CSC 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 are connected  New wireless technologies does not consider applications

Physical Layer You are here

Physical Layer Transmits 0s & 1s  Only deals with transmissions of 0s and 1s 0 1

Physical Layer Examples Examples of the physical layer  Dial-up connection: modem’s phone-line  DSL/Cable: cable from computer to modem; cable from model to wall  Ethernet: cable from wall to computer

Physical Layer Examples “Physical” name can be misleading  May 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 1s  Does not know or care if other machine is on 0

Data Link Layer Ensures data received by other machine  Has no clue how data is transmitted, however 0 0 Physical Layer

Physical & Data Link Example

Network Layer You are here

Network Layer Assumes connections between machines work  No problem; handled by model’s lower levels Network layer handles local-area network  Typically referred to using its acronym: LAN  LAN 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 disadvantages  No 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 Source 0 Target Source 1 Target 1 1 1

Ring Topology Hard to add new nodes (computers) Can be very slow Source Target

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 Source 1 Target Source 0 Target Oops!!

Bus Topology Machines connect to central bus Data flows to every machine on the bus Source Target

Bus Topology Easiest network to build, can be fastest But needs sophisticated collision handler Source 1 Target Target 0 Source 0 Big Oops!!!

Transport Layer You are here

Transport Layer Transport layer deals with internetworking  Internetworking – Communication between networks  When 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 together  Rarely 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 networks  Backbone 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 networks  Each portion of data may be routed differently

Two Forms of IP Address Numerical  Usually written as four 8-bit numbers  Typically something like  Easiest for computers to use Hostname  Usually written as three or four words separated by a “.”  Often something such as aries.canisius.edu  Easiest for humans to use

IP Address Consider aries.canisius.edu  Each 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 name  Example domain names: canisius.edu, cnn.com, whitehouse.gov, wendys.ca, google.com, google.co.uk Each domain name is unique to the organization  Must be registered yearly through central registrar for the TLD

Computer Name First word of hostname is the computer name  Computer 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 addresses  DNS is an example of a distributed database  If a can server resolves the hostname, done  If not, server asks another DNS server  Every 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 computers  Using/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 simple  Remember to move all data at the same time

Ring Topology All data flows in one fixed direction All machines can send data at once Source 0 Target Source 1 Target 1 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 # , as needed) Node #4 Node # Node #3 Node #

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