Electrical computers and digital processing systems: multicomput – Computer-to-computer data routing
Reexamination Certificate
1999-01-04
2001-07-10
Harrell, Robert B (Department: 2152)
Electrical computers and digital processing systems: multicomput
Computer-to-computer data routing
Reexamination Certificate
active
06260071
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to automatic routing selection for data connections through multiple transmission lines on network access servers (NASs).
BACKGROUND OF THE INVENTION
A network access server (NAS) box typically provides dial-up internet services to users of the internet. A user dials into the access server box in order to be connected to the internet. The access server box transmits and receives data through the public switched telephone network (PSTN) over channelized transmission facilities, such as a BRI, T1, E1, T3, SONET or SDH lines.
Multi-link PPP (MLP) is a method for obtaining higher transmission bandwidth by sending data through separate serial channels using a point-to-point protocol (PPP) and then reassembling the data at a termination point of the serial channels. MLP is often applied in integrated services data network (ISDN) which use multiple B channels. Another application of MLP is using multiple modem connections in parallel to obtain greater bandwidth than that available from a single modem.
An MLP session does not require a high level of resources from the NAS or router box for support so long as all the PPP connections for the separate serial channels of the MLP session terminate on the same router box. However, a large NAS will typically be constructed from multiple remote access concentrators (RACs) each having its own transmission facilities connecting it to the PSTN. An example of an RAC is a network router with dial-in facilities for interfacing with the PSTN. If the multiple dial-up connections are terminated on different RACs serving the same NAS, then a multi-chassis multi-link PPP (MMP) is required to support the multiple link connection. In the case of an MMP session, the multiple PPP sessions of the MMP must be tunneled from different RACs to a single RAC in the NAS in order to terminate the MMP session.
MMP sessions can be expensive in terms of router resources and support for MMP can present a serious problem when scaled to large NAS systems having a large number of RACs. The MMP support problem, in some cases, drives network design to use a single large central RAC for each NAS system. However, the cost advantages to being able to scale many small RAC blocks in order to obtain a larger system are then lost.
Internet service providers (ISPs) typically have a large pool of customers, where only a fraction of the total pool is actively using the communications infrastructure of the ISP at any given time. ISPs therefore typically oversubscribe their communications equipment (i.e. modems, ISDN lines, T1/E1 facilities) in order to take advantage of the fact that a smaller proportion of their total users are active during any given period. The equipment which serves subscribers is therefore structured to have a concentration ratio which represents the proportion of subscriber connections to transmission facilities. The concentration ratio is selected so as to cost-effectively provide a reasonable quality of service to the subscribers with the least number of transmission facilities. Thus, the concentration rate can be relatively high for a pool of low usage users, but must be much lower for a pool of high usage users.
In the course of terminating calls to a given NAS system, a switch serving the NAS system will typically attempt to distribute the terminating calls as uniformly as possible across the multiple transmission facilities (i.e. T1/E1 facilities) connected to the NAS system on a next available basis. As a result, in a multi-chassis NAS, the probability of the multiple serial channels for an MMP session being distributed across transmission facilities serving different chassis of the NAS can become quite high.
In addition, the bulk of the pool of customers will not set up a multi-link session. Multi-link calls typically originate from a sub-set of customers who have specialized hardware needed to support multi-link connections and have paid for multi-link service.
Signaling System 7 (SS7) is an out-of-band common channel signaling system that is typically used to perform the signaling involved in setting up calls between switches in the publicly switched telephone network (PSTN). The SS7 protocol messages include the originating or source telephone number for a call, the terminating or destination telephone number as well as information regarding the telephone circuits to be used by the call. For instance, a phone call will originate and send out a request to terminate the call to a destination. The switch receiving the origination request will send an SS7 message into the PSTN requesting routing and termination on the switch serving the destination. The PSTN will determine the connection between the originating and terminating switches including sending an SS7 message requesting a timeslot and facility for the call to terminate on the destination switch. Other common channel signaling (CCS) protocols exist which are used for call routing.
FIG. 1
illustrates an example of a multi-chassis multi-link session in a conventional network architecture
100
. A customer premise equipment (CPE)
110
is served by switch
120
which is part of the PSTN
10
and which communicates with PSTN
10
through trunk line
122
. An IP or ATM network
150
is served by NAS
140
which consists of multiple RACs
142
and
146
which communicate via intra-chassis communication link
144
. NAS
140
is connected to switch
130
of the PSTN
10
via multiple T1 facilities
132
-
136
. The ellipses between T1 facility
132
and T1 facility
136
indicate that a large number of T1 facilities may be used to connect a NAS to the PSTN
10
.
An example of call establishment for a multi-link connection in the architecture
100
will now be described. Multi-link calls can take a variety of forms including an ISDN basic rate interface (BRI) having a pair of B channels having the same source identifier, i.e. ISDN number, or an asynchronous multi-link originating from modems having different source phone numbers. The function of the conventional architecture of
FIG. 1
is discussed in the context of an ISDN BRI connection, but the call establishment scenario shown is also relevant to other types of multi-link connections.
Typically, a multi-link call will originate with a first link and then add a second link as needed. A first data link connection originates on CPE
110
which sends an I.451/Q.931 call set-up request message over a D channel to switch
120
. The set-up requests a first B channel B
1
connected to switch
120
and designates a destination address in IP or ATM network
150
as the destination of the call. Switch
120
determines that the destination of the call is served by another switch in PSTN
10
and identifies the first exchange in the route through PSTN
10
toward the destination switch
130
serving NAS
140
. NAS
120
then sends an initial address SS7 message to the first exchange in PSTN
10
which contains the ISDN number of the destination party (i.e. the destination address in IP or ATM network
150
), the type of connection (e.g. 64 Kbps), and the identity of the selected physical circuit to the first exchange (i.e. a channel on T1 facility
122
).
Exchanges within PSTN
10
then route the call to the terminating NAS
140
based upon the ISDN number of the destination address in IP or ATM network
150
. At each intermediate exchange in PSTN
10
, the incoming SS7 call set-up message is received and analyzed. Based upon the destination address of the call and other routing information, the intermediate exchange makes a routing decision, selects a link to the next exchange, sets up a connection between the incoming and outgoing links and forwards the SS7 call set-up message to the next exchange. This process continues until the SS7 call set-up message arrives at the terminating switch
130
connected to NAS
140
.
Switch
130
will select an idle channel on one of the T1 facilities
132
-
136
serving NAS
140
as the circuit for the connection to NAS
140
. The T1 facility and timeslot will typic
Armistead R. Ashby
Hays Tod W.
Sargent Robert L.
Cisco Technology Inc.
Harrell Robert B
Marger & Johnson & McCollom, P.C.
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