from
the fixed point switch to a desired destination within the network and
outside the connection tree;
means for admitting the communication to the connection tree at the first
base station; and
means for routing the communication from the first base station through a
first route, which is one of the plurality of communication routes within
the connection tree, to the fixed point, and from the fixed point through
the fixed route to the destination, whereby the mobile user can
communicate back and forth with the destination;
further comprising, at the first switch:
means for storing a virtual channel identifier,
means for identifying a current base station,
means for determining whether the identified current base station differs
from a previously identified base station, and, in case of difference,
means for replacing the first routing information with new first routing
information from the first switch to the connection tree.
5. A system for mobile-communications connection and routing comprising:
means for detecting an attempted mobile communication in the vicinity of a
first base station in a network comprising a plurality of switches,
communication links and base stations, each base station corresponding to
a geographical cell;
means for setting up a connection tree within the network, the connection
tree comprising a plurality of communication routes, each route extending
from a first switch within the network through at least one other switch
and at least two communication links to a base station, each base station
situated within a predetermined distance of the first base station, so
that a route is defined from each of the base stations within the
predetermined distance to the first switch;
means for admitting the communication to the connection tree; and
means for routing the communication through a first route between the first
base station and the first switch, the first route being one of the
plurality of communication routes within the connection tree, and between
the first switch and a second base station within the predetermined
distance of the first base station through a second route, the second
route being one of the plurality of communication routes within the
connection tree, whereby the mobile user can communicate back and forth
along a complete communication route with the second base station within
the connection tree;
further comprising:
means for initiating a change of the mobile user's base station from the
first base station to a third base station upon the mobile user moving
from the cell corresponding to the first base station to the cell
corresponding to the third base station, the third base station
corresponding to a third route which is one of the plurality of
communication routes within the connection tree and which connects the
third base station to the first switch; and
means for changing the first route portion of the complete communication
route from the first route to the third route, comprising, at the first
switch:
means for storing a virtual channel identifier,
means for identifying a current base station,
means for determining whether the identified current base station differs
from a previously identified base station, and, in case of difference,
means for replacing the first routing information with new first routing
information from the first switch to the connection tree. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention relates generally to mobile communications systems and, more
specifically, to distributed call setup, admission, control and rerouting
in mobile communications systems capable of supporting asynchronous
transfer mode.
In recent years, interest has rapidly grown in two distinct areas of the
communications field. First, the popularity of mobile, or personal,
communications has increased immensely and is expected to grow in the near
future to the point where existing systems will be unable to support the
demand for mobile communications. The basic problem facing the future of
mobile communications systems is the lack of available bandwidth for the
mobile user's wireless transmission to a fixed network.
Current mobile communications systems employ the concept of "cells." A cell
is a geographical area which is assigned to a corresponding base station
which is in turn wired into a fixed communications network. The capacity
of a cellular system can become quite high since the available bandwidths
can be reused among the various cells. The fixed network is typically a
"mesh network." A mesh network is comprised of numerous switches connected
together by communication links. The mesh network is set up so that a
communications route may be traced from any one switch to any other in the
network through at least one, and more often, many combinations of links
and switches. Some of the switches in the network, in addition to being
connected to other switches, will also be connected via communication
links to one or more of the base stations and/or fixed termination points
such as a home telephone.
When a mobile user wishes to place a call, the call is transmitted through
a communication medium, often a radio channel, to the base station
assigned to the user's cell. From the base station, the call is carried by
the mesh network to the user's intended destination. When the mobile user
moves from one cell to another, a call handoff, or hand-over, between base
stations takes place. The handoffs are performed by a central system
controller. This is known as centralized call processing.
Due to the limited bandwidth available for the wireless transmissions of
mobile users' calls, each cell can handle only a limited number of
callers. Thus, to meet the needs of an increasing population of mobile
users, the area assigned to each cell must be decreased. Therefore, the
number of such cells, and their corresponding base stations, increases for
a given geographical area. The smaller cell size means that the rate of
call handoffs per second in a mobile communications system increases,
thereby placing a great burden on the central system controller. It is
predicted that in the near future, the centralized system controllers of
today's systems will be unable to handle the call handoff rate which will
be necessary to meet the demand for mobile communications. The need for a
distributed call processing system, rather than a centralized system, has
been identified, but no comprehensive solutions have as yet been found.
