mechanical and the electrical characteristics of the physical interface, including
connectors, network interface cards, and voltage and transmission distances.
The network protocol RS-232, once heavily utilized in telecommunications
and data networks, is defined solely by the physical layer. Other network pro-
tocols, such as Ethernet, are defined by the physical layer as well as some of
the entire next layer in the stack—the data link layer.
Data Link Layer
The data link layer takes the data bits and “frames,” and creates packets
of the data to guarantee reliable transmission. This layer adds source and
destination addresses to the data stream as well as information to detect
and control transmission errors. The data link layer has two sublayers.
One is the logical link control (LLC) sublayer, which essentially maintains
the communications link between two devices on the network. The other
is the media access control (MAC) sublayer which manages the trans-
missionofdatabetweentwodevices.ThenetworkcardonaPChas
a MAC address, essentially a unique address for every device on a local area
network.
The details of the data link layer can be specified differently and are
reflected in various network types (Ethernet, token ring, etc.). Each network
type has its own method of addressing, error detection, control of network
flow, and so forth.
Network Layer
The network layer routes data packets through the network. It deals with net-
work addressing and determines the best path to send a packet from one
network device to another. The Internet Protocol (IP) is the best example of
a network layer implementation.
Transport Layer
The transport layer is responsible for reliable transport of the data. At times it
may break upper-layer data packets into smaller packets and then sequence
their transmission. The Transmission Control Protocol (TCP), one of the
major transport protocols, is typically used with the best-known network layer
protocol, IP, and is referred to as TCP/IP.
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