Glossary
Tech Terms | Abbreviations A–Z
D
DANE DNS DNSSEC Domain Name Download DSL DSS Dual SIM
DANE
DNS-based Authentication of Named Entities (DANE) is an Internet security protocol to allow X.509 digital certificates, commonly used for Transport Layer Security (TLS), to be bound to domain names using Domain Name System Security Extensions (DNSSEC).
It is proposed in RFC 6698 as a way to authenticate TLS client and server entities without a certificate authority (CA). It is updated with operational and deployment guidance in RFC 7671. Application specific usage of DANE is defined in RFC 7672 for SMTP and RFC 7673 for using DANE with Service (SRV) records.
DNS
The Domain Name System (DNS) is a hierarchical and decentralized naming system for computers, services, or other resources connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities. Most prominently, it translates more readily memorized domain names to the numerical IP addresses needed for locating and identifying computer services and devices with the underlying network protocols. By providing a worldwide, distributed directory service, the Domain Name System has been an essential component of the functionality of the Internet since 1985.
The Domain Name System delegates the responsibility of assigning domain names and mapping those names to Internet resources by designating authoritative name servers for each domain. Network administrators may delegate authority over sub-domains of their allocated name space to other name servers. This mechanism provides distributed and fault-tolerant service and was designed to avoid a single large central database.
The Domain Name System also specifies the technical functionality of the database service that is at its core. It defines the DNS protocol, a detailed specification of the data structures and data communication exchanges used in the DNS, as part of the Internet Protocol Suite.
The Internet maintains two principal namespaces, the domain name hierarchy and the Internet Protocol (IP) address spaces. The Domain Name System maintains the domain name hierarchy and provides translation services between it and the address spaces. Internet name servers and a communication protocol implement the Domain Name System. A DNS name server is a server that stores the DNS records for a domain; a DNS name server responds with answers to queries against its database.
The most common types of records stored in the DNS database are for Start of Authority (SOA), IP addresses (A and AAAA), SMTP mail exchangers(MX), name servers (NS), pointers for reverse DNS lookups (PTR), and domain name aliases (CNAME). Although not intended to be a general purpose database, DNS has been expanded over time to store records for other types of data for either automatic lookups, such as DNSSEC records, or for human queries such as responsible person (RP) records. As a general purpose database, the DNS has also been used in combating unsolicited email (spam) by storing a real-time blackhole list (RBL). The DNS database is traditionally stored in a structured text file, the zone file, but other database systems are common.
DNSSEC
The Domain Name System Security Extensions (DNSSEC) is a suite of extension specifications by the Internet Engineering Task Force (IETF) for securing data exchanged in the Domain Name System (DNS) in Internet Protocol (IP) networks. The protocol provides cryptographic authentication of data, authenticated denial of existence, and data integrity, but not availability or confidentiality.
Domain Name
A domain name is an identification string that defines a realm of administrative autonomy, authority or control within the Internet. Domain names are used in various networking contexts and for application-specific naming and addressing purposes. In general, a domain name identifies a network domain, or it represents an Internet Protocol (IP) resource, such as a personal computer used to access the Internet, a server computer hosting a website, or the web site itself or any other service communicated via the Internet. In 2017, 330.6 million domain names had been registered.
Domain names are formed by the rules and procedures of the Domain Name System (DNS). Any name registered in the DNS is a domain name. Domain names are organized in subordinate levels (subdomains) of the DNS root domain, which is nameless. The first-level set of domain names are the top-level domains (TLDs), including the generic top-level domains (gTLDs), such as the prominent domains com, info, net, edu, and org, and the country code top-level domains (ccTLDs). Below these top-level domains in the DNS hierarchy are the second-level and third-level domain names that are typically open for reservation by end-users who wish to connect local area networks to the Internet, create other publicly accessible Internet resources or run web sites.
The registration of these domain names is usually administered by domain name registrars who sell their services to the public.
A fully qualified domain name (FQDN) is a domain name that is completely specified with all labels in the hierarchy of the DNS, having no parts omitted. Traditionally a FQDN ends in a dot (.) to denote the top of the DNS tree. Labels in the Domain Name System are case-insensitive, and may therefore be written in any desired capitalization method, but most commonly domain names are written in lowercase in technical contexts.
Download
In computer networks, download means to receive data from a remote system, typically a server such as a web server, an FTP server, an email server, or other similar system. This contrasts with uploading, where data is sent to a remote server. A download is a file offered for downloading or that has been downloaded, or the process of receiving such a file.
DSL
(Abbr.:) Digital subscriber line (originally digital subscriber loop) is a family of technologies that are used to transmit digital data over telephone lines. In telecommunications marketing, the term DSL is widely understood to mean asymmetric digital subscriber line (ADSL), the most commonly installed DSL technology, for Internet access.
