Review of Topology and
Access Techniques /
Switching Concepts
BSAD 141
Dave Novak
Sources: Network+ Guide to Networks,
Dean 2013
Overview

Three “base” wired topologies


Two wireless topologies


Bus, star, ring
Ad-hoc, infrastructure
Three basic access techniques

CSMA/CD, CSMA/CA, and token passing
CSMA and delay
 Packet switching versus circuit switching

Architecture

Basic level – any topology will support any
architecture

Star topology may support client – server
(CS) or peer-to-peer (P2P) architecture

Likewise, P2P or CS architecture may be
implemented using star, bus, or ring
topology
Architecture –vs- Topology

Topology –physical or logical design or
layout of the network


What is the difference between a physical
star and logical star topology?
Architecture – describes the functionality
and “administrative” structure of the network
How devices interact
 Whether there are servers and exactly what
these devices do

Access Technique
The access technique describes how the
devices that are using a particular
networking technology (such as Wi-Fi or
Ethernet) share or coordinate the use of,
the communication medium
 This is relevant because the vast majority
of networking technologies and
implementations involve the sharing of the
communications medium

Access Technique

CSMA/CD?

CSMA/CA?
Access Technique

Token Passing?
Distance limitation and
LANs
Distance is a fundamental consideration in
the design of any network – particularly when
it comes to popular LAN technologies such
as Wi-Fi and Ethernet
 The length of an Ethernet network cannot be
continually increased by adding new
segments using switches, hubs or repeaters

Ethernet and Delay

CSMA/CD (and CSMA/CA for that matter)
designed to function with limited latency

Scheme fails if length is too long or too many
segments are connected

Serious performance problems if you string a
bunch of Ethernet segments together with
repeaters

Discussion
Packet Concept

In data networks, the data are broken up
into smaller packets as opposed to
transmitting large streams of data
Packet Concept
Packets promote “equity”
 Enables many devices to share the
medium
 Data networks not designed to transfer
data as a stream of continuous bits
 Data are divided into small generic blocks
called packets


Dividing data into packets helps determine
which blocks have errors and which do not
Packet Concept
Think about the concept of encapsulation
 What happens when data are
encapsulated?


What is the difference between a packet
and a frame?
Switching
Determines how connections are
established between different nodes on the
network
 Focus on 2 types of switching


1)

2)
Type of switching

Why is this an issue?
Circuit switching
A connection-based transmission
technology where 2 nodes establish a
dedicated connection for the duration of the
communication session
 Requires that the sending node contact
receiving node to establish the connection
 Based on the concept of streaming large
amounts of data
 What is a potential drawback to a dedicated
connection?

Circuit switching

Some amount of bandwidth on circuit/channel is
dedicated and remains available until connection
is terminated

Conceptually a reserved path from sender to
receiver is created that provides an “open road”
for non-stop transfer

Not shared

While nodes are connected, all data follow
same path

Data arrive in order
Circuit switching
1.
2.
3.
4.
5.
6.
7.
8.
Pick up the phone and listen for a dial tone. This lets you know that
you have a connection to the local office of your telephone carrier
Dial the number of the party you wish to talk to
Your call is routed through the switch at your local carrier to the
party you are calling
A connection is made between your phone and the other party's
phone using several interconnected switches along the way
The phone at the other end of the connection rings, and someone
answers the call.
The connection opens a dedicated circuit for the duration of the call
You talk for a period of time and then hang up your phone
When you hang up, the communication circuit or channel is closed,
freeing the bandwidth along the circuit in between the caller and
the receiver
Source: http://computer.howstuffworks.com/ip-telephony2.htm
Advantages of Circuit
Switching

Dedicated connection between 2 hosts – full
use of communication channel for duration of
communication session

Highly reliable (with respect to path, variation
in delay, and constant bit rate)

Requires little packet overhead during
transmission because path / communication
circuit is dedicated and known

Makes optimal use of available bandwidth
(ratio of overhead to data is very small) during
transmission but not if channel is idle
Disadvantages of Circuit
Switching
Inflexible with respect to many internet-based
applications (data are bursty)
 Involves a lot of communication overhead and
time with respect to ESTABLISHING the
connection on the communication channel
 Can waste a lot of bandwidth if data are not
continually being sent (inefficient if idle)
 In event of failure, data are not packetized and
do not seek an alternate path – path may be
lost and all data may be lost

Packet switching

A digital networking communication method
where data are broken into small packets and
each packet can be transmitted independently

Delivers variable rate data streams

During transmission packets are buffered and
queued which leads to variable delay and
throughput

No communication is needed to explicitly
establish a connection with receiving node
ahead of time
Packet switching

Two modes

1) Connectionless (datagram)

2) Connection-based (virtual circuit)
 Implication:
modern, data-oriented,
digital, packet-switched networks can
take advantage of both
connectionless and connectionbased transfer
Packet switching

1) Connectionless mode (datagram)

Asynchronous communication – no
coordination

Each packet much include complete routing
information because each packet can be
routed individually
• Out-of-order delivery, different paths
Packet switching

2) Connection-based mode (virtual circuit)

Protocols establish a virtual connection
between sender and receiver
• This connection is established via software rather
than physically
• A temporary dedicated channel is established

Connection set up and torn down as needed

Packets include connection identifier

Packets are typically delivered in order
Advantages of Packet
switching

Bandwidth from end-to-end is optimized
• Unused circuits are torn down and bandwidth is
reallocated

Supports variable data transfer rates

In event of failure, packets can reroute

Designed to support “bursty” data transfer
Disadvantages of Packet
switching

Variable delay

Packets can be lost or corrupt

Sophisticated protocols are needed to
provide some level of reliability during
transfer

Not good for certain types of applications that
require constant bit rates, limited variation in
delay, or no data loss (e.g. real-time video)
Summary

Three “base” wired topologies


Two wireless topologies


Ad-hoc, infrastructure
Three basic access techniques


Bus, star, ring
CSMA/CD, CSMA/CA, and token passing
Packet switching versus circuit switching