Telephone transmission lines can be divided into access lines (local loops) and interconnection lines (trunks). Local loops refer to all two-wire voice-grade connections between a residence or office and the operator's serving end-office. Interconnection trunks refer to high capacity groups of circuits connecting switching sites such as end-offices or other switching centres.
Most telecom customers are served by copper cable (twisted pair or coaxial) terminated by local telephone companies in a telephone network interface box called a network termination. To reduce the number of copper pairs, telephone systems use a hybrid transmission system to allow both transmission and reception on a single pair of copper wires. By combining both transmit and receive audio signals using a special hybrid combiner, only a single pair of wires is required to operate a standard home telephone.
For voice transmission, telephone systems restrict the audio frequency range for voice-grade circuits from 300 Hz to 3300 Hz, as information transferred in a voice conversation occurs at frequencies below 3300 Hz and above 300 Hz. Using a restricted frequency range reduces the transmission line and system switching performance requirements. The limiting of the audio frequency range is accomplished through the use of devices known as band-pass filters. Band-pass filters strongly attenuate signal frequencies above and below specific frequencies.
It is possible to send digital information over a hybrid network through the use of a modem (modulator/demodulator). The modem converts digital signals into analog tones that can be transmitted over standard telephone lines.
For data transmission, a relatively large set of data is organised into a frame or block, with one or more synchronisation bits or bit patterns used to identify the beginning and end of a logical block of data. T1 transmission, for example, is synchronised through framing bits that occur at the beginning of each frame. E-1 transmission is synchronised through the use of a separate time slot zero (0). Synchronous modems coordinate the receiving terminal on the rate of transmission of the data from the sending terminal. Synchronous data protocols such as synchronous data link control and high-level data link control use a specific bit pattern to form synchronising characters that are integral to each frame. By receiving the synchronising bits or characters, a receiving device can match its speed of data receipt with the rate of data transmission across the circuit. Thereby, each bit of data and control information can be distinguished at the physical layer. Synchronous transmission is more efficient than asynchronous transmission, as only a few framing bits and synchronising bits surround a large block of data. Over the past decade, there has been an explosive growth in network traffic leading to the transition of networks from voice-centric to data-centric and a dramatic increase in the demand for more bandwidth in transmission networks.
Transmission media
There are different types of transmission media: copper cable (including unshielded twisted-pair [UTP] and shielded twistedpair [STP]) and coaxial cable, wireless media like microwave and satellite and optic fibre.
Fibre optics
Transmission in the telecommunications networks of today is becoming more and more digital and the need for broadband access has resulted in optic fibre cable increasingly becoming the transmission medium of choice. Optic fibre cables have the capacity to transmit all forms of communication (voice, data and video). Currently, the synchronous optical network (SONET) and synchronous digital hierarchy (SDH) technologies are being used for synchronous data transmission over optic fibre networks.
An optic fibre connection is faster than wireless by many orders of magnitude. For instance, a single optic fibre can carry about 3 trillion bits per second (bps) while the fastest wireless service (fixed wireless access) approaches only about 2 million bps. Advances in technology such as wavelength division multiplexing, have made it possible to send multiple signals on a single fibre, leading to upgrades to terabits per second.
While the infrastructure cost of projects utilising optic fibre could be very high, material and installation costs for optic fibre have come down substantially over the past few years due to the industry's R&D efforts. New developments, such as the availability of mechanical splices, save on the costs of expensive toolsthat were needed earlier. Connectors found at the end of fibre cables are also available in field-mountable versions now. As a result of these developments, optic fibre has become a less complex and a more affordable solution.
KDDI R&D Labs and the National Institute of Information and Communications Technology (NICT), Japan, have recently completed a trial using a new optical technology that implements orthogonal frequency division multiplexing (OFDM) in the transmission frequency to achieve higher data rates.
According to a Nikkei report, the trial was able to achieve data rates of up to 30 terabits per second (tbps) over a distance of 240 km. KDDI is likely to commercialise the technology as early as 2012. Initial uses for the technology would include telemedicine and other high bandwidth applications. OFDM is employed as a modulation format to overcome dispersion impairments in high speed optic fibre transmission systems, as the technology improves the frequency utilisation efficiency of optical transmission systems.
