Multi-Tech Systems MTASR1-100 - Manual (Page 10)

MTASR1-100 Owner’s Manual
10
1.3 Network Overview
Network architecture defines how computer equipment and other devices are linked together to form a
communications system that allows users to share information and resources. There are proprietary network
architectures and open architectures like the Open Systems Interconnection (OSI) model defined by the
International Organization for Standardization (ISO). The OSI model defines a layered model for an open
systems environment in which a process running in one computer communicates with a similar process on
another computer if they implement the same OSI layer communications protocol. The OSI protocol stack
shown in Figure 1-2 is commonly used as a reference. During a communications session, processes running
in each layer on each computer communicate with one another. The bottom layer defines the actual physical
components such as connectors and cable and the electrical transmission of data bits between systems. The
layers immediately above define data packaging and addressing methods. Still further up are methods for
keeping communication sessions alive. Finally, the uppermost layers describe how applications use the
underlying communication systems to interact with applications on other systems.
Application Layer 7
Presentation Layer 6
Session Layer 5
Transport Layer 4
Network Layer 3
Data-Link Layer 2
Physical Layer 1
Figure 1-2. OSI Protocol Stack
Protocols are loaded into a computer as software drivers. Each layer of the protocol stack defines a specific
set of functions. An application at the uppermost layer interacts with the layer below when it needs to send
information to another system on the network. The request is packaged in one layer and passed down to the
next layer, which adds information related to functions handled at that layer, creating a new packet within a
packet. This package is then passed down to the next layer and the process continues. Each layer adds
information to the message packet and this information is read by the corresponding layer in the receiving
system's protocol stack. In this way, each protocol layer communicates with its corresponding protocol layer to
facilitate communication.
The physical layer defines the physical characteristics of the interface, such as mechanical components and
connectors, electrical aspects such as voltage levels representing binary values, and functional aspects such
as setting up, maintaining, and taking down the physical link. Well known physical layer interfaces for data
communication include RS-232 and RS-449, the successor to RS-232.
The data-link layer defines the rules for sending and receiving information across the physical connection
between two systems. This layer encodes and frames data for transmission, in addition to providing error
detection and control. Because the data-link layer can provide error control, higher layers may not need to
handle such services. However, when reliable media is used, there is a perfomance advantage by not
handling error control in this layer, but in higher layers.
The Network layer defines protocols for opening and maintaining a path on the network between systems. It is
concerned with data transmission and switching procedures, and hides such procedures from upper layers.
The network layer can look at packet addresses to determine routing methods. If a packet is addressed to a
workstation on the local network, it is sent directly there. If it's addressed to a network on another segment,
the packet is sent to a routing device, which forwards it on the network. Some commom protocols occupying
the network layer are Internet Protocol (IP), X.25, Novell's Internetwork Packet Exchange (IPX), Banyan's
VINES Internet Protocol (VIP).
The transport layer prodvides a high level of control for moving information between systems, including more
sophisticated error handling, prioritization, and security features. It provides quality service and accurate
delivery by providing connection-oriented services between two end systems. The transport layer controls the
sequence of packets, regulates traffic flow, and recognizes duplicate packets. It assigns packetized
information which is a tracking number that is checked at the destination. If data is missing from the packet,
the transport layer at the receiving end arranges with the transport layer of the sending system to have
packets retransmitted. This layer ensures that all data is received and in the proper order.