The 21st Century Network (21CN) programme is the data and voice network transformation project, under way since 2004, of the UK telecommunications company
BT Group plc
BT Group plc ( trading as BT and formerly British Telecom) is a British multinational telecommunications holding company headquartered in London, England. It has operations in around 180 countries and is the largest provider of fixed-line, bro ...
. It was intended to move BT's telephone network from the
AXE
An axe ( sometimes ax in American English; see spelling differences) is an implement that has been used for millennia to shape, split and cut wood, to harvest timber, as a weapon, and as a ceremonial or heraldic symbol. The axe has ma ...
/
System X Public Switched Telephone Network (PSTN) to an
Internet Protocol
The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet.
...
(IP) system. As well as switching over the PSTN, BT planned to deliver many additional services over their new data network, such as on-demand interactive TV services.
BT originally stated that it would accrue annual savings of £1 billion when the transition to the new network was completed, and hoped to have over 50% of its customers transferred by 2008 (see
''External links'' below for current progress on the roll-out of optical fibre by
Openreach
Openreach Limited is a company wholly owned by BT Group plc, that maintains the telephone cables, ducts, cabinets and exchanges that connect nearly all homes and businesses in the United Kingdom to the national broadband and telephone network. I ...
). Capital expenditure was put at £10 billion over five years, this being 75% of BT's total capital spending plans in that period.
Architecture
The new network is based on an architectural model of five classes of network nodes. These are:
* Premises
* Access (MSAN)
* Metro
* Core
* iNode
Premises
Premises nodes includes residential, small-medium enterprise (SME) and enterprise. The presumption is that all these will have high speed connections to the network, delivered over copper (in the form of
ADSL
Asymmetric digital subscriber line (ADSL) is a type of digital subscriber line (DSL) technology, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. ...
or other
DSL
Digital subscriber line (DSL; 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 dig ...
technologies) or over fibre, as either
PON or direct fibre in the case of large enterprises. The sole exception to this presumption is for legacy PSTN, where provision will continue to be made for analogue voice.
Access node
21CN introduced the concept of the ''
multi-service access node'' (MSAN). This logical node takes the various access technologies (mentioned above) and, where possible, aggregates these onto a single
backhaul network technology. This includes converting analogue voice into ''voice over IP'' (
VoIP
Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms Internet t ...
) using the MSAN as a
media gateway (MGW). The aim is to implement a few hundred access nodes.
Note that these will not have any
IP routing capability, but will essentially be
layer 2
The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer p ...
Ethernet
Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 1 ...
devices.
Metro node
The backhaul network will terminate on the metro nodes. At this point the IP-based services will be implemented, and the metro nodes are the first location where IP traffic is routed. Call control (via a
softswitch
A softswitch (''software switch'') is a call-switching node in a telecommunications network, based not on the specialized switching hardware of the traditional telephone exchange, but implemented in software running on a general-purpose computing ...
or an
IMS CSCF) will be implemented here, although the softswitches and the IMS components won't be described as a part of the metro node – they are parts of the iNode. The metro nodes are also ''provider edge'' (PE) routers in
MPLS
Multiprotocol Label Switching (MPLS) is a routing technique in telecommunications networks that directs data from one node to the next based on labels rather than network addresses. Whereas network addresses identify endpoints the labels identif ...
terminology, encapsulating the IP traffic in MPLS tunnels for transmission over the core. The aim is to implement around 100 metro nodes.
[
]
Core node
The core nodes are MPLS switches, with the MPLS traffic carried over optical (DWDM
In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e., colors) of laser light. This techni ...
) transport. They are completely unaware of customer IP traffic, and only switch based on MPLS tags (all customer IP traffic is encapsulated with an MPLS header by the metro node PEs). Native IP is only used by the core nodes for protocols such as MP-BGP, an IGP, LDP, and RSVP to exchange routing and label information between all core and metro nodes. The aim is to have approximately 10 core nodes.[
]
iNode
iNodes are the logical nodes that provide the control for the services implemented using the other four types of node. BT has announced that it intends to create an IMS based iNode capability, although its initial PSTN replacement will not be an IMS implementation. The iNode will implement a set of standardized functions – common capabilities – that deliver layered services. Common capabilities include session management, authentication, profile, address book, presence and location. Combinations of these capability primitives will be used to deliver different service types and functionality.
The iNode is built upon the AXE telephone exchange
The AXE telephone exchange is a product line of circuit switched digital telephone exchanges manufactured by Ericsson, a Swedish telecom company. It was developed in 1974 by Ellemtel, a research and development subsidiary of Ericsson and Tele ...
TSS (Telephony Softswitch Server) and is currently using HP Alpha processors (APZ 212 50) as well as the IS-Blade in the APZ logic. After extensive field testing in the South Wales Pathfinder area, all current logical and intelligent network services now operate in conjunction with the existing PSTN and the MPLS network.
