26 CHAPTER 2: USING THE BRIDGE /ROUTER IN YOUR NETWORKUsing PPP on Dial-Up and Leased LinesAll dial-up and leased lines use PPP. If you want to use multiple lines orchannels to connect to the same destination, you can use Multilink PPP.Multilink PPP is especially useful for ISDN, as described in the followingsection.ISDN ISDN consists of two bearer (B) channels and one delta (D)channel. Each B channel can transmit up to 64 Kbps; the D channeltransmits signaling information about the call to computers at theswitching system at a rate of 16 Kbps.With Multilink PPP, you can use both B channels together for a combineddata rate of 128 Kbps. With PPP, you can use each 64 Kbps B channelseparately.Some ISDN lines support up to 56 Kbps per B channel. For moreinformation, contact your ISDN service provider.Using Packet-Switched Network ServicesYou can use packet-switched services with your bridge/router instead ofdial-up or leased lines to take advantage of high performance for areasonable price. This section describes X.25 and Frame Relay.X.25 An X.25 packet-switched network is defined by the InternationalTelecommunications Union (ITU) Recommendation X.25, a globalstandard that began development in the 1970s. The recommendationFractional T1 PPP CSU/DSU Up to 24 channels (DS0s) at 64Kbps eachDigital Data Service (DDS) PPP CSU/DSU Up to 64 Kbps* Some ISDN lines support up to 56 Kbps per B channel. For more information, contact your ISDN service provider.Table 4 Dial-Up and Leased Line ServicesTelco Line ProtocolSerial Device (Requiredon Flex-WAN Serial Ports) Data Transfer RateTable 5 Packet-Switched ServicesPacket-SwitchedServices/Protocol Telco LineSerial Device (Required onFlex-WAN Serial Ports) Data Transfer RateX.25 POTS Modem Up to 56 KbpsFrame Relay Leased line CSU/DSU Up to 2.048 MbpsSMDS Leased line CSU/DSU Up to 2.048 Mbps
