Diskusjon:Digital Enhanced Cordless Telecommunications

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The DECT physical layer uses:

This means that the radio spectrum is divided into physical channels in two dimensions: frequency and time.

The maximum allowed power for portable equipment as well as base stations is 250mW. A portable device radiates an average of about 10mW during a call as it is only using one of 24 time slots to transmit.

The DECT media access control layer is the layer which controls the physical layer and provides connection oriented, connectionless and broadcast services to the higher layers. It also provides encryption services with the DECT Standard Cipher (DSC). The encryption is fairly weak, using a 35-bit initialization vector and encrypting the voice stream with 64-bit encryption.

The DECT data link layer uses LAPC (Link Access Protocol - Control), a specially designed variant of the ISDN data link protocol called LAPD. They are based on HDLC.

The DECT network layer always contains the following protocol entities:

  • Call Control (CC)
  • Mobility Management (MM)

Optionally it may also contain others:

  • Call Independent Supplementary Services (CISS)
  • Connection Oriented Message Service (COMS)
  • Connectionless Message Service (CLMS)

All these communicate through a Link Control Entity (LCE).

The call control protocol is derived from ISDN DSS1, which is a Q.931 derived protocol. Many DECT-specific changes have been made. The mobility management protocol includes many elements similar to the GSM protocol, but also includes elements unique to DECT.

Unlike the GSM protocol, the DECT network specifications do not define cross-linkages between the operation of the entities (eg Mobility Management and Call Control). The architecture presumes that such linkages will be designed into the interworking unit that connects the DECT access network to whatever mobility-eneabled fixed network is involved. By keeping the entities separate, the handset is capable of responding to any combination of entity traffic, and this creates great flexibility in fixed nework design without breaking full interoperability.

DECT GAP is an interoperability profile for DECT. The intent is that two different products from different manufacturers that both conform not only to the DECT standard, but also to the GAP profile defined within the DECT standard, are able to interoperate for basic calling. The DECT standard includes full testing suites for GAP, and GAP products on the market from different manufacturers are in practice interoperable for the basic functions.

DECT for Data Networks[rediger kilde]

Other interoperability profiles exist in the DECT suite of standards, and in particular the DPRS (DECT Packet Radio Services) bring together a number of prior interoperability profiles for the use of DECT as a wireless LAN and wireless internet access service. With good range (up to 200m indoors and 6km using directional antennae outdoors), dedicated spectrum, high interference immunity, open interoperability and data speeds of around 500kb/s, DECT appeared at one time to be a superior alternative to Wi-Fi. The protocol capabilities built into the DECT networking protocol standards were particularly good at supporting fast roaming in the public space, between hotspots operated by competing but connected providers. The first DECT product to reach the market, Olivetti's Net3, was a wireless LAN, and German firms Dosch & Amand and Hoeft & Wessel built niche businesses on the supply of data transmission systems based on DECT.

However the timing of the availability of DECT - in the mid 1990's - was too early to find wide application for wireless data outside niche industrial applications. Whilst contemporary providers of Wi-Fi struggled with the same issues, providers of DECT retreated to the more immediately lucrative market for cordless telephones. A key weakness was also the inaccessibility of the U.S. market, due to FCC spectrum restrictions at that time. By the time mass applications for wireless internet had emerged, and the U.S. had opened up to DECT, well into the new century, the industry had moved far ahead in terms of performance and DECT's time as a wireless data transport was past.

Ironically, the failure of DECT as a data protocol became a strength when DECT 6.0 phones finally appeared in the U.S. in the late 2000's. By this time, the ISM bands had become crowded in the U.S., especially the 2.4 GHz band which is used by both the most common variants of Wi-Fi, 802.11a and 802.11g, and many cordless phones; thus interference between unlicensed devices has become common in these bands. However, since Wi-Fi does not operate in the UPCS band and DECT devices negotiate with each other for the available spectrum, DECT 6.0 phones are immune from this type of interference.

See also[rediger kilde]

References[rediger kilde]

  • Book: Cordless Telecommunications Worldwide, pub Springer 1996, ed WHW Tuttlebee
  • Book: Personal Wireless Communication with DECT and PWT, pub Artech 1998, J Philips & G MacNamee
  • «Rechnernetze - The DECT Standard».  (a summary of the DECT standard)

External links[rediger kilde]

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