Differences
The most significant differences between the legacy 20th century network (20CN) and 21CN are:
* The use of VoIP landline telephone services served digitally, rather than the previous analogue PSTN
The public switched telephone network (PSTN) provides infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local teleph ...
system. This does not mean telephone services are provided over the Internet with 21CN. In reality this will make no difference to the end user.
* The availability of ADSL2+
G.992.5 (also referred to as ADSL2+, G.dmt.bis+, and G.adslplus) is an ITU-T standard for asymmetric digital subscriber line (ADSL) broadband Internet access. The standard has a maximum theoretical downstream sync speed of 24 megabits per second ...
(including ADSL2+ Annex M) broadband services. The 20CN allows services up to ADSL Max with a maximum download throughput of 8 Mbit/s whereas ADSL2+ allows a maximum theoretical download throughput of 24 Mbit/s.
* The retirement of legacy IPstream data services previously available with 20CN.
Evolution towards FTTC
Any chain is only as strong as its weakest link, and in the case of 21CN, its weakest link – the access network
An access network is a type of telecommunications network which connects subscribers to their immediate service provider. It is contrasted with the core network, which connects local providers to one another. The access network may be further ...
– is also its most valuable. Although the architecture of 21CN simplifies the network plant, it does not look to solve the problem which will have the greatest effect on data rate, that is loop length i.e. the length of cable from the exchange to the customer. Unlike the active core network, the access network is a passive network and has no capability for self-discovery. Reasonable consumer expectations can be established based upon the length and characteristics of these wires. A more global model would require precise knowledge of wire material (e.g. copper or aluminium), where the wires are routed, and the direction in which traffic flows around the circuit. This information is not held at present, and would need significant effort to obtain.
By moving the MSANs deeper into the network, i.e. to street side cabinets, length issues are more likely to be reduced; however, line transmission characteristics are still highly variable since lines may be bridged with materials other than copper (such as aluminium) which have a degrading effect on conductivity and hence signal strength. In addition changes in wire gauge
Wire gauge is a measurement of wire diameter. This determines the amount of electric current the wire can safely carry, as well as its electrical resistance and weight.
Types of wire gauge
Wire gauges may be broadly divided into two groups, t ...
(thickness) are common and introduce signal reflection
In telecommunications, signal reflection occurs when a signal is transmitted along a transmission medium, such as a copper cable or an optical fiber. Some of the signal power may be reflected back to its origin rather than being carried all the ...
due to changes in impedance.
The original 21CN design did not look to drive MSANs deeper into the network; instead it locates them in each exchange. Without detailed information on the critical local routings, it is difficult to ascertain what capacity remains in the duct network and which connections run through it. This makes planning of future upgrades or fibre additions difficult. Openreach
Openreach Limited is a company wholly owned by BT Group plc, that maintains the telephone cables, ducts, cabinets and exchanges that connect nearly all homes and businesses in the United Kingdom to the national broadband and telephone network. I ...
considered integrating MSANs into the access network, although this was originally deemed unlikely to happen because there are only 5,600 exchange buildings and over 85,000 'primary connection points', usually in the form of street cabinets.
However, in July 2007, Sir Christopher Bland, chairman of BT, stated that BT was considering fibre to the kerb and that VDSL2
Very high-speed digital subscriber line (VDSL) and very high-speed digital subscriber line 2 (VDSL2) are digital subscriber line (DSL) technologies providing data transmission faster than the earlier standards of asymmetric digital subscriber li ...
was a 'likely development going forward'. In October 2007, Ofcom launched a consultation into Next Generation Access Networks in the UK after pressure from the government.
Subsequently, Openreach began a rollout of VDSL2
Very high-speed digital subscriber line (VDSL) and very high-speed digital subscriber line 2 (VDSL2) are digital subscriber line (DSL) technologies providing data transmission faster than the earlier standards of asymmetric digital subscriber li ...
FTTC
Fiber to the ''x'' (FTTX; also spelled "fibre") or fiber in the loop is a generic term for any broadband network architecture using optical fiber to provide all or part of the local loop used for last mile telecommunications. As fiber optic ...
technology over 21CN, deploying DSLAMs in thousands of new street-side cabinets. BT Retail launched their BT Infinity
BT Superfast Fibre (formerly BT Infinity) is a broadband service in the United Kingdom provided by BT Consumer, the consumer sales arm of the BT Group. The underlying network is fibre-to-the-cabinet (FTTC), which uses optical fibre for all excep ...
service in 2010 and expected to continue expanding availability by installing new cabinets until at least 2014. The FTTC deployment essentially installs a small (96-288 line) DSLAM into thousands of street cabinets, each backhauled using direct fibre to a larger exchange or metro node, removing much of the local-loop length limitations and allowing end-user speeds exceeding 100 Mbit/s following future upgrades.
Suppliers
In April 2005, BT announced that it had selected eight suppliers for its 21CN roll-out. These were:
* Fujitsu – MSAN
*Huawei
Huawei Technologies Co., Ltd. ( ; ) is a Chinese multinational technology corporation headquartered in Shenzhen, Guangdong, China. It designs, develops, produces and sells telecommunications equipment, consumer electronics and various smar ...
