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|CoDel (controlled delay) is a network scheduler algorithm designed to overcome bufferbloat in network links, such as routers by setting limits on the delay network packets can suffer because of buffering. It only affects congested links, and helps improve overall latency by dropping the slowest packets on links that are overloading the buffers.
CoDel improves on the performance of the RED algorithm by better recognizing bufferbloat. CoDel distinguishes between two types of packet queues, "good queues" that exibit no buffer bloat, and "bad queues" that exibit some buffer bloat (more than 5ms delay because of buffers). CoDel only comes into effect when the network buffers fill up, and the minimum delay for the TCP Window is exceeded, it then treats "bad queues" by dropping slowest packets to control link rates.
CoDel is available in the Linux kernel since version 3.5. It is also available with various open source router firmwares, such as CeroWrt, OpenWrt, dd-wrt, and AsusWrt-Merlin firmware.
|In a network, a packet collision is the result of two devices atteprting to transmit data at the same time. Collisions are common in Ethernet networks. Collisions cause packets to be retransmitted and can impede the performance of the network when in large quantities. Using switches (rather than hubs) can reduce the number of collisions/retransmissions.
See also: CSMA/CD
|CoS (Class of Service, or 802.1p) is a 3 bit field within an Ethernet frame header when using 802.1Q tagging. The field specifies a priority value between 0 and 7 that can be used by Quality of Service (QoS) disciplines to differentiate traffic.
Some switches/routers use CoS to internally classify traffic for QoS purposes.
|CPE (customer premises equipment) - any piece of equipment in a communication system that resides within the home or office of the customer. Examples include Cable/DSL modems, TV set-top boxes, telephones, etc.
|cps (characters per second) is a measure of speed used to describe the number of characters passing a designated point per second.
Usually used in measuring the speed of dot-matrix and daisy-wheel printers, where one character is printed at a time.
|cracker - an intruder who breaks into someone else's computer system, often (incorrectly) referred to as a hacker.
The term cracker is also widely used to describe a person who breaks copy protection in software applications in order to keep or distribute free/pirated copies.
Hacker is a term used by some to mean "a clever programmer" and by others, especially journalists or their editors, to mean "someone who tries to break into computer systems.". Please note hacking is the process of cleverly solving a programming problem, not necessarily associated with crackers.
|CRC (Cyclic Redundancy Check/Code) is a method of checking for errors in data that has been transmitted over a network.
The sending device applies a 16 or 32 bit polynomial division to a datagram (Ethernet uses 32-bit CRC) and appends the resulting CRC before transmitting it. The receiving end applies the same polynomial to the data, compares the CRC results and determines whether the data has been received successfully, or it needs to be retransmited.
See also: checksum
|A type of Ethernet cable in which the "send" and "receive" pairs are crossed, so it can be used to connect similar network devices (such as swithches, or PCs without a hub/switch between them) directly.
In a crossover cable, the inside wires are arranged differently (crossed over) in the RJ45 plugs on either end. It has the same effect as (and can substitute for) an uplink port in a hub/switch.
See also: How to make Network Cables
|crosstalk is an interference along a circuit or a cable pair. When such crosstalk occurs, a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel.
In essence, one signal disrupts a signal in an adjacent circuit and can cause the signals to cross over and interfere with each other.
|CSMA/CD (Carrier Sense Multiple Access / Collision Detection) is a standard set of rules determining how network devices detect and respond when a packet collision occurs in a network, such as Ethernet. In essense, after detecting a collision, a device waits a random delay time and then attempts to re-transmit the data.
|CSU (Channel Service Unit) receives and transmits signals from and to the WAN line and provides a barrier for electrical interference from either side of the unit. The CSU can also echo loopback signals from the phone company for testing purposes.
Often combined with a DSU into a single CSU/DSU (Channel Service Unit/Data Service Unit) device.
|CSU/DSU (Channel Service Unit/Data Service Unit) - a hardware device that converts a digital data frame from the communications technology used on a LAN into a frame appropriate to a WAN and vice versa.
A CSU/DSU is typically an external hardware device that sits at the customer premise at the termination point of a digital line. The CSU/DSU then connects to the customer network, normally through a router. The CSU/DSU bridges the customer network and the LEC network by performing line coding, line-conditioning, and equalization functions, and responds to loopback commands sent from the central office.
|CTCP (Compound TCP) is a newer TCP congestion control algorithm designed to aggressively adjust the sender's TCP Window to optimize TCP for connections with higher bandwidth and latency.
