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||POTS (Plain Old Telephone Service) is low-bandwidth communication, as opposed to DSL/ISDN.
POTS uses the frequencies from 300 to 3 kHz to carry voice/data and is generally restricted to ~52 Kbps (kilobits/second). Any service sharing a line with POTS must either use frequencies above 3 kHz or convert POTS to digital and interleave with other data signals.
||Powerline (a.k.a. power-line communication, or PLC) is a range of communication technologies that define network communication of data over conductor wires that are also used simultaneously for AC electric power transmission. A wide range of power-line communication technologies are needed for different applications, ranging from home automation to Internet access and home networking.
Powerline in home networking is most widely deployed under the HomePlug Powerline Alliance standard. HomePlug AV is the most current of the HomePlug specifications, and estimated 45 million HomePlug devices are deployed worldwide.
Note that the technology may require that Powerline devices are on the same electrical circuit, and surge protectors/power strips with filtering may absorb the Powerline signal.
||PPPoA (point-to-point protocol over ATM)
A standard very similar to PPPoE (point-to-point protocol over Ethernet), with some minor differences, for example a DSL modem pumping ATM is internal to the computer, rather than being an ethernet cable away. PPPoA allows for MTUs (maximum transmission units) of 1500, as opposed to PPPoE.
||PPPoE ( Point-to-Point Protocol over Ethernet ) is a method for building PPP sessions and encapsulating PPP packets inside Ethernet frames.
PPPoE is used by a number of DSL providers. It requires either routers that have built-in PPPoE support, or PPPoE software to "dial up" and establish the session. Some notable characteristics of a PPPoE connection are that it requires a username/password to connect, and adds an additional 8-byte header to packets, reducing the maximum packet size to 1492 from 1500.
See also: RFC 2516
||PPTP (Point-to-Point Tunneling Protocol - RFC 2637) is an extension to the PPP protocol designed for VPN, that allows for private "tunnels" over the public Internet.
Generally, such tunneling refers to the ability to encapsulate packets of data formatted for one network protocol in packets used by another protocol (usually transfer of TCP/IP data over a non-TCP/IP network).
PPTP uses port 1723/tcp.
||protocol is an agreed-upon format for transmitting data between two devices. The protocol determines the following:
- type of error checking to be used
- data compression method, if any
- how the sending device will indicate that it has finished sending a message
- how the receiving device will indicate that it has received a message
There are a variety of standard protocols with particular advantages and disadvantages; for example, some are simpler than others, some are more reliable, and some are faster. From a user's point of view, the only significant aspect about protocols is that your computer or device must support the right ones if you want to communicate with other computers.
||A proxy server is an intermediate gateway that sits between a client PC and the Internet and provices forwarding/caching service, as well as security, logging and administrative control.
A caching web proxy server saves content (such as Web pages) in its local cache the first time a request is made, and then serves the pages to other local clients from cache, without the need to forward the request to the Internet. Pages are only requested from the Internet if they are not found in cache. The proxy server forwards requests acting as a client, with its own IP address, and then in turn serves the pages to local clients.
Caching proxy servers offer transparent service to clients, they can save bandwidth and improve performance, however they can also cause slowdowns when congested, as well as some problems delivering dynamic and secure content.
||PSTN (Public Switched Telephone Network) refers to the international telephone system based on copper wires carrying analog voice data.
Originally a network of fixed-line analog telephone systems, the PSTN is now almost entirely digital, and includes mobile as well as fixed telephones. Most switches now use digital circuits between exchanges, with analog two-wire circuits still used to connect to most telephones.