The second area of communications which has been the focus of much research
is the standardization of broadband integrated services digital networks
(B-ISDN) utilizing the asynchronous transfer mode (ATM). Present
communications systems rely on circuit switching techniques. In these
circuit switching systems, a circuit path between the caller and his
destination is found and used exclusively for a single call until the call
terminates. On the other hand, future B-ISDN systems will employ packet,
or cell, transport techniques. This means, essentially, that a
communication is broken up into discrete "packets", or "cells" (unrelated
to the geographic cells of mobile communications systems), which are sent
one at a time through the system and received at the intended destination
as an uninterrupted communication. Packets from many different callers may
simultaneously share the same communication link, thus making these B-ISDN
systems a more efficient means of communication than the circuit switching
systems.
ATM is the target mode for future B-ISDN systems. The other, and at one
time favored, choice for B-ISDN packet communications was synchronous
transfer mode (STM) which would handle the packets from a given
communication during allocated time slots occurring on a regular basis.
For reasons unimportant to this invention, ATM was chosen over STM.
The ATM packets are themselves divided into two sets of information. One
set is the information which the user intends to transmit and the other
set is called the "header." The header contains routing information,
including a virtual channel identifier (VCI). The VCI, simply put, is a
code assigned to the packet which lets an ATM switch know where to send
the packet next, based on the switch port where the packet has been
received.
A conventional ATM B-ISDN switch, hereinafter called an ATM switch, has
several input and output ports. Embodied in the implementation of a
conventional ATM switch is a "lookup table." The lookup table may be
thought of as having four columns: input port, incoming VCI, output port,
and outgoing VCI. For every possible input port and incoming VCI
combination, there is a corresponding output port and outgoing VCI
combination programmed into the lookup table. When an ATM switch receives
a packet at a given port, the ATM switch will find the row in the lookup
table which has the incoming VCI and input port which correspond to that
of the received packet. The ATM switch will then switch or route the
packet to the output port which appears in the same row and replace the
incoming VCI in the packet header with the outgoing VCI. This process is
performed at each switch until the packet arrives at its destination.
The heightened interests in both mobile communications and B-ISDN
communication using ATM which has been described above has led to the
present invention. Up until now, no system, existing or proposed, has
provided a technique for distributed, as opposed to centralized, call
setup and rerouting in a mobile ATM based B-ISDN system with several ATM
switches.
SUMMARY OF THE INVENTION
In mobile communications, distributed call setup and rerouting are
realized. A communication route is determined which includes a connection
from a base station via a switching node. A connection tree is determined,
to include potential connections from other base stations to the switching
node. When the mobile user moves from one cell to another, corresponding
to a change in base station within the connection tree, the call is
automatically handed over or rerouted to another route within the
connection tree. By obviating the need for a central processor for
handoffs within the connection tree, a greater number of communications
may be supported than in conventional mobile communications systems.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features and advantages of the invention will become
apparent, and its construction and operation better understood, from the
following detailed description when read in conjunction with the
accompanying drawings, in which:
FIG. 1 is a representation of a connection tree in a mesh network according
to an exemplary embodiment of the present invention;
FIG. 2 is a representation of a simple connection tree according to a
preferred first embodiment of the present invention;
FIG. 3 is a representation of an ATM Switch/VCI Translator;
FIG. 4 is a representation of a simple connection tree according to a
preferred second embodiment of the present invention in which one of the
ATM switches is an ATM Switch/VCI Translator, and FIGS. 4A and 4B are
representations of the lookup tables which correspond to two of the
switches;
FIG. 5 is a representation of an ATM switch/rerouter;
FIG. 6 is a representation of a simple connection tree according to a
preferred third embodiment of the present invention in which there are
three ATM Switch/Rerouters, and FIGS. 6A and 6B are representations of the
lookup tables which correspond to two of the switches;
FIG. 7 is a representation of a simple a connection tree according to a
preferred fourth embodiment of the present invention in which one of the
switches is a Switch/Rerouter, and FIGS. 7A and 7B are representations of
the lookup tables corresponding to two of the switches;
FIG. 8 is a flow diagram of a communications method in accordance with a
preferred embodiment of the present invention;
FIG. 9 is a schematic of an ATM switch in the root of a connection tree in
a preferred embodiment of the invention;
FIG. 10 is a schematic of a VCI monitor/translator included in the root
switch of FIG. 9.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
One embodiment of the general architecture of the present invention may be
seen in FIG. 1. At call setup time, a connection tree 10 within a
communications network 12 having a packet switching architecture is
defined for a mobile user. The communications network 12 is a mesh network
as described in the Background section hereinabove.
