TELECOM Digest OnLine - Sorted: Re: Private Line History

Re: Private Line History
8 Feb 2007 19:23:58 -0800

Charles Gray wrote:

> In retrospect, the CCSA was a predecessor to the "software defined
> network" that was introduced by AT&T, Sprint and MCI in the late
> 1980s.

Once again, thanks for your excellent and accurate description.

> I don't know for sure, but I expect the switches were the Western
> Electric 4E tandems.

Here's an extract from the Bell System history:

CCSA -- Common Control Switching Arrangement

"Growth in public switching was paralleled by expansion of private
line services, and, as might be expected, this required special
services and featurs. In particular, arrangements were developed for
No. 5 crossbar and No. 1 ESS so that the central office provided not
only Centrex service for one or more customers but acted in nationwide
two-level hierarchical private networks that served the
telecommunication needs of large businesses. By dialing an access
code, usually 8, the customer's employees could reach company or
agency locations on the network. In order for the switching offices
to be shared by several private networks, common-control switching was
required, hence the name CCSA. A special 4-wire version of the No. 5
crossbar system was developed in 1960 to serve as the first or higher-
level (tandem) in these networks."

"In many cases, CCSA networks include not only PBXs but also Centrex
service from the same switching systems. CCSA service includes not
only access to private network switching and transmission facilities,
but also local and distant (foreign exchange) access to public network
and dial or tie trunk facilities between PBXs connected to the

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