Multiplex communications – Communication techniques for information carried in plural... – Adaptive
Reexamination Certificate
2001-10-10
2002-12-03
Marcelo, Melvin (Department: 2663)
Multiplex communications
Communication techniques for information carried in plural...
Adaptive
C370S474000
Reexamination Certificate
active
06490296
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to the field of computer networks. More particularly, the invention relates to private computer networks based on asymmetric transfer mode technology. Specifically, a preferred implementation of the invention relates to multi-link segmentation and reassembly for bonding multiple permanent virtual circuits in an inverse multiplexing arrangement.
2. Discussion of the Related Art
Asymmetric Transfer Mode (ATM) is a popular technology for providing secure and reliable Virtual Private Network arrangements. The use of ATM Switches permits the Service Provider (such as an Inter-Exchange Carrier, or “IXC”) to better utilize the inter-machine trunking facilities by providing trunk bandwidth only when there are ATM cells available for transmission. This is in contrast with the notion of a (truly) Private Network wherein the Service Provider is required to dedicate the prescribed bandwidth for the end customer in all inter-machine trunking facilities provisioned for the connection.
Consider, for example, an end-user that requires a Private Network linking three separate locations, A, B, and C. Assume that a 56 kbps connection is required between each pair of locations. One approach to providing this service is to have dedicated 56 kbps access links between the customer premises and the Service Provider Network (often called the “cloud”) Each location would have two such dedicated “DDS” (Digital Data Service) links. The Service Provider would use Digital Access and Cross-connect Systems (“DACS”) to manage the private network. The “cloud” could then be visualized as a multitude of such DACS machines interconnected by trunks. These trunks are usually DS
1
links that may be further amalgamated into DS
3
or SONET multiplexed assemblies for transmission over long haul facilities. Each 56 kbps link is treated as a D
10
and the DACS machines ensure that each 56 kbps link is maintained across the Network by establishing the correct cross-connections. Fundamentally, a 56 kbps link between points A and B require that all intervening DACS machines be appropriately provisioned and that a D
10
be reserved in all intervening facilities for that particular link. This reserved D
10
represents bandwidth unavailable for any other use. The advantages (to the end-user) of such an arrangement are privacy and availability.
Referring to
FIG. 1
, an example of a conventional virtual private network is shown where the 56-kbps link between A and C is treated as a D
10
(a D
10
is a 64 kbps channel). One D
10
in the inter-machine trunk labeled IMT-A that interconnects DACS machines D-
1
and D-
2
is reserved for the link and the cross-connect maps in D-
1
and D-
2
ensure the connectivity. Since each D
10
is a “time-slot” within a DS
1
, the networking method is referred to as TDM (time division multiplexing). Similarly, the link between A and B requires that a D
10
be dedicated in IMT-B and IMT-D and the cross-connect maps in D-
1
, D-
3
and D-
4
must be coordinated to ensure connectivity. Likewise, the link between B and C requires the reservation of D
10
s in IMT-C and IMT-D and the coordination of cross-connect maps in D-
2
, D-
3
, and D-
4
.
Clearly, the access method can be enhanced to DS
1
(“T
1
”) whereby the two 56 kbps links at a location are assigned to two D
10
s in the access DS
1
. With DS
1
(1.544 Mbps) access, the same form of Network (DACS machines interconnected by high-speed trunks) can be deployed to provide links of the form N×64 kbps or N×56 kbps by utilizing multiple D
10
s per link (N is between
1
and
24
, inclusive).
The example depicted in
FIG. 2
represents a situation where there is a 8×64=512 kbps link between A and C, a 6×64=384 kbps link between C and B, and a 12×64=784 kbps link between B and A. The corresponding bandwidth must be reserved on the various IMTs connecting the DACS machines. Clearly, no single link in the above example can be greater than 24×64=1536 kbps since we are assuming DS
1
access.
A problem with this technology has been that the bandwidth is wasted when there is no data available for transmission. The DSU/CSU used at the customer premise to drive the access segment will fill in null data (such as flags or equivalent fill-in units) to maintain the synchronous data rate (1.544 Mbps). The Service Provider network is unaware of such idle data fill-in units and the associated bandwidth is thus required to transport such useless data across the cloud. Generally speaking, in a TDM-based private network, connectivity is provided at the bit level; in the cloud no determination is made as to whether the bits being transported correspond to actual data or to fill-in units.
The use of ATM technology allows the sharing of access and inter-machine trunks by multiple (logical) links. The underlying premise of ATM is that a data stream can be segmented into cells. The ATM standard calls out for cells that contain 48bytes of user data. Appended to each cell are 5 bytes of overhead that includes an identifier of the destination. This identifier is encapsulated as a combination of “VPI” and “VCI” (for Virtual Path Identification and Virtual Channel Identification). Instead of the DACS machines in the prior example, ATM Switches are deployed and the inter-machine and access trunks carry cells rather than channelized information. The equivalent of cross-connection is performed by the ATM Switches on a cell-by-cell basis, using the VPI/VCI as the underlying pointer to match the ingress and egress trunks from the Switch. A Permanent Virtual Circuit (PVC) is established by provisioning the intervening ATM Switches between the two (or more) points of customer (end-user) access into the ATM cloud. In the configuration of three end-user locations considered above, cells from location A destined to location B will have a prescribed VPI/VCI in the cell-overhead when launched from location A. The 48 bytes of user-data are transported across the cloud though the overhead (i.e., the cell header) may be modified. Cells associated with a specific PVC will always traverse the same route and thus cell sequencing is not an issue. If there is no data available for transmission, the access multiplexer will insert “filler” cells to maintain the synchronous transmission rate of the access link but these filler cells can be discarded by the network. This arrangement is depicted in FIG.
3
.
It is certainly possible to create private networks wherein the Network Service Provider maintains TDM links between the various access multiplexers and the ATM (or equivalent) switching capability resides in the CPE equipment. This form of private networking is quite common and, more often than not, the TDM links between multiplexers are T
1
links and the access multiplexers in this situation are referred to as T
1
multiplexers.
Whereas in TDM-based network arrangements the address or identity of a link is defined by its position (in time) within the DS
1
stream, in an ATM-based network the address of the destination is encoded appropriately by the access multiplexer on a cell-by-cell basis. Thus at Location A, data (cells) destined for Location B will be assigned a VPI/VCI, say “a”. Likewise access multiplexers in all locations are assigned VPI/VCI codes for each of their PVCs depending on the end points of the PVC (they do not have to be the same code at the two end points of the PVC). The ATM Switches D-
1
and D-
2
are programmed such that a cell from Location A with VPI/VCI=“a” will be delivered to Location C and the VPI/VCI there may be “c”. Whereas it is natural to establish the “shortest” path for a link, there is no fundamental restriction to that effect. In fact, the link between A and C may be established by creating a permanent virtual circuit that traverses D-
3
as an intermediate step.
Inter-machine trunks can thus carry cells associated with a multiplicity of virtual circuits. Since the bandwidth is used on an “as-neede
Jacobsen Gary
Kraba Kamila
Lai Chien-Chou
Shenoi Kishan
Sommer Jeremy
Gray Cary Ware & Friedenrich LLP
Marcelo Melvin
Mehra Inder Pal
Symmetricom, Inc.
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