The connection tree 10 established within the mesh network 12 extends a
connection from a fixed point 11 of the network, called the root, to each
one of the base stations 17 in the neighborhood 20 of the mobile user. The
base station in charge of a mobile user is called the mobile user's access
point, base station 22 in this example. The neighborhood 20 may be defined
as the area within a predetermined distance of the mobile user's access
point 22. A connection tree 10 provides a route, referred to as a virtual
channel connection, to the mobile user's access point 22 as well as a
virtual channel connection for each of its neighboring mobile access
points 17 such that a concatenation of one of these virtual channel
connections with another virtual channel connection 25 provides an
end-to-end connection in which the mobile user can be the source or
destination of the traffic. At the time a connection tree 10 for a mobile
user is set up, a unique connection tree ID is assigned to the user. The
significance of the connection tree ID will be explained later.
As discussed above, the connection tree 10 is set up with reference to the
mobile user's location. This is the case whether the mobile user is the
source or destination of a call. In both instances, the connection tree 10
will be set up at the mobile user's first interaction with a base station
17.
Further explanation of the connection tree architecture will be made with
reference to FIG. 1. FIG. 1 represents just one of a large number of
possible connection tree configurations. Connection tree links are shown
with thick lines, such as link 3, and other links of the network are shown
with narrow ones, such as link 4. For the remainder of this disclosure,
the existence of links not included in the connection tree will be
ignored. This, of course, does not mean that they should not be thought of
as part of the network. Also, although they appear in the figures, the
links which are included in the connection tree will not be discussed and
should be assumed to connect the switches and base stations as shown in
the figures.
Returning to FIG. 1, at the call setup time, the mobile user is
communicating to a base station 22 connected to switching node 5.
Switching nodes 7, 8, and 9 are the switches to which all the neighboring
base stations 17 are connected. In any end-to-end connection for which the
mobile user is the source or the destination, the communication path is
divided into two parts. One part of the path is contained within the
connection tree 10. This part of the path changes as the result of the
mobile user's connection handoffs. Details of this procedure are described
hereinbelow. The other part of the path, which is from the root 11 of the
connection tree to the other end of the connection 18, remains fixed
throughout most of the connection lifetime. It should be noted, however,
that the fixed part of the path will not always be necessary. A mobile
user may wish to communicate with a destination, fixed or mobile, within
the same connection tree. In this scenario, the communications will be
routed to the root of the connection tree, but instead of then being
switched out of the tree and through a fixed route, the communications
will instead be routed back into the tree to the intended recipient. Thus,
the two-part path consists of two connection trees rather than a
connection tree and a fixed route.
Whenever the user reaches the boundary of the connection tree 10, a new
connection tree is established so that the neighboring base stations of
the mobile user's access point belong to a new connection tree. This
procedure is called the connection tree handoff. Any time there is a
handoff from one base station 17 to another, the current virtual channel
connection is disabled and the virtual channel connection terminating at
the new mobile access point is enabled. The part of the path in the wired
network originating at the mobile user's call destination point 18 and
which terminates at the root 11 of the connection tree, denoted by 25, is
fixed and reused.
An example may be illustrated referring to FIG. 2. FIG. 2 depicts a simple
connection tree having three switching nodes 14, 15, 16 and four base
stations 33, 34, 35, 36. The mobile user is represented by an automobile
29. Upon placing a call in the vicinity of base station 34, the connection
tree is set up. Switch 15 is the root of the connection tree and four
virtual channel connections 50, 51, 52, 53 to the four base stations 33,
34, 35, 36 are provided to the mobile user 29. At the outset, the mobile
user's access point is base station 34 and virtual channel connection 51
is enabled. As the mobile user 29 moves out of the geographical cell 40
assigned to base station 34 and into the cell 41 assigned to base station
35, a call handoff takes place whereby virtual channel connection 51 is
disabled, virtual channel connection 52 is enabled and base station 35
becomes the mobile user's access point.
In this embodiment of the invention, call handoff is initiated by the
mobile user. This is called mobile initiated handoff. A handoff may be
initiated by any one of several well known means. For example, the user
may monitor power from each of the base stations in its connection tree
and initiate a handoff whenever the signal strength received from the
user's current base station approaches some threshold of unacceptability
and there is at least one base station in the tree from which an
acceptably high power level is detected. In such a case, the mobile user
would initiate a handoff to any base station from which acceptably high
power is detected. The rerouting of the connection is done within the
connection tree and the involvement of a central network call processor
for rerouting is unnecessary. Since a distributed rerouting of connections
is provided, a large number of handoffs can be realized in a
"micro-cell/pico-cell" environment which has a high frequency of handoffs.