The basic digital circuit in the PSTN is a 64 kbit/s channel called Digital Signal 0 (DS0). To carry a typical phone call the audio sound is digitized at an 8 kHz sample rate using 8-bit pulse code modulation (PCM). The call is switched using a signaling protocol (Signaling System 7) between the telephone exchanges under an overall routing strategy.
||PSU (Power Supply Unit) is the component that provides power for a computer to function. The power cord from a standard electrical outlet connects to the PSU, which converts the AC current to DC and regulates voltages that the separate devices in the computer need to function. The PSU usually has a wattage rating, which indicates the maximum power output of the PSU under load. Faster computers with more components require more powerful (in terms of watts) power supplies to provide adequate power to all components.
||QAM (Quadrature Amplitude Modulation) is a modulation technique in which there are 16 possible four-bit patterns, determined by the combination of phase and amplitude.
||A cable modem scans the entire frequency spectrum looking for a 6 MHz channel where the MPEG-2 frames are stamped with a PID of 0x1ffe. Once the cable modem finds a 6 MHz channel that carries this type of MPEG-2 frame, it achieves what is called a QAM lock.
||QoS (Quality of Service) is a term that describes the ability of a network to provide a particular level of service availability. QoS levels can range from best-effort to dedicated bandwidth. QoS is a technical term associated with ATM, but is commonly used to describe the concept of guaranteed availability for a wider range of technologies.
||RADIUS (Remote Access Dial-Up User Service) is an authentication standard technology, often used to protect access to wireless networks. RADIUS is a user name and password scheme that enables only approved users to access the network; it does not affect or encrypt data. The first time a user wants access to the network, secure files or internet locations, he or she must input his or her name and password and submit it over the network to the RADIUS server. The server then verifies that the individual has an account and, if so, ensures that the person uses the correct password before she or he can get on the network. RADIUS can be set up to provide different access levels or classes of access.
||RADSL (Rate-Adaptive Asymmetric Digital Subscriber Line) is an ADSL implementation that automatically adjusts the connection speed according to the quality of the telephone line, allowing RADSL to function over longer distances than ordinary ADSL.
||RAID - Redundant Array of Independent (or Inexpensive) Disks. Type of disk storage configuration often used on servers, where several physical disks are combined into an array for better speed and/or fault tolerance.
RAID 0 (data striping) - file blocks are written to separate drives. Does not provide fault tolerance, because failure of one drive will result in data loss.
RAID 1 (data mirroring) - data is duplicated on two drives either through software or hardware. Provides faster read performance than a single drive and good fault tolerance.
RAID 2 (not used in practice) - data is split at bit level at written to multiple drives.
RAID 3 (requires 3+ drives) - data block is striped at byte level across drives and error correction codes (parity info) is recorder on another drive. Provides fault tolerance but slower writing performance.
RAID 4 - similar to Level 3 but provides faster performance because it uses blocks for striping.
RAID 5 - similar to Level 4 but improves performance by also striping parity info across multiple drives.
RAID 6 - similar to Level 5 but also uses second parity scheme for better fault tolerance.
RAID 7 - proprietary RAID design by Storage Computer Corporation. Faster than other levels because it uses multiple levels of cache and asynchronous I/O transfers.
||RARP - Reverse Address Resolution Protocol (RFC 903).
RARP is a Link layer networking protocol used by a host computer to obtain its Internet Protocol (IPv4) address when it has available its link-layer address, such as an Ethernet MAC address.
||RC4 is a symetric stream cipher with an arbitrary key size designed by RSA Security, Inc. in 1987. It is essentially a pseudo random number generator, with the output of it being XORed with the data stream to produce encrypted data. The source code for an RC4 algorithm (or a code that produces the same results) was posted on Usenet in 1994. It is fast, it has been extensively analyzed and proven to be secure.
The RS4 cipher is used in SSL (Secure Socket Layer), WEP (Wireless Equivalent Privacy), WPA (Wi-Fi Protected Access), TKIP, MS XBOX, Oracle SQL, Adobe Acrobat, etc.
||RCPI (Received Channel Power Indicator) in 802.11 wireless networking is a measure of the received RF power in a selected channel over the preamble and the entire received frame. Note that a power level metric like RCPI generally can't comment on the quality of the link by itself.
See: IEEE 802.11k-2008
||Regedit is a commonly used editor (provided with Windows) for accessing the Windows Registry. It can be accessed from the Start Menu ( START > Run > type "regedit" ). Many of the registry values require rebooting the PC before they take effect.