In October this year, a team of scientists from Monash University, USA, developed an optic fibre technology which also utilises OFDM and allows transmission of up to ten times the current volume of data over existing cables. The innovation, known as optical OFDM (oOFDM), will boost the capacity of broadband networks and give better download time. The technology applies asymmetric digital subscriber line principles – already used to expand data transfer capacity over traditional copper and wireless broadband – to optic fibre cables, potentially increasing their data capacity.
According to the team leader Professor Arthur Lowery, the appeal of oOFDM is that it offers an inexpensive means of dramatically increasing long-haul capacity from the current transmission rate of 10 Gigabits per second (Gbps) to more than 100 Gbps over new and existing optic fibre.
Other transmission media
Other transmission media include copper cables, which includes unshielded twistedpair (UTP), shielded twisted-pair (STP), and coaxial cable, and wireless media like microwave and satellite. While copper and coaxial cables are primarily used for low to moderate frequency transmission over a few miles, microwave transmission, in which information is transmitted over a microwave link, is used in locations that cannot be cost effectively served by wires. Mobile and broadband connectivity, digital television and Wi-Max are some key market segments driving the wireless transmission sector today.
Microwave solutions for high capacity applications are particularly important with the introduction of universal mobile telecommunications system (UMTS) and high speed downlink packet access (HSDPA) in mobile networks; improved television networks with the launch of digital video broadcasting – terrestrial (DVbT) and digital video broadcasting – handheld (DVB-H); and the popularity of broadband networks. In fact, microwave access based on point-to-point microwave radios is the dominant technology in base station access networks and offers the fastest means for network rollout and capacity expansion.
Competition in the microwave market has led to a drop in prices making this technology even more attractive for both greenfield ventures and existing operators. Wireless backbones for communication carriers in emerging countries and for mission-critical applications in mature markets still represent a significant driver for growth.
The role of transmission systems in wireless telecommunications is to provide a mechanism for transmitting and receiving the microwave radio signals that carry telecom traffic. The core of the system is the transceiver radio itself. Other components include antennas and network management hardware and software. Microwave radio transmission is used across the network infrastructure: in the backbone network over long distances, in the backhaul network for transport of traffic to and from base stations, and in the access network to and from base stations and terminals.
In a microwave network, the thumb rule is that the utilisation factor (which determines how much capacity of a certain transmission link has been used and the growth margin reserved for future expansion) should not be more than 70-80 per cent, leaving the remaining 20-30 per cent for capacity upgrades. However, in the case of many microwave links connected in tandem or operator-specific requirements, this rule may be modified.
Moreover, to accommodate a rise in network traffic, the total number of links in a chain or ring of nodes should enable easy upgradation of the transmission links within the chosen topology without a major rerouting of transmission paths. For a small, low capacity network, a plesiochronous digital hierarchy (PDH) solution is sufficient while an SDH network is preferred for networks of higher capacity.
Recent technology developments
SDH and SONET have been the trusted workhorses of telecommunication networks for more than a decade, and with good reason. SDH/SONET provides network operators with a highly reliable transport medium, giving them complete control of traffic resources and extensive monitoring facilities that enable fast protection switching and performance management. With more than $400 billion invested in such networks globally, SDH/SONET is also a resource that carriers are keen to reuse as much as possible.
Nevertheless, it is widely accepted that SDH/SONET will need to be replaced. These protocols are primarily designed for circuit switched voice networks and while SDH/SONET networks have been adapted to transport packets effectively, and are indeed the dominant transport mechanism today, migration to a fully packet-based network is imminent and will provide the best solution in terms of scalability and costs, as well as statistical multiplexing advantages – in other words, overprovisioning of connections.
In addition, the proliferation of 3G networks with increased capacity requirements globally and the onset of 4G trials have led to the development of more competitive technologies such as packet transport network (PTN) and reconfigurable optical add-drop multiplexer (ROADM) which may prove to be potential replacements for SDH/SONET.
Packet transport network
Connection-oriented packet transporttechnologies include transport MPLS (tMPLS) and provider backbone transport/provider backbone bridge-transport engineering (PBT/PBB-TE). While the industry initially intended to implement IP/MPLS directly over Ethernet without the need for a transport network such as SDH/SONET, most operators were concerned that using IP/MPLS routers on such a large scale could lead to higher complexity and cost. MPLS, invented as an inserted layer between Ethernet (Layer 2) and IP (Layer 3) to lighten the load on IP core processor chips, brings connection-oriented capabilities to packet networks and both IP and MPLS are wellestablished technologies in core networks.
