Principles and technologies of local area networks - Intelligent Buildings and Building Automation - page 52

to the medium type in use. It provides a physical path for electrical signals
representing bits of transmitted information. It also defines the characteristics
of these signals, such as voltage and current levels, frequencies and timing. It
specifies the mechanical properties of network cables and connectors. From
the point of view of upper layers, the Physical Layer ensures that streams of
bits produced at the interface between the Data Link and Physical Layers by
the transmitting node arrive at the interface between the Physical and Data
Link Layers in all receiving nodes in a bus-topology LAN. The Physical Layer
is the only real interconnection between network nodes.
Data Link Layer: The Data Link Layer defines rules for sharing the use
of the Physical Layer among network nodes. On a bus-topology LAN, the
medium (Physical Layer) is shared in time-division fashion. Information is
transferred in addressed frames, one frame at a time. The format of these
frames and the method by which a node decides when to transmit or accept
a frame are defined. Two general types of frames are used. Data frames,
which convey the upper-layer messages, are also referred to as packets. Other
frames used by the Data Link Layer, such as token or acknowledge frames,
are called control frames. Error-detection and error-correction techniques are
used to ensure that packets are transferred error-free from source to destina-
tion nodes. From the point of view of upper layers, Data Link and Physical
Layers provide error-free transmission of data packets. For LAN implemen-
tations, the Data Link Layer is further divided into two sub-layers, namely
the Media Access Control (MAC; also known as Medium Access Control),
and the Logical Link Control (LLC) layers.
￿ Media Access Control: The MAC is peculiar to the particular access
method employed on the LAN. The MAC itself performs many func-
tions such as the reception of data from the upper layers (LLC), and the
encapsulation of this into frames according to the requirements of the
LAN access method employed. In addition, the MAC monitors the com-
munications channel to determine when the channel is clear, and then
passes the frame to the Physical Layer for transmission. Equally, in terms
of reception, the MAC accepts incoming data frames from the Physical
Layer. It then decapsulates the data and checks its integrity. Finally, in the
case of Ethernet/IEEE 802.3 LANs, the MAC is responsible for collision
detection, and the recovery from these conditions.
￿ Logical Link Control: The LLC sub-layer itself is designed to provide a
consistent level of service to the network layer regardless of the MAC in
use.
Network Layer: The Network Layer is beyond the scope of any LAN stand-
ard, but it provides users with a means of communicating between logical
networks (as opposed to LANs). It therefore includes facilities such as net-
work routing, addressing and, in some cases, flow control.
Transport Layer: This layer provides the basic interface between the