FIG. 8 shows in simple flow chart form the steps performed in the
connection tree switching scheme as described above. A communications
request is detected by a base station in block 160. This is followed by
connection tree setup and setup of the fixed portion of the end-to-end
connection in blocks 161 and 162 respectively. The specific route within
the connection tree between the root and the base station where the
communication was detected is defined in block 163. In block 164, the
system looks for a change in the user's base station. If the user is still
within the vicinity of the base station, the communication continues in
block 165. If the user has moved into the vicinity of a new base station,
the system looks to see if the new base station is within the connection
tree in block 166. If so, control returns to block 163 and a new route
within the connection tree is defined. If the new base station is not
within the connection tree, control returns to block 161 and a new tree is
set up. This will continue until the communication is terminated, which
can occur at any time.
Two schemes for rerouting of the connections in the connection tree will be
discussed herein. The schemes are devised for a packet switched B-ISDN
asynchronous transfer mode (ATM) system as described in the Background
section of this specification. This should not be interpreted to limit the
use of the connection tree system discussed above to a packet switched
B-ISDN ATM system. It is possible to use the connection tree architecture
described above in other types of packet switched communications systems.
The term "switch" used in the description of these schemes should be
interpreted to include any switching node or network, such as a local area
network (LAN) or metropolitan area network (MAN), which is capable of ATM
packet transport based on virtual channel identifiers. The terms
"communication" or "call" as used in the description should be interpreted
to include phone, fax, and any other type of communication which can be
supported by a B-ISDN ATM system.
A. Scheme 1
As described above, at call setup time, a connection tree is defined for
the user. In this embodiment of a connection tree, the root of the tree is
a switch with an added feature which is described herein. This switch is
shown in FIG. 3 and is called an ATM Switch/VCI Translator.
An ATM Switch/VCI Translator performs the virtual channel connection
rerouting in the mobile user's connection tree. It is an ATM switch having
input ports 55 and output ports 56 with additional components which will
be discussed with reference to FIG. 3. At each input port 55 of the
switch, there is a VCI monitor/comparator 57 which has read/write access
to the switch's routing lookup table 58. The concept of a routing lookup
table was discussed in the Background section of this specification.
The VCI monitor/comparator 57 copies all incoming packet headers, each of
which includes a virtual channel identifier, to its local memory and
compares them to the lookup table 58. It associates a virtual channel
connection from the root of the tree to the base station of a mobile
connection with an incoming and outgoing VCI as well as the incoming and
outgoing switch ports of the connection. To clarify this procedure an
example is given in FIG. 4.
FIG. 4 shows a simple connection tree. The root of the connection tree is
an ATM Switch/VCI Translator 75 having three input ports 59, 62, 73 and
three output ports 60, 61, 74. The other switches 76, 77 are ordinary ATM
switches. Switch 76, through which our call will travel, has three input
ports, 63, 66, 68 and three output ports 64, 65, 67. Portions of the
lookup tables of switches 75 and 76 are also shown in FIGS. 4A and 4B. The
lookup table of an ATM Switch/VCI Translator differs from that of a
conventional ATM switch in that only certain rows will be enabled
depending on which of the base stations within the tree is serving as the
mobile user's access point.
In FIG. 4, when the mobile user is communicating to the base station 78,
there is a virtual channel connection 85 established between the root 75
of the connection tree and the base station 78. Communications leaving
base station 78 travelling along path 85 leave base station 78 with
virtual channel identifier VC1. The packets are transmitted via output
port 70 into input port 66 of switch 76. Switch 76 reads VC1, consults the
third row of its lookup table and assigns virtual channel identifier VC3
to the packet to let ATM Switch/VCI Translator 75 know where the
communication is coming from and what the packet's final destination is.
The packet is transmitted via output port 64 of switch 76 and enters input
port 62 of ATM Switch/VCI Translator 75. When the VCI Monitor/Comparator
at input port 62 reads VC3, the ATM Switch/VCI Translator enables rows one
and three which correspond to path 85. The packet is then assigned virtual
channel identifier VC4, as per row three of the lookup table, to tell the
fixed portion of the end-to-end connection where the packet is going. The
packet then leaves the connection tree via output port 74.
