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Transcript
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 are connected New wireless technologies does not consider applications Physical Layer You are here Physical Layer 0 1 Transmits 0s & 1s Only deals with transmissions of 0s and 1s Physical Layer Examples Examples of the physical layer connection: modem’s phone-line DSL/Cable: cable from computer to modem; cable from model to wall Ethernet: cable from wall to computer Dial-up Physical Layer Examples 0 1 “Physical” name can be misleading May not involve physical items such as wireless using radio signals to/from computer Data Link Layer You are here Data Link Layer 0 Physical layer transmits 0s and 1s Does not know or care if other machine is on Data Link Layer 0 Physical Layer 0 Ensures data received by other machine Has no clue how data is transmitted, however 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 1 Target0 Source1 1 Target1 1 1 Source0 Each machine connected to 2 others All data flows in one fixed direction Ring Topology 1 1 1 1 Target 1 1 Source Hard to add new nodes (computers) Can be very slow Star Topology Target 1 1 1 Source Machines connected to center node All data flows must through this machine Star Topology Target1 1 0 Target0 1 0 Oops!! Source0 0 1 Source1 Fairly quick & easy to add new machines Need to be careful to handle collisions Bus Topology Target 1 1 1 1 1 1 Source Machines connect to central bus Data flows to every machine on the bus Bus Topology Target1 Source0 1 1 1 1 Big Oops!!! 1 1 Target0 Source1 Easiest network to build, can be fastest But needs sophisticated collision handler 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 Canisius.Buf Canisius.Amh Gateway – computer in a LAN that connects to a different LAN 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 192.28.12.1 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 Only one computer named www at Canisius Names names unique within a domain 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 1 Target0 Source1 1 Target1 1 1 Source0 All data flows in one fixed direction All machines can send data at once Star Topology Target 1 1 1 Source Central node can receive from only one machine at a time Can send & receive data at same time, however 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 Star Topology Node #2 1 0 Node #2000 1 0 Node #3 0 1 Node #4 Since 3 < 4, handle data from Node #3, then Node #4 (then Nodes #5 - 2000, as needed) 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