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T1 E1 A-Law mu-law Testing

To comply with a prior directive that all Siemens and Rolm family of PBX's be capable of communicating amongst each other in a CorNet WAN, switches marketed in Europe and North America had to be able to communicate amongst each another in as transparent of a manner as possible.

Testing was to be done by the test team in Munich, Germany but at the last minute, Munich test personnel became unavailable and I was chosen to travel there for the testing.

Specific areas of concern were feature transparancy and carrier compatibility. Europe used E1 carriers and A-law voice companding while North America used T1 carriers and mu-law voice companding. Anticipated features of such international WAN were thought to be transparent station features, international call center applications and video conferencing.

T1 versus E1

North America uses the 24 channel 1.544 Mbit/s T1 carrier whereas Europe uses the 32 channel 2.048 Mbit/s E1 carrier. The T1 standard is about a decade older than the E1 standard so, the lessons learned from T1 combined with improved technology in theory made E1 a better carrier standard. In practice, the T-carrier could be steadily enhanced and today, both carrier technologies serve equally well in their respective markets.

A-Law versus mu-Law

The European telephone system uses a system of companding known as A-law whereas North America uses mu-law. No system being perfect, both have their advantages and disadvantages. Using modern video terminology the difference between A-law and mu-law is analogous to that of MPEG and AVI. Neither is perfect but to the user, the diffference is for all practical purposes, indistinguishable.

My focus was to verify that German and North American features worked as well over an international CorNet WAN as they did locally. By international treaty, A-law companding is used when communicating between North America and Europe. In the real world, compander selection is done by an international telephone exchange.

A challenge in testing the interoperability of a T-carrier PBX and an E-carrier PBX is carrier incompatibility. We neither had an international exchange available nor were we going to reinvent any wheels so, off the shelf A-law/mu-law converters which also performed T1-E1 conversion would sit between the German Hicom PBX and an American Hicom PBX during testing.

After arriving at the test lab in Munich, Germany, I was shown the lab. A German Hicom PBX was available and fairly well provisioned. The American Hicom PBX was sitting in a corner powered down with nothing connected. No 9200 or 9751 was present. International WAN was to be tested only between Hicom PBX's. So, before I could begin any testing, the American Hicom PBX had to be provisioned and the T1 spans connected to the Mu-law/A-law converters.

Testing was fairly straight forward. The German counterparts had been reassigned and no test cases were written so, I wrote test cases "on the fly". I set up a test position with American devices on one side, German on the other and administration terminals in-between. As usual, I found quite a few problems but most were not related to international CorNet WAN. They just happened to be problems I encountered as part of CorNet WAN testing.

When it came to call center testing, a German ACD was available but not an American system. Instead, I used a protocol analyzer on the American side to send and receive messaging back and forth to the German call center. Two video conference units were available, one on the German Hicom PBX and the other I provisioned on the American Hicom PBX. Video conferencing was in it's infancy and the video conferencing setup attracted considerable attention in the lab.

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