Packet communications returning to base station 78 from outside the
connection tree will enter the connection tree at ATM Switch/VCI
Translator 75 input port 73 with a virtual channel identifier of VC1. The
VCI Monitor/Comparator at input port 73 reads VC1, finds row one enabled
and assigns a virtual channel identifier of VC2 to indicate to switch 76
that the packet is destined for base station 78. The packet leaves via
output port 61 and goes into input port 63 of switch 76. Switch 76 reads
VC2 and finds it in the first row of its lookup table. The packet is then
given virtual channel identifier VCo, is switched to output port 65 and is
transmitted into base station 78 input port 69.
When the mobile user hands off to base station 79, it sends ATM packets
with the virtual channel identifier VC5 from output port 72. Since the
lookup table for switch 76 assigns virtual channel identifier VC8 to
packets which enter the switch at input port 68 with virtual channel
identifier VC5, the packets generated by the mobile user will be received
at input port 62 of ATM Switch/VCI Translator 75 with the virtual channel
identifier VC8. Since this is not the virtual channel identifier VC3 which
was assigned by switch 76 to packets received from base station 78, the
VCI monitor/comparator at ATM Switch/VCI Translator 75 realizes that a
handoff has taken place and that the mobile user is communicating to base
station 79. Therefore, ATM Switch/VCI Translator 75 updates the lookup
table of the switch such that packets going from ATM Switch/VCI Translator
75 output port 61 to base station 79 input port 71 along path 86 are
switched accordingly as are packets going from base station 79 output port
72 to ATM Switch/VCI Translator 75 input port 62. In other words, the VCI
monitor/comparator enables the second and forth rows of the lookup table
and disables the first and third rows.
To sum up the major features of the scheme described above: First, the
mobile user initiates the rerouting as the result of its handoff by
changing the VCI of its packets. It receives all the VCIs associated with
each base station in its connection tree at the call setup time. By
sending packets with VCIs associated to a specific base station, it
initiates the rerouting of its connections to that base station. Second,
the ATM Switch/VCI Translator updates the lookup table of the switch at
the root of the connection tree so that the reverse connections are
established accordingly. There, rerouting operations are all performed
without the use of a centralized call processor in a distributed manner,
thus enabling the system to handle frequent call handoffs.
B. Scheme 2
In a second embodiment of the connection tree, the call set up and routing
is distributed by adding one or more special ATM switches which are called
Switch/Rerouters, shown in FIG. 5. An ATM Switch/Rerouter is any switch
capable of ATM Virtual Channel switching as specified by CCITT standards
with an additional feature. In addition to conventional input ports 92 and
output ports 93, it has a specific port, called the Reroute Port 90, which
has read/write access to the switch's routing lookup table 91.
In any mobile network with ATM packet transport, one or more ATM
Switch/Rerouters may be deployed in order to support distributed, fast
connection set up and rerouting, the details of which will be described
below. Again, at the connection setup time, a connection tree is
established which connects all base stations in the neighborhood of the
mobile user.
Rerouting of connections as a result of handoffs is done by sending special
packets called control packets to all Switch/Rerouters of the connection
tree. It is important to note that in this scheme, the mobile user's
access point will almost always see the same VCI for a mobile user's
connection. As a result, a handoff will be totally hidden from the mobile
user and its wireless MAC protocol.
A connection cannot, however, keep its VCI at the user-network interface at
all times for all mobile access points. Since VCI's are reusable, a
specific user might move into an access point area in which another
connection has been assigned to the same VCI at the user-network
interface. This scenario is called "terminating VCI collision." This
problem can be overcome in the following way. Each connection can be
ensured a unique VCI at the mobile access point in a given geographical
area. Each area could cover many geographic packets. When a user moves out
of the domain of its unique VCI, his or her connection is assigned a new
VCI at the mobile access point interface and its adaptation layer is
notified to take actions accordingly. Also, these unique user-network
interface virtual channel identifier domains should overlap so that there
would be no oscillating phenomenon at their boundary. As long as the
mobile user's connection is handed off to the neighboring base stations of
its connection tree, rerouting is done entirely by one or more ATM
Switch/Rerouter nodes.
Scheme 2 may be carried out with either several or only one
Switch/Rerouter. The method employing several Switch/Rerouters within the
connection tree will be discussed first followed by a discussion of the
method requiring only one Switch/Rerouter at the root of the connection
tree.
