Class Notes #17
READING: Manovich, Chapter 4 (The Operations) and Chapter 5 (The Illusions)
Networks, Internet, Web, Networked Media Systems
Part I: Networks - Introduction
A computer network is a system for communication among two or more computers.
That sounds so simplistic as to be nearly void of meaning but in fact this is the essence of all computer networks including the seemingly countless number of programs, servers, and personal computers on the Internet today.
What exactly is communicated?
Bits. Bits in the form of electrical impulses that travel close to the speed of light (with certain limitations we can ignore here). These bits are formed into "packets" or "frames" that are recognizable by the sending and receiving hardware. The topology for connecting and communicating between computers is rather complex in that there are a myriad of communications methods - all of which serve some specific purpose.
For example, we don't normally think of a "printer" as being a "network computer" though more and more that is how they function. Historically, printers were connected through standard "interface technologies" including: parallel (or centronix) ports, serial ports, USB ports, and even SCSI (small computer systems interface) ports though these were normally reserved for very high volume transfers such as for hard discs, scanners, audio digitizing units.
Speed, cost and and wide-scale standardization of interfaces has led to the current state where most peripherals (keyboards, mice, printers, scanners, audio interfaces, MIDI devices, game consoles, thumb drives, etc. etc) are connected via USB (universal serial bus). USB is not, however, used for computer-computer networking.
The need for faster computer-device transfers also led to the standardization of IEEE 1394 or "firewire" as it is commonly called. This protocol is mainly used for high-volume, high-speed devices such as digital imaging equipment and mult-channel high-bandwith audio devices. There are many storage peripherals as well that are "firewire" (or both USB and firewire).
The following chart gives you some idea of the transfer speeds of these devices:
http://macspeedzone.com/archive/5.0/usbcomparison.html
As with most components of personal computers, there is little doubt that the highest possible transfer rates will become available at reasonable cost and that the need for these very high rates will be dominated by the role of digital video in the consumer computer arena. (Recall here that there are as yet no upper limits on the resolution of high-definition video and that we can expect to see extremely high rates for HD video transfers.)
Notice that only the 10-Base-T and 100-Base-T device rates listed in the chart above refer to "networking". USB, SCSI, and Firewire are all "interfaces" that support a breathtaking array of peripherals but are rarely used for network connections other than a means to plug in an external network card. Notice also that these interfaces are not suitable for the connection of displays and CRT's at this time. The rapidly emerging standard for computer-display interfacing is called DVI (digital video interface).
see DVI for high-speed LCD panel connections and for the following table:
Specifications- DVI-D (digital)
- Minimum clock frequency: 21.75 MHz
- Maximum clock frequency: 165 MHz
- Pixels per clock cycle: 1 (single link) or 2 (dual link)
- Bits per pixel: 24
- Maximum display resolutions (single link):
- HDTV (1920 × 1080) @ 60 Hz (5% LCD blanking)
- UXGA (1600 × 1200) @ 60 Hz (GTF blanking)
- SXGA (1280 × 1024) @ 75 Hz (GTF blanking)
- SXGA (1280 × 1024) @ 85 Hz (GTF blanking)
- Maximum display resolutions (dual link):
- QXGA (2048 × 1536) @ 75 Hz (GTF blanking)
- HDTV (1920 × 1080) @ 85 Hz (GTF blanking)
How are computer networks constructed?
http://www.comptechdoc.org/independent/networking/protocol/protlayers.html
The site indicated above provides the fundamentals of modern networking called a "network layer model"
The Network Layer Model
The layered concept of networking was developed to accommodate changes in technology. Each layer of a specific network model may be responsible for a different function of the network. Each layer will pass information up and down to the next subsequent layer as data is processed.The ISO Network Model Standard
The ISO network model layers are arranged here from the lower levels starting with the physical (hardware) to the higher levels.
1. Physical Layer - The actual hardware.
2. Data Link Layer - Data transfer method (802x ethernet). Puts data in frames and ensures error free transmission. Also controls the timing of the network transmission. Adds frame type, address, and error control information. IEEE divided this layer into the two following sublayers.
1. Logical Link control (LLC) - Maintains the Link between two computers by establishing Service Access Points (SAPs) which are a series of interface points. IEEE 802.2.
2. Media Access Control (MAC) - Used to coordinate the sending of data between computers. The 802.3, 4, 5, and 12 standards apply to this layer. If you hear someone talking about the MAC address of a network card, they are referring to the hardware address of the card.
3. Network Layer - IP network protocol. Routes messages using the best path available.
4. Transport Layer - TCP, UDP. Ensures properly sequenced and error free transmission.
5. Session Layer - The user's interface to the network. Determines when the session is begun or opened, how long it is used, and when it is closed. Controls the transmission of data during the session. Supports security and name lookup enabling computers to locate each other.
6. Presentation Layer - ASCII or EBCDEC data syntax. Makes the type of data transparent to the layers around it. Used to translate date to computer specific format such as byte ordering. It may include compression. It prepares the data, either for the network or the application depending on the direction it is going.
7. Application Layer - Provides services software applications need. Provides the ability for user applications to interact with the network.Many protocol stacks overlap the borders of the seven layer model by operating at multiple layers of the model. File Transport Protocol (FTP) and telnet both work at the application, presentation, and the session layers.
The 7 layer network model is then implemented by manufacturers of networking equipment and networking software. For example, when you "ftp" a file from somewhere, all of the component layers described above come into play to cause that file transfer to occur with a minimum of errors and as rapidly as possible.
When you use iChat or MSN Messaging, you are running an application (Layer 7) that knows how to send your typed characters to a server that then passes these on to whomever you may be chatting with. This is equally true for audio and video in "chat" or "instant message" mode.
How does the Web work?
The origins of the www are well known - physicists and researchers at the Swiss research center called CERN needed a better way to share images and text such that the format of the materials would remain constant and be completely independent of the software running on the researcher's computer (many used UNIX workstations that did not support any MSoft products so file sharing was not so easy). But you should envision a world where most if not all computer users "typed" everything - that is, there were no universal graphical user interfaces (GUI's) that could effortlessly present information in graphical form. Of course the Mac, NeXT and SUN workstations had wonderful graphics but they did not have a "sharable" data format.
The central developments that concern us are 1. the invention of hypertext transfer protocol (http) and hypertext markup language (html) 2. the invention of Mosaic - the first graphical html browser and 3. the concept of hyperlinked anything (predates the web).
From above we learned that ISO Layer 7 is the application layer and indeed, a browser is an application. But behind the browser is the http server - the device that can deliver html informatio and that can resolve the address links called "URL" or "universal resource locator".
A useful site that outlines this extraordinary invention is: How it all started. (w3c 10th anniversary). We will step through this in class. For a more detailed historical outline see: http://www.netvalley.com/intval1.html
An excerpt:
There are two ages of the Internet - before Mosaic, and after. The combination of Tim Berners-Lee's Web protocols, which provided connectivity, and Marc Andreesen's browser, which provided a great interface, proved explosive. In twenty-four months, the Web has gone from being unknown to absolutely ubiquitous.
A Brief History of Cyberspace, by Mark Pesce, ZDNet, October 15, 1995
Internet timeline - a brief synopsis of the whole history.