– MSAN and core optical
*Alcatel Alcatel may refer to:
* Alcatel, a former French telecommunications equipment company, which became Alcatel-Lucent and is now part of Nokia
* Alcatel Mobile, a brand of mobile phones, tablets and wearables, formerly a joint venture between Alcatel ...
– metro
*Lucent
Lucent Technologies, Inc. was an American multinational telecommunications equipment company headquartered in Murray Hill, New Jersey. It was established on September 30, 1996, through the divestiture of the former AT&T Technologies business u ...
– core MPLS, using Juniper components
*Cisco
Cisco Systems, Inc., commonly known as Cisco, is an American-based multinational digital communications technology conglomerate corporation headquartered in San Jose, California. Cisco develops, manufactures, and sells networking hardware, ...
– metro and core MPLS
* Siemens – metro, using Juniper components
*Ciena
Ciena Corporation is an American telecommunications networking equipment and software services supplier based in Hanover, Maryland. The company has been described by ''The Baltimore Sun'' as the "world's biggest player in optical connectivity." ...
– core optical
*Ericsson
(lit. "Telephone Stock Company of LM Ericsson"), commonly known as Ericsson, is a Swedish multinational networking and telecommunications company headquartered in Stockholm. The company sells infrastructure, software, and services in informa ...
– iNode
Alcatel and Lucent merged in December 2006 to form Alcatel-Lucent
Alcatel–Lucent S.A. () was a French–American global telecommunications equipment company, headquartered in Boulogne-Billancourt, France. It was formed in 2006 by the merger of France-based Alcatel and U.S.-based Lucent, the latter being a s ...
.
The fact that Britain's Marconi received no major 21CN contract was a surprise to commentators and sent the company's shares tumbling. An example of analysis before BT's announcement is Dresdner Kleinwort Wasserstein's: " arconi isso advanced with its products and so entrenched with BT Group plc that its selection looks certain."
Criticism
BT's plans for broadband internet access services were criticised. BT's main plan for providing Internet access was to upgrade their DSLAM
A digital subscriber line access multiplexer (DSLAM, often pronounced ''DEE-slam'') is a network device, often located in telephone exchanges, that connects multiple customer digital subscriber line (DSL) interfaces to a high-speed digital ...
s to ADSL2+
G.992.5 (also referred to as ADSL2+, G.dmt.bis+, and G.adslplus) is an ITU-T standard for asymmetric digital subscriber line (ADSL) broadband Internet access. The standard has a maximum theoretical downstream sync speed of 24 megabits per second ...
in the exchange. This was controversial because ADSL2+ was already an old technology and is limited to 24Mbit/s
In telecommunications, data-transfer rate is the average number of bits ( bitrate), characters or symbols ( baudrate), or data blocks per unit time passing through a communication link in a data-transmission system. Common data rate units are mu ...
downstream, and would be even more dated by the time 21CN is completed. This led to criticisms that BT was not future proof
Future-proofing is the process of anticipating the future and developing methods of minimizing the effects of shocks and stresses of future events. Future-proofing is used in industries such as electronics, medical industry, industrial desig ...
ing their network, since fibre, while more expensive to lay, is a lot more future-proof. Critics argue that fibre to the home would be more appropriate, which would mean replacing the current copper wire which supplies individual homes (the "last mile").
In late 2009, British ISP Andrews & Arnold reported severe issues with 21CN, citing multi-hour outages due to single points of failure and said that 21CN was not "fit for purpose" due to this and numerous other problems.
21CN's core technology of using VoIP has resulted in many problems with machine to machine communications - faxes, telemetry, alarm systems etc. These systems are much fussier than the human ear, and as such the conversion chain from analogue (between premises and local exchange) to 64 kbps digital, to VoIP and back again combined with IP routes of varying quality reduces the signal quality enough such that the devices at either end cannot communicate effectively. These problems are very difficult to fault find as any individual section of the call functions perfectly well; the effect only becomes apparent when all sections are joined together to make the end-to-end call. T38 fax promises to eliminate the fax issues (the fax is essentially sent as a JPEG with error correction), however the other realtime m2m comms has no solution other than to move to IP-based devices at each end, a mammoth and costly undertaking for the users. As such, there is talk of a 'network of last resort' being retained using TDM technology (64 kbps digital all the way through) to provide service for m2m calls until such a point as devices are replaced.
See also
*Next generation network
The next-generation network (NGN) is a body of key architectural changes in telecommunication core network, core and access networks. The general idea behind the NGN is that one network transports all information and services (voice, data, and al ...
References
External links
Rollout of fibre by Openreach
by Graeme Wearden, ZDNet.co.uk, 28 April 2005
BT transforms phone network
, BBC News, 9 June 2004
First call on new phone network
, BBC News, 28 November 2006
BT inks 21CN supplier contracts
, by Tim Richardson, in The Register, 22 December 2005
{{DEFAULTSORT:Bt 21cn
Telephony
Broadband
BT Group
History of telecommunications in the United Kingdom