Method 1: Distributed Call Setup and Reroute with Several Switch/Rerouters
In FIG. 6, a simple connection tree, which has as its root ATM switch 114,
is shown. This connection tree includes all the neighboring base stations,
each having an address, which are connected to switches 112 and 113. For
simplicity, only base stations 110 and 111 are shown. All three switching
nodes 112, 113, 114 in this example are ATM Switch/Rerouters. For specific
illustration, it will be assumed that a mobile user is communicating to
the base station 110. The mobile user is assigned a unique Connection ID,
100, which remains the same everywhere within the connection tree. For
this case, the lookup tables at ATM Switch/Rerouters 112 and 114 are shown
in FIGS. 6A and 6B. Each of these ATM Switch/Rerouter lookup tables has
the capacity to have its rows either enabled or disabled just as in the
ATM Switch/VCI Translators. In the ATM Switch/Rerouters, the updating
process is accomplished via control messages received at the reroute port,
while in the ATM Switch/VCI translators, the updating was done using a VCI
Monitor/Comparator at each input port.
In this scheme, each base station has a unique ID, distinct from the user's
connection ID. Both IDs are included in the payload of the control packet,
or control message. In each tree there are predefined routes for control
messages called control virtual channel connections which start from each
base station and terminate at the Reroute port of each ATM
Switch/Rerouter. The control messages are generated either by the mobile
user or by the base station accessed by the mobile user at the time a
handoff occurs. These messages are switched like any other ATM packet and
arrive at the reroute ports. Upon arrival of a control message at a
reroute port, the ATM Switch/Rerouter identifies which row of its lookup
table should be enabled based on the Connection ID and the base station ID
contained in the payload. Where, as in our example, multiple ATM
Switch/Rerouters are employed, the switch may take no action at all.
Returning to the example, when the mobile user first communicates to the
connection tree, control signals are sent out to each of the ATM
Switch/Rerouters 112, 113, 114. The control signals enable the first and
third rows of the lookup tables for ATM Switch/Rerouters 112 and 114. This
sets up connection path 120 through the connection tree. Thus, any packet
with virtual channel identifier VCI coming into the connection tree on
input port 115 of the ATM Switch/Rerouter 114 is switched to output port
95 and given virtual channel identifier VC2. The packet then enters ATM
Switch/Rerouter 112 at input port 97. Since row one of the lookup table
for the ATM Switch/Rerouter 112 has been enabled by the control signals,
the packet is assigned the virtual channel identifier VCo, which is unique
to the mobile user's connection, and sent via output port 101 to the input
port 105 of base station 110.
In the reverse direction, an ATM packet entering ATM Switch/Rerouter 112 at
input port 102 from output port 106 of base station 110 with the virtual
channel identifier VC1 is transported through ATM Switch/Rerouter 112, via
output port 98 with the virtual channel identifier VC3, into ATM
Switch/Rerouter 114 input port 96 and out of output port 116 of the ATM
Switch/Rerouter 114 with the virtual channel identifier VC4.
Further, for specific illustration, it will now be assumed that the mobile
user establishes a connection with base station 111. In this case, control
messages are sent to the Reroute Ports of ATM Switch/Rerouters 112, 113,
and 114 by the base station 111 indicating the base station ID and the
mobile user's connection tree ID, in this example 100.
Only the lookup table of ATM Switch/Rerouter 112 is changed by the control
messages received from base station 111 at the Reroute Port. The control
message essentially tells the switch to enable connection 121 and disable
connection 120. To do this, the lookup table's first and third rows are
disabled and its second and forth rows are enabled. Since Switch/Rerouter
112 has the capability to have its lookup table updated, there is no need
to update the lookup table of ATM Switch/Rerouter 114 because all packets
intended for the mobile user are routed from ATM/Switch Rerouter 114 to
ATM Switch/Rerouter 112 whether the user is accessing base station 110 or
base station 111 and therefore only one set of connection data need be
changed.
Now, the packets entering input port 115 of ATM Switch/Rerouter 114 with
virtual channel identifier VC1 are transported through ATM Switch/Rerouter
112, via output port 103 to input port 107 of base station 111 with
virtual channel identifier VCo along path 121. The reverse connection
along path 121 from output port 108 of base station 111 to ATM
Switch/Rerouter 112 input port 104 and on to output port 116 of ATM
Switch/Rerouter 114 is maintained as well. It is important to note that
this distributed r | | |
