Network Interface Layer
The bottommost layer of the Internet protocol suite (TCP/IP) is the Network Interface Layer. It defines the software and hardware used to carry TCP/IP traffic across an underlying network technology. In the language of the OSI Reference Model (OSI-RM), the Network Interface Layer corresponds to the Physical Layer and Data Link Layer (DLL). In the old adage, "IP over everything," the Network Interface Layer is the everything (or anything) over which IP can run. That includes any local area network (LAN), metropolitan area network (MAN), or wide area network (WAN) technology. Examples include:
- Any LAN Media Access Control (MAC) scheme: This includes IEEE standards such as IEEE 802.3 (Ethernet), IEEE 802.4 (Token Bus), IEEE 802.5 (Token Ring), IEEE 802.11 (Wireless LAN, or WLAN), IEEE 802.16 (WiMAX), and IEEE 802.17 (Resilient Packet Ring, or RPR). It also includes such things as Fiber Distributed Data Interface (FDDI). Many of these technologies have now become all but obsolete (or completely obsolete). Ethernet and WLAN (or Wi-Fi) predominate. Some carriers are implementing RPR within their networks. WiMAX is also being deployed by some carriers as a wireless Internet access alternative.
- Point-to-Point Protocol (PPP): As the name implies, PPP was developed for use over any serial point-to-point circuit. That includes switched access circuits such as ISDN and POTS, as well as dedicated circuits (also known as leased lines or private lines) using technologies such as DS-0, DS-1, DS-3, or any of the SONET speeds.
- Multilink Point-to-Point Protocol (MLPPP): Also known as Multilink PPP (MLP, or simply MP) is a version of PPP that supports inverse multiplexing (at the Data Link Layer) two or more PPP connections into a single logical communications channel.
- Frame relay: This fast packet service was initially developed to replace the older X.25 services, supporting transmission rates from DS-0 through DS-3. Today, frame relay is considered a legacy service that is rapidly giving way to Metro Ethernet services.
- Asynchronous Transfer Mode (ATM): This cell relay technology is also considered a fast packet service, but it is capable of supporting data, voice, and video. ATM can operate at transmission rates from DS-1 through the higher SONET speeds, although many carriers cap their services at OC-12. As with frame relay, ATM is widely considered a legacy technology.
- Asymmetric Digital Subscriber Line (ADSL): ADSL is one of many digital subscriber line (DSL) technologies. This variant is aimed at residential and small office, home office, (SOHO) markets. Some versions deliver up to 25 Mbps downstream and up to 5 Mbps upstream, hence the term asymmetric. It is commonly used for Internet access.
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