Jay, I'd like to add another fruit to the basket.
>> There's lots of apples and oranges comparisons being made here.<<
There's more than one flavor of DSL access and aggregation being offered today. Your analysis, I believe, takes into account only the classic DSLAM model, the one that is supposed to be "dedicated and unobstructed," as it was originally put forth by Bellcore and the ANSI/ITU folks.
But we should keep in mind in these discussions that this fundamental set of attributes doesn't always translate into a dedicated point to point connection, unobstructed, to the network's edge and core. The congestion problems and the other parameters between the ISP and the Core are well known today. But that's not what I'm referring to. Rather I am referring to the Central Office collection device used for DSL, itself, which is more nebulous.
More often than not a standards based device is used by the RBOCs and larger CLECs (read: ATM-backplaned DSL Access Multiplexers, or DSLAMs). But there are also DSL vendors whose platforms employ a form of DSL "concentration" in their design, where contention and collision issues are just as prevalent as those found on the shared media cable TV segment. You are more likely to see this kind of device used by emerging Data CLECs, or DLECs, and ISPs, although, I don't know who they are or what the breakdown is.
The distinctions lie in the means of aggregation and forwarding, where one is a DSLAM and the other a DSLAC or DSL Access "Concentrator," often employing an ethernet-like fabric in the backplane. There are advantages and disadvantages to both approaches.
For more on this, see the Telephony Magazine article that was written about a year ago. You'll have to read between the lines on this one since it was written by a vendor, thus you may only see the bright side on the surface.
internettelephony.com
The article is copied below for posterity, but the url site contains some informative graphics that I can't reproduce here.
Regards, Frank Coluccio
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"Powers of concentration"
As the number of ADSL users increases, an access concentrator can help provision and manage them more efficiently
BENJAMIN "TAC" BERRY
In the heated battle for market share, the local service provider's most formidable weapon is the twisted-pair copper that already connects all likely customers--now approaching 700 million--to a switching center. Asymmetrical digital subscriber line lets carriers capitalize on this singular asset and acquire rapid market share.
ADSL offers more than enough bandwidth for high-speed access to the burgeoning dial-up PC market. It also solves the network problems that extended data downloading times create and offers a migration path to the latest network infrastructures such as asynchronous transfer mode. Cable TV companies are bound to hybrid fiber/coax and increased subscriber investment; ADSL converts existing lines to megabit access ports.
To capitalize on the potential that ADSL offers, access to the customer infrastructure must be upgraded. The architecture for the data backbone must be implemented rapidly. And carriers must use an infrastructure that supports higher subscription rates over time.
An essential component of an infrastructure that can meet all these needs is a data/video access concentrator, which will permit a service provider to consolidate data traffic over the high-speed ADSL connections into a single high-speed data pipe for network interface, much like a digital subscriber line access multiplexer (DSLAM).
The key differentiating point between a DSLAM and a data/video access concentrator is that manufacturers offering or planning to offer DSLAMs typically envision an ATM-based design, whereas a data/video access concentrator is Internet protocol-based. Support for IP may facilitate the adoption of data/video access concentrators. A data/video access concentrator also enables a single ADSL connection to be used simultaneously for video and data transmission.
A flexible design
ADSL was originally intended for video transmission over phone lines. To meet this application, early ADSL services, as defined by Bellcore, were to include multiple downstream channels operating at 1.5 Mb/s and multiple duplex upstream channels totaling 640 kb/s.
Now the emphasis has shifted to data services, specifically Internet access. Rather than offering a single giant pipe to the user, however, a data/video access concentrator leverages the multipath capability defined in the early Bellcore specifications, offering two downstream paths totaling as much as 8 Mb/s, and two upstream data paths totaling as much as 640 kb/s over every ADSL link.
A data/video access concentrator can provide several advantages to the local communication services provider including consolidating the high-speed backbone connections for multiple subscribers.
Additionally, an access concentrator can allow better usage of rack space in the central office. In a typical application, the carrier would devote multiple shelves in the CO to support ADSL service. Each shelf could hold two concentrator cards that could support six modems each and would include a management interface card (Figure 1). The carrier could add concentrator, modem and management cards as needed to support new users.
The data/video access concentrator incorporates data level bridging of multiple TCP/IP data streams onto a common network interface.
In Amati's design, for example, the output from the data/video access concentrator to the network is a combination of 10BaseT connections for data and RS-422 connections for video. Using the multiple data channels per modem, an access system can then provide data and video channel access to each subscriber. The CO shelf concentrates the data traffic to a network Ethernet interface and provides individual serial interfaces for the video traffic.
At the CO, the data/video access concentrator includes external POTS splitters to separate the voice and data traffic from the combined telephone/ADSL signals in the twisted pair cable connection (Figure 2). A shelf configuration for the POTS splitter simplifies maintenance and CO distribution of the services.
Under control
As the number of ADSL customers increases, it will be increasingly important for carriers to manage a data/video access concentrator through a standard interface such as the simple network management protocol (SNMP). This can be accomplished by including an enterprise management information base (MIB) and an SNMP interface agent on the management interface card installed in each shelf.
The interface agent will be used to convert information about the transmission line, modem and configuration into the SNMP MIB structure. A management interface, supporting up to four shelves, or 48 modems, would deliver this information to an SNMP interface manager in a central location where all ADSL connections could be administered and maintained.
The management interface would support commands such as "get" to retrieve data, "set" to set a data parameter and "trap" to set an alarm parameter. The MIB would provide access to parameters such as performance of each ADSL line and modem, line statistics, noise margins and general system alarms. A 10BaseT connection on the master shelf should be reserved for transmitting management data.
The development of an access concentrator will be key to supporting widespread use of ADSL. A shelf system can provide a platform for future ADSL modem designs based on new generations of semiconductors and ADSL software. An access concentrator will enable upgrades to newer system protocols as service providers require them. From today's packet-based interface, an access concentrator can be extended to provide frame- or cell-based access in the future.
Benjamin "Tac" Berry is Vice President of Marketing for Amati Communications Corp., Sunnyvale, Calif.
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