Chapter Two: Getting Started | 3. Choose a connection protocol 21 |
| |
Choosing a connection protocol
Since the DIVA LAN ISDN Modem supports so many potential connections, a set of rules is needed to control how the two ISDN B-channels are shared between connections. These rules are based on the type of connection protocol you decide to use for your data traffic. The DIVA LAN ISDN Modem supports the two protocols: PPP and V.120.
•PPP is a widely-used standard for transferring data on point-to-point links. The DIVA LAN ISDN Modem supports four variations of the PPP protocol: PPP, Multilink PPP, Multilink PPP+BACP, and Multilink PPP+AO/DI. Each version has its own specific features as detailed later in this section.
•V.120 was designed to allow non-ISDN devices to communicate with the ISDN network. The DIVA LAN ISDN Modem provides support for V.120, so you can connect with such legacy systems. For example, in many countries around the world, Compuserve offers access to their network over ISDN, using the V.120 protocol.
By choosing the correct protocol variation for your connection, you control how the ISDN channels are assigned and how the DIVA LAN ISDN Modem provides support for analog calls.
PPP
PPPoperates on a single ISDN B-channel. Since two B-channels are available, the DIVA LAN ISDN Modem can support two PPP connections at the same time. This is useful if you need to have two connections that need to be established at the same time. For example, to maintain a connection to the Internet and to a remote corporate network, or if you intend to support incoming data calls.
By default, the DIVA LAN ISDN Modem is configured to support call bumping. However, a PPP connection will never be disconnected to allow an incoming or outgoing analog call to connect. If two PPP connections are active, no analog calls can be made or received.
Multilink PPP
When you activate multilink PPP (MLPPP), the DIVA LAN ISDN Modem combines both your B-channels into a single 128 kbps communications link. Both B-channels remain in use as long as the connection is active, which may result in excessive ISDN connection charges if the link is frequently idle.
MLPPP reserves both B-channels for its exclusive use. This means that if your Internet connection uses MLPPP, you will not be able to make or receive other data calls.
Analog calls, however, are governed by a different set of rules. By default, the DIVA LAN ISDN Modem is configured to support call bumping. Call bumping causes MLPPP to drop one B-channel when you make or receive an analog call. This reduces the throughput of the data call to 64 kbps. When the analog call is done, MLPPP re-establishes the second B-channel, returning the data connection to 128 kbps.
Only one PHONE port can be used when an MLPPP connection is active. This can be either PHONE 1 or PHONE 2.
Multilink PPP + BACP
To save on communications charges, you can use BACP in combination with MLPPP. The DIVA LAN ISDN Modem uses BACP to monitor incoming and outgoing traffic, and to open or close the second B-channel as the traffic volume warrants. This reduces your communications charges by ensuring that two ISDN channels are never active when one is sufficient. For information on how to configure BACP, see “Configuring BACP” on page 61.
As with MLPPP, MLPPP + BACP reserves both B-channels for its exclusive use. This means that if your Internet connection uses MLPPP + BACP, you will not be able to make or receive other data calls.
As with MLPPP, call bumping causes MLPPP + BACP to drop one B-channel (if both are active), when you make or receive an analog call.
