LIS 525 - Hardware

Transfer Rates

In the past, transfer rates were often measured in baud (pronounced "bawd" and named after J.M.E. Baudot, the inventor of the Baudot telegraph code). This is the number of signaling elements per second. At slow speeds (below 1200 baud), each signaling element carries only one bit; so the baud rate is equivalent to the bits per second. At higher speeds, however, each signaling element may carry more than one bit. So, at present, data transfer rates are usually simply expressed in bits per second (bps). Allowing for control bits, the number of bytes per second is about one tenth of the number of bits per second.

For digital devices, bits per second is the common measure of bandwidth, the amount of data that can be transmitted in a fixed amount of time.

Communication Devices

A modem (modulator-demodulator) is a device that enables a computer to transmit data over analog lines (such as traditional telephone lines). The standard interface for connecting external modems to computers is RS-232; almost every personal computer has an RS-232 port.

Telephone modems, used for dial-up access, support transfer rates up to 56Kbps.

A cable modem is a modem designed to operate over cable TV lines. Speeds are up to 10Mbps download and 2Mbps upload.

ISDN (Integrated Services Digital Network) is a standard for sending voice, video, and data over either digital or analog telephone lines. ISDN supports data transfer rates of up to 128 Kbps.

DSL (Digital Subscriber Line) refers to a family of technologies that use sophisticated modulation to pack data onto the wires connecting a telephone switching station to a home or office. DSL allows much higher speeds than ISDN: up to 32 Mbps upstream, and from 32 Kbps to over 1 Mbps downstream. Each DSL technology requires its own type of modem.

T1 is a dedicated phone connection that actually consists of 24 individual channels, each of 64Kbps. If only some channels are used this is known as fractional T-1. T1 supports data rates of 1.544Mbps.

T3 (or DS3) is a dedicated phone connection that actually consists of 672 individual channels, each of 64 Kbps. T3 supports data rates of about 43 Mbps. T3 is used mainly by ISPs connecting to the Internet backbone and for the backbone itself.

OC (Optical Carrier) refers to fiber optic lines conforming to the SONET standard. Speeds range from 51.85 Mbps (OC-1) up to 13.21 Gbps (OC-255).

Network Hardware

Computers on a local area network generally need to be connected with a network card such as an Ethernet card. Data transfer rates are 10Mbps (10BaseT or 10Base2), 100Mbps (100BaseT), and 1Gbps (Gigabit ethernet).

A hub is a common connection point for devices in a network. A hub contains multiple ports. When a packet arrives at one port, it is copied to the other ports so that all segments of the LAN can see all packets. A passive hub serves simply to let the data go from one device (segment) to another. An intelligent hub (or manageable hub) includes additional features allowing an administrator to monitor the traffic and to configure each port in the hub.

A switch in a network is a device that filters and forwards packets between segments.

A repeater is a device that regenerates or replicates a signal. In a data network, a repeater can relay messages between subnetworks that use different protocols or cable types.

A bridge is a device that connects two LANs or two segments of a LAN. Bridges simply forward packets without analyzing and re-routing.

A router is a device that connects LANs, uses headers and a forwarding table to determine where packets go, communicates with other routers, and configures the best route between two hosts.

A gateway is a combination of hardware and software that links two different networks. Some routers can be used as the hardware portion of gateways.

Server Hardware

Processor

The processor carries out such tasks as connecting, loading pages from disk or memory, sending pages, and disconnecting. Speed is not generally crucial if the connection is slow. For multiple connections or services, multiple processors might be needed: one processor can be used for Web connections and a second processor to process database queries.

Memory

Memory is used by all programs or processes. All pages need to be loaded to memory before they are sent. Memory can also be used to cache frequently requested pages. A busy site with multiple pages or domains needs more memory than a busy site with few pages.

Hard Disk

IDE (Intelligent Drive Electronics or Integrated Drive Electronics, also known as ATA or AT Attachment) hard disks are relatively cheap, but they are limited to 2 (sometimes 4) to a computer, the disk controller requires the main processor's help, and only one disk can be accessed at a time. They are suitable for small servers with little traffic.

SCSI (Small Computer System Interface) (pronounced "scuzzy") hard disks are more expensive, may require that an additional board be installed in the computer, and do not all follow the same standard; but many drives (up to 15) can be used in one computer, multiple disks can be accessed at one time, and main processor time is not required. They are suitable for busy servers.

Multiple pages require multiple disk accesses. If the pages are spread across multiple disks, speed of loading is increased.

RAID (Redundant Array of Inexpensive Discs or Redundant Array of Independent Disks) drives can spread data redundantly over a set of disks, improving both performance and reliability, but are more expensive than ordinary disks. RAID drives are often connected to the computer via a SCSI interface.

Backup Device

Backups are needed in order to restore data lost through software crashes, hardware failures, or human error. A slow but cheap means is to use high capacity tape drives during low activity times.

Network Card

The card must be capable of providing the necessary bandwidth. Slow connections can use 10Mbps Ethernet cards; faster connections might need 100Mbps Ethernet cards or ATM (Asynchronous Transfer Mode) cards. To host multiple domains on the same server (virtual domains), multiple network cards might be needed.

Removable Media

Removable media devices may be needed in order to install software, though sometimes this can be done over the network. CD-ROM drives are most popular for installing programs, and floppy drives or, more recently, keychain drives (also known as USB drives or flash drives) for quick data transfers (for example, of pages and images).

Other Hardware

Server Appliances

Server appliances combine features of many networking components; e.g., browser-based management, intranet server, e-mail server, firewall, network address translation, and Web caching. Dedicated server appliances have both advantages and disadvantages in comparison with using a general-purpose computer as a server:
Server appliance General purpose computer
Advantages
  • Cheaper (both initial cost and power consumption)
  • Little setup required
  • More secure
  • Can be very sophisticated
  • Usually faster
  • Can be very secure if properly administered
  • Very customizable
  • Great expandability
Disadvantages
  • Difficult to customize
  • Limited expandability
  • Limited functionality
  • May require remote configuration
  • Difficult to set up
  • More expensive
  • Insecure if not properly administered
Possible uses
  • Small offices with low web content requirements
  • Schools
  • Tourist information booths
  • Large organizations
  • Sites with dynamic web content
  • Sites whose structures change/expand often
  • Sites that host multiple domains

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Last updated April 16, 2007.
This page maintained by Prof. Tim Craven
E-mail (text/plain only): craven@uwo.ca
Faculty of Information and Media Studies
University of Western Ontario,
London, Ontario
Canada, N6A 5B7