A basic PC logical flowchart is shown in Fig. 1. The Keyboard and Application are shown
as inputs to the CPU that would request access to the hard-drive. The Keyboard requests accesses to the hard-drive through user enquiries
such as "DIR" commands and the Application through "File Openings" and "Saves". The CPU, through the Disk Operating System, sends/receives
data from the local hard-drive ("C:" in this example).
A PC setup as a network workstation has a software "Network Redirector" (actual name depends on the network - we will use a generic term)
placed between the CPU and DOS as in Fig 2. The Network Redirector is a TSR (Terminate and Stay Resident) program which presents the network
hard-drive as another local hard-drive ("G:" in this example) to the CPU. Any CPU requests are intercepted by the "Network Redirector".
The Network Redirector checks to see if a local drive is requested or a network drive. If a local drive is requested, the request is passed
on to DOS. If a network drive is requested, the request is passed on to the network operating system (NOS).
Electronic mail (E-Mail), client-server databases, games played over the network, print and file servers, remote logons and network management
programs or any "network aware" application are aware of the network redirector and can communicate directly with other "network applications
" on the network. The "Network Aware Applications" and the "Network Redirector" make up Layer 7 - the Application layer of the OSI Model
as shown in Fig 3.
The Application layer identifies the network aware service by assigning a unique number to it. The actual number and identifier will depend on
the Network Operating System (NOS) used. For example, the suite of TCP/IP protocols calls the Application layer identifier a port or socket as shown
in Figure 4.
Assigning unique identifiers allows multiple network aware applications or services to be run on the same machine. This means that you can run
your email client while surfing the web at the same time. The unique identifier will direct the network information to the appropriate network
aware program. TCP/IP Well Known Ports are standardized and well documented. Commonly used well known ports are:
- 23 : Telnet - remote access
- 25 : SMTP - Simple Mail Transport Protocol
- 80 : HTTP - HyperText Transport Protocol
- 443 : HTTPS - Secure HyperText Transport Protocol
The Application layer has several subsections:
TCP/IP Protocol - The OSI Model is a theoretical model consisting
of 7 layers. TCP/IP is an actual protocol and doesn't match perfectly to the OSI model. We can map TCP/IP's 4 layers to the OSI model to understand
SNMP - Simple Network Management Protocol - This protocol provides
a good example of an application layer interface for remotely managing network devices.
Gateways - The term gateway is a generic term used in
networking. It can mean a "default gateway" or an application layer device. Let's find out what the term actually means.