13.1 Concepts of Optical Networks
513
connections, data link control, and routing and relaying functions that support the
actual transmission of data. The upper layers support user applications by structuring
and organizing data for the needs of the user.
In the classical OSI model, the various layers carry out the following functions:
Physical Layer The physical layer refers to a physical transmission medium, such
as a wire or an optical fiber, which can handle a certain amount of bandwidth. It
provides different types of physical interfaces to equipment, and its functions are
responsible for actual transmission of bits across an optical fiber or metallic wire.
Data Link Layer The purpose of the data link layer is to establish, maintain, and
release links that directly connect two nodes. Its functions include framing (defining
how data is structured for transport), multiplexing, and demultiplexing of data. Examples of data link protocols include the point-to-point protocol (PPP), Ethernet, and
the high-level data link control (HDLC) protocol.
Network Layer The function of the network layer is to deliver data packets from
source to destination across multiple network links. Typically, the network layer
must find a path through a series of connected nodes, and the nodes along this path
must forward the packets to the appropriate destination. The dominant network layer
protocol is the Internet Protocol (IP).
Transport Layer The transport layer is responsible for reliably delivering the
complete message from the source to the destination to satisfy a quality of service
(QoS) requested by the upper layer. The QoS parameters include throughput, transit
delay, bit-error rate, delay time to establish a connection, cost, information security,
and message priority. The transmission control protocol (TCP) used in the Internet
is an example of a transport layer protocol.
Higher Layers The higher layers (session, presentation, application) support user
applications, which are not covered here.
Note that there is nothing inherently unique about using seven layers or about the
specific functionality in each layer. In actual applications some of the layers may be
omitted and other layers can be subdivided into further sublayers. Thus the layering
mechanism should be viewed as a framework for discussions of implementation and
not as an absolute requirement.
13.1.4 Optical Layer Functions
When dealing with optical network concepts, the term optical layer is used to
describe various network functions or services. The optical layer is a wavelengthbased concept and lies just above the physical layer, as shown in Fig. 13.4. This
means that whereas the physical layer provides a physical connection between two
nodes, the optical layer provides lightpath services over that link. A lightpath is an
513
connections, data link control, and routing and relaying functions that support the
actual transmission of data. The upper layers support user applications by structuring
and organizing data for the needs of the user.
In the classical OSI model, the various layers carry out the following functions:
Physical Layer The physical layer refers to a physical transmission medium, such
as a wire or an optical fiber, which can handle a certain amount of bandwidth. It
provides different types of physical interfaces to equipment, and its functions are
responsible for actual transmission of bits across an optical fiber or metallic wire.
Data Link Layer The purpose of the data link layer is to establish, maintain, and
release links that directly connect two nodes. Its functions include framing (defining
how data is structured for transport), multiplexing, and demultiplexing of data. Examples of data link protocols include the point-to-point protocol (PPP), Ethernet, and
the high-level data link control (HDLC) protocol.
Network Layer The function of the network layer is to deliver data packets from
source to destination across multiple network links. Typically, the network layer
must find a path through a series of connected nodes, and the nodes along this path
must forward the packets to the appropriate destination. The dominant network layer
protocol is the Internet Protocol (IP).
Transport Layer The transport layer is responsible for reliably delivering the
complete message from the source to the destination to satisfy a quality of service
(QoS) requested by the upper layer. The QoS parameters include throughput, transit
delay, bit-error rate, delay time to establish a connection, cost, information security,
and message priority. The transmission control protocol (TCP) used in the Internet
is an example of a transport layer protocol.
Higher Layers The higher layers (session, presentation, application) support user
applications, which are not covered here.
Note that there is nothing inherently unique about using seven layers or about the
specific functionality in each layer. In actual applications some of the layers may be
omitted and other layers can be subdivided into further sublayers. Thus the layering
mechanism should be viewed as a framework for discussions of implementation and
not as an absolute requirement.
13.1.4 Optical Layer Functions
When dealing with optical network concepts, the term optical layer is used to
describe various network functions or services. The optical layer is a wavelengthbased concept and lies just above the physical layer, as shown in Fig. 13.4. This
means that whereas the physical layer provides a physical connection between two
nodes, the optical layer provides lightpath services over that link. A lightpath is an