The algorithm is generally an improvement over the more traditional slow-start and congestion avoidance TCP algorithms. It is especially effective with broadband internet connections, or any connection with a high bandwidth * delay product.
CTCP has been implemented in Windows Vista/2008, and is also available for Windows 2003 server, and XP(64-bit) via hotfixes. The algorithm has been ported to Linux as well.
|CTIA Cellular Telecommunications and Internet Association (www.ctia.org). Formerly the Cellular Telecommunications Industry Association.
The CTIA is a membership organization, involved with regulatory and public affairs issues in the wireless industry. It represents all elements of wireless communication, including cellular, mobile satellite services and personal communications services. The CTIA deals with taxation, roaming, safety, fraud and evolving technologies.
|Defense Advanced Research Projects Agency
|The term has been generally replaced by the term packet. Datagrams or packets are the message units that the Internet Protocol (IP) transports over the Internet.
According to RFC 1594 a datagram is "a self-contained, independent entity of data carrying sufficient information to be routed from the source to the destination computer without reliance on earlier exchanges between this source and destination computer and the transporting network."
|Direct Broadcast Satellite
|DCCP (Datagram Congestion Control Protocol) is a computer networking transport layer protocol.
DCCP provides provides bidirectional unicast connections of congestion-controlled unreliable datagrams. It is suitable for applications that transfer fairly large amounts of data and that can benefit from control over the tradeoff between timeliness and reliability. Such applications include streaming media, multiparty online games and Internet telephony (where getting new data is preferred to resending lost packets).
DCCP is published as RFC 4340, a proposed standard by the IETF in March 2006. See also: RFC 4336, RFC 4341, RFC 4342. Linux had an implementation of DCCP first released in Linux kernel version 2.6.14
|Data Link Connection Identifier (DSLAM) is a number of a private or switched virtual circuit in a frame relay network. Located in the frame header, the DLCI field identifies which logical circuit the data travels over, and each DLCI has a Committed Information Rate (CIR) associated with it.
The DLCI number is local to the FRAD and frame relay switch it connects to, and it is generally changed by the switch within the network, because the receiving switch uses a different DLCI for the same connection.
|DCOM (Distributed Component Object Model) is a protocol that enables software components to communicate directly over a network. Previously called "Network OLE," DCOM is designed for use across multiple network transports, including Internet protocols such as HTTP. DCOM is based on the Open Software Foundation's DCE-RPC spec. For more information about DCOM visit the following Web site: http://www.microsoft.com/com/tech/dcom.asp
|DDNS (Dynamic Domain Name System) - the capability of having server software on a dynamic IP address to have a fixed domain name.
|DDoS (Distributed Denial of Service) is a type of external Internet attack, in which multiple sources attack a single target system, with the goal being denial of service for its users. DDoS attacks flood the target system with incoming messages at a rate much higher than it can process, therefore slowing it down to a level where it is rendered useless to users.
Usualy the intruders that initiate DDoS attacks break into a system (or a few systems) and make it the DDoS master that (by using some DDoS daemon software) manages other (100s even 1000s) "slave" compromised systems, whose sole purpose is to distribute the attack over multiple sources, when initiated. Whenever the DDoS attack is initiated, the multiple slave systems are instructed by the master to launch the DDoS attack and flood the end target system simultaneously, rendering it useless for legitimate users.
Note: The multiple systems involved in a DDoS that initiate the flood are most often compromised systems that can be considered victims of the attacker as well as the final target system.
|DECT (Digital Enhanced Cordless Telecommunication) is an ETSI standard for digital cordless phones.
The DECT standard fully specifies a means for a portable unit such as a cordless telephone to access a fixed telecoms network via radio, but unlike the GSM standards does not specify any internal aspects of the fixed network itself. Connectivity to the fixed network (which may be of many different kinds) is done through a base station or "Radio Fixed Part" to terminate the radio link, and a gateway to connect calls to the fixed network. In most cases the base station connection is to the public switched telephone network or telephone jack, although connectivity with newer technologies such as Voice over IP has become available. There are also other devices such as some baby monitors utilizing DECT, and in these devices there is no gateway functionality.
|Conversion of a carrier signal or wave form (analog) into an electrical signal (digital).
|DES (Data Encryption Standard) is a cryptographic algorighm used to protect data when transmitted through an unsecured network.
|DHCP (Dynamic Host Configuration Protocol) is a protocol for automatic assignment of IP addresses in an organization's network.
Using the Internet Protocol, each device that can connect to the Internet needs a unique IP address assigned to it. The DHCP server assigns a dynamic IP address to each client, using the concept of a "lease" (amount of time) that the IP will be valid for a particular network node. Assigning short lease times to clients allows for a dynamic network with more nodes than available IP addresses.
