SYSTEM ADMINISTRATION
Chapter 2
The OSI Model
The OSI Model
• The OSI Model was designed by the International
Standards Organization (ISO) as a structural
framework for the rules of data communications.
• This model allows any vendor who adheres to the
framework to design a component for data
communications with the assurance that it can be
used with products from any other vendor and
provide seamless interoperability.
The Physical Layer
• The Physical layer defines the way data is
transmitted over the physical connections of the
network.
• Other component characteristics of this layer
include physical topology, connection types,
signal type, baseband and broadband, and bit
synchronization.
Physical Topology
• The physical topology of the network defines the
way the network is laid out, including:
– Cabling (star, bus, mesh, ring, and wireless)
– Interconnection of segments
– Devices
Star
• The star topology is the most commonly used physical
layout for networks today.
• When building a star topology, a hub or switch will be the
central device connecting all nodes on the segment.
• When a node failure occurs, there are three places
where the failure can take place: the port on the hub or
switch, the cable connecting the node and the hub or
switch, or the network interface on the node itself.
• A star topology is relatively easy to set up, and can be
expanded so long as the physical limitations are not
exceeded.
Bus
• The bus topology is the easiest to install and the
most difficult to troubleshoot.
• Cables must be terminated.
• The bus topology has strict physical limitations of
cable length and distance from other nodes.
Mesh
• Mesh networks provide redundant links between all
nodes on the network, thus creating a fault-tolerant
data communications topology.
• The mesh network is time-consuming and
somewhat expensive to install because of the
multiple connections between nodes.
• Troubleshooting mesh networks is relatively easy
because the failure points are easily identified.
Ring
• The ring topology connects nodes in a circle,
allowing only nodes directly connected to talk to
each other.
• Data is transmitted in a single direction only.
• A node can accept and respond to packets
addressed to it, then pass the packets on to the next
node on the ring.
Wireless
• A wireless topology relies on either radio frequency
(RF) or infrared (IR) frequencies, or channels, to
connect directly to each other or to access points
(APs).
• Wireless communication devices connect, or
network, using either ad hoc or infrastructure
networking.
Connection Types
• Point-to-point connections occur when two computers
are connected and exchange information. An example is
modem connections.
• Multipoint connections use many devices connected to
transmission media, sharing the available bandwidth.
The corporate network is an example.
Signal Types
• There are two broad categories of signal types:
digital and analog.
• Analog signals use constantly varying voltages or
waves.
• Digital signals use electrical pulses.
• All signals are subject to deterioration of signal over
distance or attenuation.
• Each category of signal has a set of specific
encoding schemes that allow for the efficient
transmission of the data.
Baseband vs. Broadband
• Baseband and broadband refer to the way the
signals are passed across the media.
• Baseband transmission allows a single
transmission to use the entire bandwidth.
• Broadband transmission requires that many
signals share the same bandwidth.
• Broadband traffic is more efficient for data
transmission.
Bit Synchronization
• Baseband and broadband refer to the way the
signals are passed across the media.
• Baseband transmission allows a single transmission
to use the entire bandwidth.
• Broadband transmission requires that many signals
share the same bandwidth.
• Broadband traffic is more efficient for data
transmission.
The Datalink Layer
• The Datalink layer is subdivided into two sub-layers
known as the Logical Link Control layer (LLC) and
the Media Access Control layer (MAC).
• The LLC is responsible for the standards that
govern how network communication will take place.
• The MAC layer maintains the physical addressing
scheme used by network-connected nodes, allowing
each node to be uniquely identified as a participant
on the network.
The Network Layer
• The Network layer is responsible for two important
functions: logical addressing of nodes on the
network or internetwork and routing of packets from
source to destination.
• The logical addressing scheme used most
frequently is IP addressing, a binary method for
building large numbers of unique addresses.
• Routing is a store and forward action that allows the
best path to be chosen when sending a packet from
source node to a destination node.
The Transport Layer
• The Transport layer makes sure that the data is
transmitted reliably between nodes.
• This layer also segments large packets based on the
type of network.
• Each segment is given a sequence number so that the
receiving node can recreate the message correctly.
• Transport layer function also includes flow control, or the
management of the rate of data transmission.
• To maintain reliable delivery of segments, this layer will
issue either an ACK (acknowledgement) or a request for
retransmission if errors are detected.
The Session Layer
• Session layer functions include the management of
sessions or conversations between nodes.
The Presentation Layer
• The Presentation layer makes sure that both nodes
understand in what format the data will arrive.
• Its function is that of translator when two different
data formats are present.
• Encryption/decryption are managed at this layer.
• Compression/decompression of data is managed at
this layer.
The Application Layer
• The Application layer provides services to software
applications used by a user.
• Some of the services include file access services,
printing services, e-mail services, file transfer
services, and file management services.