Environmental Engineering Reference
In-Depth Information
moves from an ACAD programme running on a Sun Microsystems workstation
(computer A) to an ACAD or similar application running on a Dell Dimension
4550 Hyper-Threading Technology PC (computer B). The intent of an open
architecture is that the communications process is transparent to the hardware used.
The OSI seven layers are defined as follows:
1.
Physical
. The modem and wire or other channel connecting to nodes. The
channel may be coaxial cable, twisted pair copper wire, fibre-optic cable, radio
or infrared links etc.
2.
Data link
. Describes how the nodes of the network obtain access and share the
physical connections to the channel. Physical address of a node, or its media
access control, is defined at layer 2. For example, Internet service provider
point to point control for dial up access is defined at this data link layer.
3.
Network
. This layer takes care of routing data packets to nodes that may not
even be on the same LAN as computer A, for instance. The network layer
contains logical rather than physical addresses and the routing mechanisms
needed to access remote LANs or wide area networks (WANs). This is where
Internet protocol (IP), for example, resides.
4.
Transport
. The layer that acknowledges message transmission across the net-
work and validates that transmission has occurred without loss of data. If a data
packet is lost, this layer is responsible for resending the data packet and con-
firming it was received and placed back into the correct sequence. This layer is
called TCP for transmission control protocol.
5.
Session
. The session layer maintains an open communications channel
between two separate nodes during transmission. An initial packet is trans-
mitted to establish the connection after which subsequent packets are sent. The
session layer ensures that the context of all packets is preserved.
6.
Presentation
. This is the application isolation layer. Layer 6 reconciles dif-
ferences between application data encryption/decryption techniques. For
example, the application on computer A may use EBCDIC for character code
conversion, while computer B may be using ASCII to encrypt character data.
The presentation layer isolates the application layer from the particulars of the
environment in which the application on node B, for example, resides in.
7.
Application
. This is where executable applications such as AutoCAD, or
Microsoft Word, or any other application resides. This layer is the human-
machine interface (HMI).
To continue the communications process described in Figure 4.48, we follow
the arrows from layer 7 on computer A to layer 7 on remote computer B. Node A
hands off the data packet (file transfer (FTP) to remote computer B) to its pre-
sentation layer 6, which isolates the application from node B's application. Layer 6
does its processing, adds headers and trailers to the packet and passes it down to
layer 5, which does the same, adding its headers and trailers etc. down to the
physical layer. The physical layer modem sends its 1s and 0s across the commu-
nications channel to node B. Layer 1 on node B collects up all the 1s and 0s, packs
them up and begins the hand-off to its layer 2. At layer 2 on node B it strips off