With DHCP there is no need to statically assign IPs to each node locally, and there is no need to reconfigure clients if they are moved to a different place in the network. The only disadvantage of using DHCP is the fact clients have no permanent IP addresses; when the DHCP lease expires IPs can be reassigned to a different client.
See also: What is the difference between dynamic and static IP addresses ?
|DLC (Digital Loop Carrier) technology makes use of digital techniques to bring a wide range of services from the telecommunications network provider to users via twisted-pair copper phone lines.
|Data Local Exchange Carrier
|DMZ (De-Militarized Zone) refers to a computer/device on an internal protected network that is placed outside the firewall, but still available to the LAN.
The advantage of a DMZ computer is it has all communicatons ports open and it can use any special aplication that communicates on non-standard ports, such as many VoIP programs and online games. The disadvantage is that the device might be vulnerable to attacks, not being protected by the firewall.
|DNS (Domain Name System) is a protocol used to translate Internet domain and host names (such as www.speedguide.net) to IP addresses. DNS implements a distributed database to store name and address information for all public hosts on the Net, assuming IP addresses are statically assigned.
|DOCSIS ( Data Over Cable Service Interface Specification ) is the Cable Modem standard that defines the interface requirements of high-speed data transmission over cable networks. Cable modems often have a "CableLabs Certified" logo on the devices to indicate they are DOCSIS compliant.
DOCSIS 1.x (v1.0 1997, v1.1 1999) 38 Mbps downstream, 9 Mbps upstream. Version 1.1 standardized QoS support.
DOCSIS 2.0 (2001) 38 Mbps downstream, 27 Mbps upstream.
DOCSIS 3.0 (2006) 38 Mbps downstream, 27 Mbps upstream per channel, 4+ channel bonding, support for IPv6.
DOCSIS 3.1 (2013) 10 Gbps downstream, 1 Gbps upstream using 4096 QAM.
See also: CableLabs.
|domain - a specific name for a network of computers.
|downstream (or download, downlink) is a transmission from a server toward the end user and upstream/upload is a transmission toward the server. Data rate can differ in the downstream and upstream directions.
|DS0 (digital signal 0) is digital transmission rate of 64 Kbps, the bandwidth normally used for one telephone voice channel. It is the base multiple for both T (North America) and E (Europe and Japan) DS (digital signal) carriers.
A T1 for example is 24 DS0 (64 Kbps) signals, or 1.544 Mbps. E1 = 32 DS0s, or 2.048 Mbps.
|DSL ( Digital Subscriber Line ) is a method of high-speed data transfer over ordinary copper telephone lines. xDSL refers to different variations of DSL, such as ADSL, HDSL, SDSL and RADSL.
|DSLAM (Digital Subscriber Line Access Multiplexer) is a network device, usually at a telephone company central office, that receives signals from multiple customer DSL connections and puts the signals on a high-speed backbone line using multiplexing techniques.
Depending on the product, DSLAM multiplexers connect DSL lines with some combination of ATM (asynchronous transfer mode), frame relay, or IP networks.
|DSSS (direct-sequence spread spectrum) is a modulation technology used in wireless network transmissions. DSSS is used as a kind of radio transmission that distributes the signal over many frequencies.
DSSS generates a redundant bit pattern (chipping code) for every bit of data to be transmitted. Data is then combined with the chipping code according to some spreading ratio. This chipping code allows for recovering lost data without retransmissions. DSSS improves pricacy, increases the signal's resistance to narrowband interference, and provides data redundancy.
See also: FHSS
|DSU (Data Service Unit) is a modem-like interface device that connects the computer as end user Data Terminal Equipment (DTE) to digital access lines (through a CSU), and which provides framing of sub-64Kbps customer access channels onto higher rate data circuits.
Often combined with a CSU into a single CSU/DSU (Channel Service Unit/Data Service Unit) device.
|DTE (Data Terminal/Termination Equipment) - a single piece or an interconnected subsystem of multiple pieces of equipment that perform all the required functions necessary to permit users to communicate. Typically the device that transmits data (computer, data terminal).
|Refers to the ability to transfer data in both directions. Full-duplex allows for data transfer in both directions simultaneously; half-duplex allows for data transfer only in one direction at the same time.
Most current NICs allow for full-duplex communication, theoretically doubling their throughput (i.e. for 100BaseT - 100 Mbps in one direction and 100 Mbps in the other at the same time). Curent Cable/DSL modems are usually half-duplex.
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