Multiplex communications – Fault recovery – Bypass an inoperative switch or inoperative element of a...
Reexamination Certificate
2000-08-11
2004-12-21
Nguyen, Steven (Department: 2664)
Multiplex communications
Fault recovery
Bypass an inoperative switch or inoperative element of a...
C370S221000, C714S002000
Reexamination Certificate
active
06834038
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to high-speed data communication systems, and specifically to Digital Subscriber Line systems.
BACKGROUND OF THE INVENTION
Digital Subscriber Line (DSL) is a modem technology that enables broadband digital data to be transmitted over twisted-pair wire, which is the type of infrastructure that links most home and small business subscribers to their telephone service providers. DSL modems enable users to access digital networks at speeds tens to hundreds of times faster than current analog modems and basic ISDN service. DSL thus opens the most critical bottleneck in local-loop access to high-speed networks, such as Asynchronous Transfer Mode (ATM) and Internet Protocol (IP) networks, without requiring major investments in new infrastructure. A range of DSL standards have been defined, known generically as “xDSL,” wherein the various standards have different data rates and other associated features but share common principles of operation.
DSL subscribers are connected to high-speed networks through Digital Subscriber Line Access Multiplexer (DSLAM) systems. Because of the high cost of network bandwidth, a single DSLAM must typically be designed to serve between 100 and 1000 subscribers and to concentrate their traffic through one or a few network trunks. The need to serve such a large and potentially variable number of subscribers in the one DSLAM has led to the development of “multi-shelf” access architectures. Multi-shelf systems are built around a master unit, which communicates with a core network, such as an ATM network. The master unit multiplexes downstream and upstream packets, or cells, among multiple shelves, referred to herein as slave units. Each master or slave unit comprises a shelf unit, a concentrator unit, a plurality of subscriber ports serving respective subscriber premises via suitable DSL modems, and other ports used to connect the shelves to one another and/or to the network.
In order to maintain the transmission speeds and quality of service prescribed by xDSL standards, the modems associated with the ports of the slave units must be within a certain maximum distance (expressed in terms of the required run of copper-wire cable) from subscriber premises. On the other hand, the elements of the DSLAM are costly, and it is uneconomic to deploy the entire system in an area in which there are relatively few subscribers to serve. Therefore, some multi-shelf systems are designed for extension, whereby one or more slave units (shelves) are located remote from the master unit and provide network access to subscribers in the remote area. A moderate- to high-speed link connects each of the remote slave units to the master.
FIG. 1
is a block diagram that schematically illustrates a DSLAM system
21
offering access to an ATM network
22
. System
21
comprises a master unit
20
, which communicates with network
22
via a high-speed trunk
23
and multiplexes among slaves
30
. The master unit comprises a network interface block
24
, which includes a physical layer (PHY) and data link layer (ATM) interface, communicating with the network core. It also performs virtual path/virtual circuit identifier (VPI/VCI) translation and other higher-layer functions. These elements are well known in the ATM art.
A failure in the network interface of master
20
will interrupt service to the hundreds or thousands of subscribers who are served by system
21
. For this reason, in DSLAM systems known in the art, block
24
typically comprises two network interfaces (not shown explicitly in the figure), one active and one standby, both connected to network
22
. Generally, these interfaces are designed to function in accordance with standard ITU-T G.783 of the International Telecommunications Union (1994), entitled “Characteristics of Synchronous Digital Hierarchy (SDH) Equipment Functional Blocks,” which is incorporated herein by reference. In a “1+1” configuration, which is most commonly used for this purpose, master unit
20
receives data on both of the interfaces and can send data on either interface, as well. When another entity with which the master unit is communicating over network
22
finds that the performance of the active interface has degraded, it signals the master unit to switch interfaces, in a manner that is described by the standard. Alternative redundancy schemes for this purpose, such as 1:1 and 1:N, are also defined by the standard. All of these schemes require substantial hardware and software complexity, as well as duplication of resources, but the associated costs are justified by the need to avoid down-time of the master unit.
A concentrator
25
multiplexes among slave units
30
, which contain the subscriber ports. Subscribers who are geographically close to master unit
20
are typically served by local slave units (not shown), collocated with master unit
20
, or even by ports on the master shelf itself. Remote subscribers, however, are served by remote slave units
30
, which communicate with master unit
20
via IMA links, as described above. Each of the remote slave units is served by an IMA interface card
26
, via a respective link
40
. IMA is described in document AF-PHY-0086.001, promulgated by the ATM Forum (1999), entitled “Inverse Multiplexing for ATM (IMA) Specification Version 1.1,” which is incorporated herein by reference. IMA allows data to be transferred to and from one virtual ATM port by multiplexing over a number of parallel physical, point-to-point lines, such as E1 (2.048 Mbps) or T1 (1.544 Mbps) lines. Up to 32 such lines may be used, as stated in the IMA standard, although it is generally economical to use no more than eight lines, with the actual number of lines depending on the required bandwidth. IMA is typically used to serve small, remote sites, such as slaves
30
in
FIG. 1
, for which a single E1 or T1 line does not give sufficient bandwidth, while a high-rate interface is not needed or economically justified. Card
26
is referred to herein as an IMA-C card. Links
40
typically belong to an existing plesisynchronous digital hierarchy (PDH) network
28
, such as E1 or T1 lines provided over a public switched telephone network (PSTN). In accordance with the above-mentioned standard, each of the IMA links may comprise a plurality of such lines, which are multiplexed by card
26
.
Each slave unit
30
comprises an IMA interface card
32
, referred to herein as an IMA-R card, which communicates over the respective link
40
with the corresponding IMA-C card and similarly multiplexes among the multiple lines making up the link. A concentrator
34
serves a plurality of line cards
36
, each of which typically includes a number of subscriber ports. Unlike the above-mentioned SDH protection standards for the connection between master unit
20
and ATM network
22
, there is no standardized method of protection against failures in the IMA links between slave units
30
and master unit
20
. There is therefore a need for a solution to support protection of remote sites from failure of network interfaces or equipment at the main site.
SUMMARY OF THE INVENTION
It is an object of some aspects of the present invention to provide improved methods and apparatus for multiplexed network access by remote subscribers.
It is a further object of some aspects of the present invention to provide methods and apparatus for protection of such remote subscribers against equipment failures, including failures associated with a network interface at a main site.
In preferred embodiments of the present invention, a multi-shelf access system serves network users, who are coupled thereby to access a high-speed core network, typically an ATM network. The system comprises a master sub-system, which includes two master units, at a main site and at least one remotely-located slave unit, which communicates with the master units over a link, preferably an IMA link through a PDH network. The users communicate with the slave via respective ports, which preferably comprise Digital Subscriber Lin
Aloni Eli
Kaplan Menahem
Sdayor Ron
Zelig David
Darby & Darby
Nguyen Steven
Orckit Communications Ltd.
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