Review of Topology and Access Techniques / Switching ConceptsBSAD 141Dave Novak

Sources: Network+ Guide to Networks, Dean 2013

Overview Three “base” wired topologies

Bus, star, ring Two wireless topologies

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

architectureStar topology may support client – server

(CS) or peer-to-peer (P2P) architectureLikewise, 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 networkWhat is the difference between a physical

star and logical star topology? Architecture – describes the functionality

and “administrative” structure of the networkHow devices interactWhether 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 latencyScheme 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 switching1)

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 sharedWhile nodes are connected, all data follow

same pathData arrive in order

Circuit switching1. 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 carrier2. Dial the number of the party you wish to talk to3. Your call is routed through the switch at your local carrier to the

party you are calling4. A connection is made between your phone and the other party's

phone using several interconnected switches along the way5. The phone at the other end of the connection rings, and someone

answers the call.6. The connection opens a dedicated circuit for the duration of the call7. You talk for a period of time and then hang up your phone8. 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 connection-based 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 identifierPackets are typically delivered in order

Advantages of Packet switching

Bandwidth from end-to-end is optimized • Unused circuits are torn down and bandwidth is

reallocatedSupports variable data transfer ratesIn event of failure, packets can rerouteDesigned to support “bursty” data transfer

Disadvantages of Packet switching

Variable delayPackets can be lost or corruptSophisticated 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 topologiesBus, star, ring

Two wireless topologiesAd-hoc, infrastructure

Three basic access techniquesCSMA/CD, CSMA/CA, and token passing

Packet switching versus circuit switching