Page 296 - Optical Communications Essentials

Optical Networks

286 Chapter Seventeen

User application
program

Provides general services
Layer 7 Application related to applications (e.g.,
file transfer, user access)
Formats data (encodes,
Layer 6 Presentation
encrypts, compresses) Support
Responsibility
user
of host system
Maintains dialog between applications
Layer 5 Session
communicating devices
Provides reliable end-to-end
Layer 4 Transport
data transmission
Switches and routes
Layer 3 Network
information units
Responsibility Provides data exchange Govern the
of network Layer 2 Data link between devices on the same medium communication
facilities
Transmits bit stream
Layer 1 Physical
to physical medium
Figure 17.4. General structure and functions of the seven-layer OSI reference model.

a path through a series of connected nodes, and the nodes along this path
must forward the packets to the appropriate destination. Currently the dom-
inant network layer protocol is the Internet Protocol (IP).
■ 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, error
rate, delay time to establish a connection, cost, security, and priority. The trans-
mission control protocol (TCP) used in the Internet is an example of a trans-
port layer protocol.
■ The higher layers (session, presentation, application) support user applications,
1
which are not covered here (see the network books by Keiser or Forouzan 2
for more details).

In actual systems there are many implementation and protocol variations on
the classical layering model. A certain layer may work together with lower or
higher layers, or a layer may be divided into several sublayers. As an example,
consider the widely used IP over SONET architecture. In this case, IP operates
at the network and data link layers to format packets in such a way that they

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