WHAT ARE DATA PROTOCOLS?
establishment, maintenance, and release of data-link connections. These protocols control
the flow of data and supervise error recovery. A most important function of this layer
is recovery from abnormal conditions. The data-link layer services the network layer or
logical link control (LLC; in the case of LANs) and inserts a data unit into the INFO
portion of the data frame or block. A generic data frame generated by the link layer is
illustrated in Figure 10.7.
Several of the more common data-link layer protocols are: CCITT LAPB, LAPD; IBM
SDLC; and ANSI ADCCP (also the U.S. government standard).
Network layer. The network layer moves data through the network. At relay and
switching nodes along the traffic route, layering concatenates. In other words, the higher
layers (above layer 3) are not required and are utilized only at user end-points.
The concept of relay open system is shown in Figure 10.23. At the relay switching
point, only the first three layers of OSI are required.
The network layer carries out the functions of switching and routing, sequencing,
logical channel control, flow control, and error-recovery functions. We note the duplication
of error recovery in the data-link layer. However, in the network layer error recovery is
network-wide, whereas on the data-link layer error recovery is concerned only with the
data link involved.
The network layer also provides and manages logical channel connections between
points in a network such as virtual circuits across the public switched network (PSN). It
will be appreciated that the network layer concerns itself with the network switching and
routing function. On simpler data connectivities, where a large network is not involved,
the network layer is not required and can be eliminated. Typical of such connectivities
are point-to-point circuits, multipoint circuits, and LANs. A packet-switched network is
a typical example where the network layer is required.
The best-known layer 3 standard is CCITT Rec. X.25 (Ref. 22).
Transport layer. The transport layer (layer 4) is the highest layer of the services asso-
ciated with the provider of communication services. One can say that layers 14 are the
responsibility of the communication system engineer. Layers 5, 6, and 7 are the respon-
sibility of the data end-user. However, we believe that the telecommunication system
engineer should have a working knowledge of all seven layers.
Only the first three OSI layers are required at an intermediate relay (switching) point.