|  | 4.1 | PPP
| 4.1.3 | PPP layer functions

PPP uses a layered architecture, as shown in the Figure. With its lower-level functions, PPP can use: Synchronous physical media, such as those that connect Integrated Services Digital Network (ISDN) networks. Asynchronous physical media, such as those that use basic telephone service for modem dialup connections. With its higher-level functions, PPP supports or encapsulates several network-layer protocols with NCPs. These higher-layer protocols include the following: BCP -- Bridge Control Protocol IPCP -- Internet Protocol Control Protocol IPXCP -- Internetwork Packet Exchange Control Protocol These are functional fields containing standardized codes to indicate the network-layer protocol type that PPP encapsulates.
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