Note: Modifying some registry settings can cause serious problems and damage your operating system to a point where it needs to be reinstalled. Make backups and use the information at your own risk.
Again, backup the Windows Registry before changing anything !
||In telecommunications, a repeater is a device that receives a signal and retransmits it, so that the signal can cover longer distances. A repeater can either retransmit it at a higher level/power, or it can simply extend the range by repeating the signal over some obstacles. If placed at the edge of a network, it can extend the coverage area.
In wireless networking, a wireless repeater (a.k.a wireless range extender) takes an existing signal from a wireless router or access point and rebroadcasts it to/from wireless clients to extend the range of the network. It differs from access points in that it is wirelessly connected to all devices on both ends of the connection.
||RFCs (Requests for Comments) are the official documents of the Internet Engineering Task Force (IETF) and the Internet Society that specify the details for protocols included in the TCP/IP family.
RFCs are authored by computer scientists and engineers in the form documents describing research, methods, and innovations applicable to the working of the Internet and Internet-connected systems. RFCs are submitted either for peer review or simply to convey new concepts and information. The IETF adopts some of the proposals published as RFCs as Internet standards.
IETF has published over 7000 RFCs, all of which can be viewed at IETF.org, or RFC-Editor.
||RFI (Radio Frequency Interference) - signal interference on the radio frequency electromagnetic spectrum (from 3 KHz to 300 GHz).
||RIAA (Recording Industry Association of America) is a special interest trade group formed in 1952, representing the US recording industry.
The RIAA has been at the heart of the P2P file-sharing controversy, as well as any new technology that can be used to record media content (the VCR for example). RIAA is continuously lobbying and waging an aggressive legal campaign attempting to defend the interests of its members (the larger record labels).
||RIR (Regional Internet Registry) is an organization overseeing the allocation and registration of Internet number resources within a particular region of the world. Resources include IP addresses (both IPv4 and IPv6) and autonomous system numbers (for use in BGP routing).
There are currently five RIRs in operation:
American Registry for Internet Numbers (ARIN) for North America and parts of the Caribbean
RIPE Network Coordination Centre (RIPE NCC) for Europe, the Middle East and Central Asia
Asia-Pacific Network Information Centre (APNIC) for Asia and the Pacific region
Latin American and Caribbean Internet Addresses Registry (LACNIC) for Latin America and parts of the Caribbean region
African Network Information Centre (AfriNIC) for Africa
||RJ-11 (Registered Jack 11) - a standard connector that is used to connect to the telephone line, the most common telephone jack.
||RJ-45 (Registered Jack 45) is a standard connector for the ends of a Category 5 network cable. Though it is similar in apperance to a phone plug (RJ-11), it has 8 wires rather than 6, and is slightly wider.
||Rolling On Floor, Laughing
||rootkit is a type of malicious software that is activated each time your system boots up. Rootkits are generally difficult to detect, since they run before the Operating System has completely booted up.
||A router is the central switching device in a packet-switched network that directs and controls (routes) the flow of data.
A traditional router routes packets within a single address realm, while a NAT router directs datagrams between different address realms. A NAT router sits on the border between two adress realms and performs such "transparent routing" by modifying addresses in IP headers, so that when packets enter another address realm they can be valid and routed properly.
Routers are differentiated from data communications switches by the ability to perform higher-level functions necessary to the interconnection of different networks.
||RPC (Remote Procedure Call) is a protocol that a program can use to request a service from a program located on another computer in a network. RPC helps with interoperability because the program using RPC does not have to understand the network protocols that are supporting communication. In RPC, the requesting program is the client and the service-providing program is the server.
Two newer object-oriented methods for programs to communicate with each other, CORBA and DCOM, provide the same types of capabilities as traditional RPCs.
||RSN (Robust Security Network) is a WPA (Wi-Fi Protected Access) replacement built on 802.1x and the AES (Advanced Encryption Standard).
||RSS (Really Simple Syndication) is a popular, lightweight XML format designed for sharing headlines, and other web content between sites. It is most often used for gathering and distributing news and other web content.