DSL service can be delivered simultaneously with wired telephone service on the same telephone line since DSL uses higher frequency bands for data. On the customer premises, a DSL filter on each non-DSL outlet blocks any high-frequency interference to enable simultaneous use of the voice and DSL services.
The bit rate of consumer DSL services typically ranges from 256 kbit/s to over 100 Mbit/s in the direction to the customer (downstream), depending on DSL technology, line conditions, and service-level implementation. Bit rates of 1 Gbit/s have been reached.
In ADSL, the data throughput in the upstream direction (the direction to the service provider) is lower, hence the designation of asymmetric service. In symmetric digital subscriber line(SDSL) services, the downstream and upstream data rates are equal. Researchers at Bell Labs have reached speeds over 1 Gbit/s for symmetrical broadband access services using traditional copper telephone lines, though such speeds have not yet been deployed elsewhere.
DSS
(Abbr.:) Dynamic Spectrum Sharing. Allows simultaneous use of 4G and 5G in the same frequency range. By sharing existing 4G infrastructure, 5G coverage can be achieved relatively quickly. The available bandwidth of a specific frequency band is divided between the 4G and 5G users active in a radio cell according to demand. A slightly higher average data rate can generally be expected with 5G DSS compared to 4G.
Dual SIM
Some mobile phones support use of two SIM cards, described as dual SIM operation. When a second SIM card is installed, the phone either allows users to switch between two separate mobile network services manually, has hardware support for keeping both connections in a "standby" state for automatic switching, or has individual transceivers for maintaining both network connections at once.
Dual SIM phones are mainstream in many countries where phones are normally sold unlocked. Dual SIMs are popular for separating personal and business calls, in locations where lower prices apply to calls between clients of the same provider, where a single network may lack comprehensive coverage, and for travel across national and regional borders. In countries where dual SIM phones are the norm, people who require only one SIM simply leave the second SIM slot empty. Dual SIM phones will usually have two unique IMEI numbers, one for each SIM slot.
Devices that use more than two SIM cards have also been developed and released, notably the LG A290 triple SIM phone, and even handsets that support four SIMs, such as the Cherry Mobile Quad Q70.
History
The first phone to include dual SIM functionality was the Benefon Twin, released by Benefon in 2000. It wasn't until the late 2000s, however, when more dual SIM phones entered the marketplace and started to attract mainstream attention, most of them coming from small Chinese firms producing phones using Mediatek systems-on-a-chip.
Such phones were initially eschewed by major manufacturers due to potential pressure from telecommunications companies, but in the early 2010s Nokia, Samsung, Sony and several others followed suit, with the Nokia C2-00, Nokia C1-00 and Nokia C2-03 and most notably the Nokia X, phones from Samsung's Duos series, and the Sony Xperia Z3 Dual, Sony Xperia C and tipo dual. Apple added dual SIM support in its 2018 iPhone XS models, with models sold in China containing two physical SIM slots, and models sold elsewhere supporting dual SIM by means of Embedded-SIM alongside a single physical SIM.
Adapters
Prior to the introduction of dual SIM phones, adapters were made for phones to accommodate two SIMs, and to switch between them when required.
Passiv
Dual SIM switch phones, such as the Nokia C1-00, are effectively a single SIM device as both SIMs share the same radio, and thus are only able to place or receive calls and messages on one SIM at the time. They do, however, have the added benefit of alternating between cards when necessary.
Standby
Dual standby phones, such as those running on Mediatek chipsets, allows both SIMs to be accessed through time multiplexing. When making or receiving calls, the modem locks to the active channel; the other channel would be ignored and thus unavailable during the duration of the call. Examples of Dual SIM Standby smartphones include the Samsung Galaxy S Duos, the Sony Xperia M2 Dual, and the iPhone XS, XS Max and iPhone XR.
Active
Dual SIM active phones or dual active (DSDA) phones, however, come with two transceivers, and are capable of receiving calls on both SIM cards, at the cost of increased battery consumption. One example is the HTC Desire 600.
Unequal Connectors
Some telephones distinguish a primary SIM slot that allows for 4G/3G connectivity and a secondary slot limited to 3G/2G connectivity. However, selecting either of the SIMs as primary is usually possible without physically swapping the SIMs.
Some phone models utilize a "hybrid" SIM tray, which allows two SIM cards or one SIM card and one MicroSD memory card. Huawei's Mate 20 range introduced a proprietary memory card format called Nano Memory, which exactly matches the dimensions of a nano SIM card.
Some devices accept dual SIMs of different form factors. The Xiaomi Redmi Note 4 has a hybrid dual SIM tray that accepts one micro SIM card and one nano SIM card, the latter of which can be swapped for a MicroSD card.