Troubleshooting in IP/MPLS networks in particular is a problem because the existing standards do not have monitoring, fault location and other management functions. This is a source of concern for carriers with large SONET/SDH network investments who are keen to minimise the disruption that the replacement of SONET/SDH might entail.
As a result, in most carrier organisations today, IP/MPLS networks are managed and operated by a group that is distinct from the transport group that manages and operates SDH/SONET. This is where connection-oriented packet transport technologies like T-MPLS and PBT have a role to play.
T-MPLS is based on a subset of MPLS features with added operation, administration and maintenance (OAM) functionality to allow automatic protection switching within the required 50 milli second (ms) time-frame. Connections are set up deterministically, using either a management system or a suitable control plane. IP-specific functionality in MPLS is removed, making OAM implementation simpler. tMPLS is thus a pure transport profile of existing MPLS standards but with the addition of OAM and automatic protection switching features.
The other contender for connectionoriented packet transport is based on Ethernet and is known as PBB-TE. PBbTE adds OAM and automatic protection switching functionality to Ethernet to mimic the capabilities of SONET/SDH networks, but uses a PBB as a starting point. PBB introduces a new frame format to make Ethernet more scalable and robust.
PBB-TE also uses a management system or suitable control plane to set up point-topoint connections deterministically.
Thus, T-MPLS and PBB-TE offer similar functionality: end-to-end bidirectional connections, connections established deterministically using a management system or control plane; end-to-end monitoring using OAM; and automatic protection switching with sub-50 ms response. As a result, T-MPLS and PBbTE's features and operation resemble the SONET/SDH network they intend to replace, which will enable the migration to packet-based networks.
ROADM
ROADMs, used in systems that employ wavelength division multiplexing, can add, block, pass or redirect modulated infrared and visible light beams of various wavelengths in an optic fibre network. While this technology has been in the news for some time, it has moved into the limelight only recently as operators have started offering triple-play services.
In a conventional ROADM, switching is accomplished without optical-to-electrical or electrical-to-optical conversion using three operations called add, drop and cut-through. An outgoing IR or visible beam can be generated (the add operation) or an incoming beam terminated (the drop operation). A beam can also be passed through the device without modification (the cut-through operation). In combination, these functions allow optical signal routing of considerable complexity.
The configuration of the system can be changed remotely. Two major ROADM technologies are currently in use. They are called wavelength blocking (WB) and planar lightwave circuit (PLC). However, neither of these facilitates true optical branching, in which beams of any wavelength can be directly routed to any desired port without the need to perform multiple intermediate operations. Optical branching capability is important in the deployment of efficient, reliable, high volume optical networks, which can provide services such as videoon-demand. An evolving technology called enhanced ROADM (eROADM) makes true optical branching possible.
In legacy dense wavelength division multiplexing (DWDM) networks without ROADM technology, frequent traffic disruptions and truck rolls are needed to address capacity upgrade and traffic churn.
ROADM solutions alleviate this opex through software-configurable wavelength add/drop functionality. In the highly competitive metro environment, ROADM gives the service provider a competitive edge by enabling swift response to customer requests. Many service providers, however, believe that there is a trade-off between the flexibility attained from ROADMs and the multiplexers' initial cost. Therefore, any ROADM deployment must have a good return on investment (RoI): low opex, low first cost and low total cost of ownership.
The right time to deploy ROADMs is when an optical metro or regional network is expected to transport converged services (video, data and voice). Today, every service provider strives to offer such triple-play services. The competitive nature of this market and the newness of the services imply tremendous growth and traffic churn in the metro/regional environment. Therefore, ROADMs should not be considered an option; they are now a requirement for networks with dynamic traffic patterns.
Clearly, as bandwidth demands continue to rise, new transmission technologies will continue to emerge. As carriers weigh their different requirements, while certain technologies will become the poster boys for the industry, others may not find favour and still others will find their place in the network. This segment clearly represents a very dynamic industry that will continuously come up with new developments.