RSS (Receive-side Scaling) is also a method allowing for parallelized processing of received network packets on multiple processors, while avoiding packet reordering.
||RSSI (Recieved Signal Strength Indicator) is a common name for the received radio signal power level in a wireless network. It is often measured in either decibels (db), or simply numbers between 0 and 100. RSSI can be expressed as either a negative or positive value, however, in both instances smaller values closer to zero indicate a stronger signal.
Some older chipsets used RSSI from 0 to 256, or 0 to 127. Most newer chipsets use 0 to 100.
In 802.11 wireless, RSSI is being replaced by RCPI (Received Channel Power Indicator).
||RTT ( RoundTrip Time ) is the amount of time it takes for a packet of data to get from one designated point to another and back to the sender. RTT is usually used to measure latency.
||In Ethernet networks, any frame shorter than the minimum 64 bytes but with a valid CRC is considered a runt.
See also: jabber
||RWIN (TCP Receive Window) is a buffer that determines how much data the receiving computer is prepared to get at one time.
The sending side will only send data up to the size of the RWIN, and wait for acknowledgement before sending additional packets.
A RWIN value that's too large will result in greater loss of data if a packet is lost or damaged. A too small RWIN will be very slow, as each packet will have to be acknowledged before the next packet is sent.
RWIN is one of the most important parameters in tweaking any TCP/IP connection.
||SackOpts (Selective Acknowledgements - RFC 2018). SACKs allow a receiver to acknowledge non-consecutive data. This is particularly helpful on paths with large TCP Windows (large RWIN / BDP). Selective acknowledgements also allow TCP to recover from IP packet loss without resending packets that were already received by the client.
While SACK is now supported by most operating systems, it may have to be explicitly turned on by the system administrator.
||SAN (Storage Area Network) is a high-speed special-purpose (sub)network that interconnects different types of data storage devices with associated data servers on behalf of a larger network of users. Typically, a storage area network uses Fibre channel framework, or Gigabit Ethernet and is part of the overall network of computing resources for an enterprise.
||Serial ATA (Serial Advanced Technology Attachment). SATA is a newer standard for connecting hard drives in computer systems. It is based on serial signaling technology (as opposed to Parallel ATA, or PATA). SATA cables have only 7 conductors and are more flexible, thinner, use lower voltage, have lower potential for crosstalk/EMI, and can be longer than PATA ones (40-conductors).
Recent SATA hard drives have started implementing NCQ (Native Command Queuing, a technology designed to increase performance by allowing the disk to internally optimize the order in which read/write commands are executed. This bring brings SATA drives performance a step closer to SCSI than IDE PATA drives.
||SCSI (Small Computer System Interface) is an electronic interface allowing computers to communicatd with peripheral hardware such as disk drives, CD-ROM drives, scanners, etc.
SCSI is a very fast, and flexible parallel interface supported by all major Operating Systems. The latest Ultra320 SCSI standard allows for 15 devices to be connected to a single SCSI cable/port with length of up to 12 meters(LVD), having transfer rates of up to 320 MBps (Megabytes per second).
SCSI hard disk drives are often used in server environments, have better specs, and are higher priced than their IDE counterparts.
Top 10 reasons why SCSI is better than IDE:
10. more devices per cable/port (up to 16)
9. you can connect external devices to the SCSI bus (scanners/printers, etc.)
8. better multi-user capabilities (simultaneous access)
7. much lower CPU utilization
6. SCSI ensures data integrity with CRC checks, etc.
5. four times higher MTBF (mean time between failure)
4. longer is better (cables up to 12 meters with LVD)
3. lower access/seek time (HDD spindle speed up to 15K RPMs)
2. higher transfer rates (up to 320 MBytes/sec.)
and the number one reason why SCSI is better than IDE is....:
1. SCSI is just a smarter bus than IDE
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