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IT Essentials — Assessing Infrastructure and Networks

Layered Service Network Model

When referencing networks, it helps to conceptualize the different network layers using a model.
Sometimes collectively referred to as the network stack, the most commonly used network-layered
model is the Open System Interconnection Model (OSI) Seven-Layer model.

Like many IT concepts, this model is not universal, but it may be helpful when thinking about
services provided by a network stack. Most OSs provide a network stack that contains a series of
applications allowing for remote connections and for sending/receiving data to remote devices.
Each layer has a responsibility and operates independently of other layers. Additionally, each layer
accepts data from the higher level and performs its required functions before passing it to a lower
level. This is referred to as passing information down the network stack and allowing developers to
assume that necessary services will have been provided by lower layers. It also requires that the
services they develop provide stable interoperation up the stack.

Information passed down from a higher layer is most often intact. It can be divided or combined as
needed at the new layer because all data from the higher level is simply a field of data. Control
information, called metadata (data about data) is added. Metadata is usually referred to as a header.

The OSI Seven-layer Model
Source: IIA GTAG: “IT Essentials for Internal Auditors”

Layer 1 — Physical
The physical layer can be very expensive to update. Many legacy network methods are maintained to
prevent replacement of Layer 1 infrastructure. The physical layer exists in all network spans and in
the nodes themselves. Older routers and switching equipment can provide limited function even
with software updates because of their Layer 1 limitations. Older network interface cards (NIC’s) can
have similar limitations. Newer equipment maintains backward compatibility to allow network
operation on older infrastructure.
Function: The job of the physical layer is to provide a path for data transmission.
Media implementing this layer: Copper wire, fiber optic cable, radio waves, or any other method
capable of transmitting data.

Copyright © 2020 by The Institute of Internal Auditors, Inc. All rights reserved.