CSC 110 - Intro. to Computing Lecture 23: Networks
Jan 13, 2016
CSC 110 -Intro. to Computing
Lecture 23:
Networks
Announcements
Quiz #5 on Thursday Homework #6 (on networks) due Friday
Quiz #6 (also on networks) will be next week
OSI Reference Model
Open Systems Interconnect Reference Model Established by International Organization for
Standardization (ISO) Models how computers connect in networks
Allows different systems (e.g., Windows and Macs) to connect on a single network
Proprietary networks may not use OSI Reference Model But then may be unable to connect to other networks!
Using OSI Reference Model
Each layer is independent of others New technologies created at
appropriate layer New IM client need
not consider how computers areconnected
New wireless technologies does not consider applications
Physical Layer
You are here
Physical Layer
Transmits 0s & 1sOnly deals with transmissions of 0s and 1s
0 1
Physical Layer Examples
Examples of the physical layerDial-up connection: modem’s phone-lineDSL/Cable: cable from computer to modem;
cable from model to wallEthernet: cable from wall to computer
Physical Layer Examples
“Physical” name can be misleadingMay not involve physical items such as wireless using radio signals to/from computer
0 1
Data Link Layer
You are here
Data Link Layer
Physical layer transmits 0s and 1sDoes not know or care if other machine is on
0
Data Link Layer
Ensures data received by other machineHas no clue how data is transmitted, however
0 0Physical Layer
Physical & Data Link Example
Network Layer
You are here
Network Layer
Assumes connections between machines workNo problem; handled by model’s lower levels
Network layer handles local-area networkTypically referred to using its acronym: LANLAN is connects relatively small number of
“physically” interconnected machines
LAN Topologies
Defines how machines in LAN connected 3 dominant topologies exist
Each has advantages and disadvantagesNo single solution works in all situations
Ring topology – like a traffic rotary/circle Star topology – like flying on an airline Bus topology – similar to computer buses
Ring Topology
Each machine connected to 2 others All data flows in one fixed direction
Source0
Target0
1
1
Source1
Target1
1
1
Ring Topology
Hard to add new nodes (computers) Can be very slow
Source
Target
1
1
1
1
1
1
Star Topology
Machines connected to center node All data flows must through this machine
Source
Target
1
1
1
0
Star Topology
Fairly quick & easy to add new machines Need to be careful to handle collisions
Source1
Target1
1
1
1
Source0
Target0
0
0
Oops!!
Bus Topology
Machines connect to central bus Data flows to every machine on the bus
Source
Target
11
1
1
1
1
Bus Topology
Easiest network to build, can be fastest But needs sophisticated collision handler
Source1
Target1
11
1
1
1
1
Target0
Source0
BigOops!!!
Transport Layer
You are here
Transport Layer
Transport layer deals with internetworking Internetworking – Communication between
networksWhen we combine LANs we call the resulting
structure a WAN (wide-area network)
Wide-Area Network
Gateway – computer in a LAN that connects to a different LAN
Canisius.Buf
Canisius.Amh
Sidebar: internet vs. Internet
an internet –network which joins multiple LANs togetherRarely used synonym for WAN
the Internet – global network created by joining many networks together using IP for transport layer IP (Internet Protocol) defines how each computer
can be uniquely identified and how to pass information across networks
Internet
Internet relies on backbone of high-speed connections to carry traffic between networksBackbone supported by AT&T, IBM, & others
Routing data between networks relies upon knowing computer’s IP address IP software breaks up data and routes it across
networksEach portion of data may be routed differently
Two Forms of IP Address
NumericalUsually written as four 8-bit numbersTypically something like 192.28.12.1Easiest for computers to use
HostnameUsually written as three or four words separated by a
“.”Often something such as aries.canisius.eduEasiest for humans to use
IP Address
Consider aries.canisius.eduEach of these words helps identify the
computer in question Similar setup exists for numerical
addresses
Top-Level Domains
The last word of the hostname (edu) is its top-level domain (TLD) name
Country Codes
Most TLDs controlled by a US-regulated corporation
Other countries maintain own TLD
Domain Name
Last two words of hostname are the domain nameExample domain names: canisius.edu, cnn.com,
whitehouse.gov, wendys.ca, google.com, google.co.uk
Each domain name is unique to the organizationMust be registered yearly through central registrar
for the TLD
Computer Name
First word of hostname is the computer nameComputer names unique within a domain
Only one computer named www at Canisius
Names may not be unique between domains Lots of computers on Internet named www
Domain Name System
The domain name system (DNS) translates hostnames into numeric IP addressesDNS is an example of a distributed database If a can server resolves the hostname, done If not, server asks another DNS serverEvery TLD has 1 – 6 root DNS servers which
contain all records If necessary, can eventually ask one of these
OSI Reference Model so far
Only discussed moving data between computersUsing/interpreting data occurs at higher levels
Back to the Network Layer
Common question is to find how long network would take to process requests
For ring topologies, this is simpleRemember to move all data at the same time
Ring Topology
All data flows in one fixed direction All machines can send data at once
Source0
Target0
1
1
Source1
Target1
1
1
Star Topology
Central node can receive from only one machine at a time
Can send & receive data at same time, however
Source
Target
1
1
1
Star Topology Timing
Nodes in star network begin by sending data Collision occurs when multiple nodes send data
to center node at same time Bad news: central node cannot differentiate all
the different data Good news: central node serves as traffic cop
Orders nodes by their ID number Requests data from first node at first time unit after
collision Reads new data and sends out results in following
units
0
Star Topology
Since 3 < 4, handle data from Node #3, then Node #4 (then Nodes #5 - 2000, as needed)
Node #4
Node #2
1
1
1
Node #3
Node #2000
0
0
Bus Topology Timing
No central node to determine the order node should send data after a collision
Nodes use simple heuristic to avoid further collisions Lots of different heuristics exist, but class uses simple
one After collision, each node waits number of time units
equal to its node ID number before send data Note: this may result in some wasted time units
(unless more traffic is generated)
For Next Lecture
Read chapter 16 Do homework #5 Get your service learning finished ASAP!
Start polishing your essay before its due