B-CHANNELS - ISDN Service comes provisioned with 3 "Channels". There is one "D Channel" used for setting up the ISDN call and other purposes. MCN ISDN does not currently support the use of the D Channel for other purposes, although this has been done with other companies. There are two "B Channels" used for the transmission of Data (or voice, etc.) over the ISDN lines. These are configurable by the end user/customer's equipment to optionally use either 1, or 2 Channels at any given time. In other words, a good piece of equipment would be able to "throttle" the ISDN Terminal Adaptor/Router to 1 B Channel only, or, always use 2 B Channels regardless of the bandwidth requirements, or, dynamically bring up the second B Channel as required by bandwidth requirements.

BANDWIDTH ON DEMAND - Better ISDN equipment comes with the ability to only use one B Channel if that is all that is required during the current "session". Typically, sending and receiving email (other than "attachments") does not require more than one B Channel. Downloading an Attachment or a graphics file or an FTP session will typically cause the second B Channel to be activated "on demand".

CENTREX - This definition is only accurate within the context of MCN's ISDN Centrex Option for Internet Access. The Centrex Option allows MCN customers to place ISDN calls to MCN's ISDN Internet equipment without having to pay Pacific Bell for "usage" charges which are typically 3¢ for the first minute of the current "session" and 1¢ for each additional minute for the current "session".

It is very important to understand that the "current session" is for the duration of the current B Channel in question AND for the duration of the current ISDN telephone call for that particular B Channel. Without Centrex, if you configure your ISDN Terminal Adaptor/Router to drop the ISDN call after being "idle" for, say, 3 minutes, then that is the end of that "session" for that B Channel. Therefore, when you reconnect on a new "session", you will be charged 3¢ for the first minute again. This means that you may be better off staying connected for 3 minutes rather than being charges 3¢ to set up another call. If you are not careful, your ISDN telephone bill could run as much as 5 times the normal due to excessive bringing up/tearing down call charges. If you have the Centrex option included with ISDN Pro, you do not need to worry about "Usage" charges. Please carefully consider if you will need this option when ordering your ISDN line. If you chose the Centrex Option (included within the ISDN PRO service), you MUST have MCN order the line for you. You will NOT be able to "upgrade" an existing ISDN line to a Centrex ISDN line. If you do NOT need this option, then you may order your line directly from Pacific Bell ISDN Service Center.

IDLE - Your connection to MCN (either analog or ISDN) is considered "idle" if there has been no IP packets either sent or received within the "Idle Timeout" period configurable in your equipment.

SESSION - The current telephone call, either analog or ISDN. ISDN calls are capable of 2 concurrent sessions. If your modem/TA/Router drops the connection, then the session has ended. This is an important concept in managing the cost of ISDN service.

Mendocino Community Network
P.O. Box 2445, Mendocino, California 95460
707-937-1444 Technical Support
707-937-0733 Fax