1. The Computer Continuum4b-1 Communication Basics of Networks n How is it possible to measure the capacity of communications links? Bandwidth : Digital –Number of bits per second (bps) that can be sent over a link. –The wider the bandwidth, the more diverse kinds of information can be sent. –Simplest is voice, most sophisticated is movie videos (with sound). Bandwidth : Analog –The difference between the highest and lowest frequencies that can be sent over an analog link (like phone lines). –Measurement is given in hertz (Hz). For both: The wider the bandwidth, the more information can flow over the channel.

2. The Computer Continuum4b-2 Communication Basics of Networks Typical cable bandwidths used in local area networks. Cable:Typical Bandwidth: Twisted Pair10 to 100 Mbps Coaxial Cable10 to 100 Mbps Fiber-optic cable100 to 200 Mbps The bandwidths of different services offered by a telephone company: Service:Bandwidth (Mbps) ISDN0.064 /channel DSL (dn)1.5 DSL (up)0.64 Cable10-30 Satellite (dn)0.6-1.4 Satellite (up)0.064 T11.5 STS-151

3. The Computer Continuum4b-3 The Physical Organization of Networks n Node: The generic name given to all devices hooked up to a network. How does the network distinguish each node? –Each node has a unique address assigned to it by the network. –Direct-connected network: Those whose nodes have direct connections through either physical or wireless links. Point-to-point: Simplest version of direct-connected network. Connecting two computing systems. »Example of point to point: Home to ISP. »Computer 1 to computer 2: null modem cable

4. The Computer Continuum4b-4 The Physical Organization of Networks n The bus network - A continuous coaxial cable to which all the devices are attached. All nodes can detect all messages sent along the bus. n The ring network - Nodes linked together to form a circle. A message sent out from one node is passed along to each node in between until the target node receives the message. Linking nodes:

5. The Computer Continuum4b-5 The Physical Organization of Networks n The star network - Each node is linked to a central node. All messages are routed through the central node, who delivers it to the proper node. n The tree network - (hierarchical network) Looks like an upside-down tree where end nodes are linked to interior nodes that allow linking through to another end node. Linking nodes:

6. The Computer Continuum4b-6 The Physical Organization of Networks n The fully connected network - All nodes are connected to all other nodes. n Internetworking - Connecting together any number of direct-connected networks. The largest: Internet. Linking nodes: NetworkA NetworkB NetworkC

7. The Computer Continuum4b-7 The Physical Organization of Networks n Categorizing networks according to size: DAN (Desk Area Network) LAN (Local Area Network) MAN (Metropolitan Area Network) WAN (Wide Area Network) n Benefits of networks : Sharing of hardware resources. Sharing of software and data. Consolidated wiring/cabling. Simultaneous distribution of information. More efficient person-to-person communication. -Your study area -College Building -Campus network -All Universities (Merit)

8. The Computer Continuum4b-8 The Physical Organization of Networks n Security of a Network Firewall: A set of programs that monitor all communication passing into and out of a corporation’s intranet. Helps prevent, but doesn’t eliminate, unauthorized access.

9. The Computer Continuum4b-9 Software Architecture of Networks n Problem: Different operating systems Connect several different machines running different operating systems (Windows, OS/2, MacOS, UNIX, VMS...) Now, try to: send email, data or files between them. n Solution: Create a standardized set of rules, or protocols, that, when followed, will allow an orderly exchange of information. A collection of these programs is called a protocol suite. –Must be on all computers or nodes in the network. Network’s architecture: The protocol suite and the general scheme that guides the network’s rules.

10. The Computer Continuum4b-10 Software Architecture of Networks n Problem: Collisions of information are caused by two computers simultaneously attempting to send information in the network. n Solution: Different networks have different protocol suites: Apple Computer’s LocalTalk Protocol - Permission must be granted before information can be sent along the network. Token-Ring Protocol (IBM and others) - A token is “picked up” by a node signifying that a message is about to be sent, the computer sends the message, then, replaces the token so that others can use the network. Ethernet Protocol (Xerox and others) - Collisions are not avoided. When they occur, both detect the others’ presence, stop sending, wait a random amount of time, and send again.

11. The Computer Continuum4b-11 Software Architecture of Networks n The Architecture of the Internet Four-layer architecture: FTP HTTP NV (network video) TFTP (Trivial ftp) TCP UDP (User datagram protocol) IP Network #1 Network N Network #2

12. The Computer Continuum4b-12 Software Architecture of Networks n The Internet is referred to as a packet-switching network. Packet: A unit of information created by the Transfer Control Protocol (TCP) software for transmission over the Internet. –Once a file is requested, it is split into packets. Each packet is assigned a number. Each packet contains information regarding content, where it came from, where it is supposed to go. –As the packet travels through the Internet from network to network: Each packet may not travel through the same path through the Internet to its destination. Each network has its own “packet-limiting” size. Packets are often “packaged” and “repackaged.” –They are reconstructed in order when they reach the destination. To you From me

13. The Computer Continuum4b-13 Software Architecture of Networks n Problem: If someone wants his own WWW site, he must find a home for it. n Solution: Find a Server willing to store your homepage. Server: A dedicated computer that is part of a network. –The hard drive contains web files that are “served” to whatever requests them. –Could be data, programs, or home pages for the WWW. –The server normally runs the networking software. Client/server model: One computer, the client, requests information from another computer, the server. –Client computers can run any type of operating system as long as they have the ability to use Internet protocols.

14. The Computer Continuum4b-14 Web terms you should know n Types of nodes important to networks. HubA device that repeats or broadcasts the network stream of information to individual nodes (multiplexes to personal computers) SwitchA device that receives packets from its input link, and then sorts them and transmits them over the proper link that connects to the node addressed. (think mailman) BridgeA link between two networks that have identical rules of communication. GatewayA link between two different networks that have different rules of communication. RouterA node that sends network packets in one of many possible directions to get them to their destination. (ISPs use these)

15. The Computer Continuum4b-15 Trace route n Trace route: A program that follows the tracing of packets over the Internet or any network using TCP/IP protocol. One version will tell you the names of the hops. Uses a special number - TTL (Time to Live) - contained in a place at the beginning of each packet sent over the network. –The number is originally set to 255. –Each time it is received by a router, it decrements by 1. –If the TTL number becomes 0 before reaching its destination, the router where this happened sends back an error message (time exceeded) with the address of the router. Stops messages from circulating forever.