In the first installment of this three-part series examining the evolution of the line card, we looked at the technology's humble beginnings and its evolution, driven primarily by semiconductor IC advancements. Now we turn our attention to the pivotal role the line card is playing in the battle being waged for consumers' communications dollars. Today, as telcos and cable companies around the world compete for new subscribers, the line card is the critical platform on which new services are built. The next generation of line cards are designed and developed with the triple play of voice, data, and video in mind.
In Part 2 of this series then, we will discuss the ways in which the line card has evolved to work with broadband services such as ADSL and VDSL2, and the integration of voice compression schemes to the line card as part of the move from the conventional class 5 switch to the next generation soft-switch. Finally, we'll discuss what next-generation line cards will mean for service providers as well as subscribers.
But before we dive-in, we need to establish some context. The evolution of the line card has not occurred in a vacuum. Rather, it has evolved with the overall network. In order to fully understand the evolution of line card and its roles today, we must take a few minutes to describe the backhaul mechanisms and the network switches, which enable the connection from one end to another, as well as the communications physical infrastructure, including the components enabling connections between one subscriber to another.
Although all networks were created using analog voice connections, today most network switches use digital circuits between exchanges, with the traditional two-wire circuit connecting two telephones. The basic digital circuit in the public switched telephone network (PSTN) is a 64 kbits/s channel originally designed by Bell Labs, called Digital Signal 0(DS0). To carry a typical phone call from one location to another, the analog voice is digitized at 8kHz sample rate (two times 4kHz voice frequency based on Nyquist sampling frequency) using 8-bit pulse code modulation (PCM). The call is switched using a signaling protocol SS7. DS0s are also known as timeslots because they are multiplexed together using time-division multiplexing (TDM). Multiple DS0s are multiplexed together to form a DS1 signal, carrying 24 DS0s on a North American or Japanese T1 line, or 32 DS0s (30 for calls plus 2 for framing and signaling) on an E1 line, which are used in most other countries.
As the line card and networks have evolved over the years, this digitizing and multiplexing is now performed at the edge of the network (closer to the end user). The collections of multiplexed timeslots - also known as the access network - use a synchronous optical transmission or SONET and SDH technology. Access networks rely on standards established by organizations such as the European Telecommunications Standard Institute (ETSI), and in North America, The American National Standards Institute (ANSI).
All the backhaul connections are made through the network switch. In the United States, a Class 5 switch refers to the exchange located at the central office directly servicing subscribers. The main features of the Class 5 switch, in addition to the basic dial tone, include the calling features. Categories, or classes, of switches were created by AT&T many years ago, with only Class 4 and 5 still existing as separate categories today.
In all, the categories created by AT&T included:
- Class 1: International gateways, handling all traffic from outside the US. This category is also referred to as a Regional Center.
- Class 2: Exchanges created to interconnect whole regions of the US networks. This category is also referred to as a Sectional Center.
- Class 3: This class was created for exchanges within an exchange, connecting a group within a specific population area. This category is also referred to as the Primary Center.
- Class 4: This category was created to connect various parts of a city or town. This class is also referred to as the Toll Center.
- Class 5: The Class 5 switch is the exchange in which subscribers and end telephone lines are connected. This category is also referred to as the Local Exchange. The fundamental key feature of the class 5 switch that is distinguishable from other categories is that the class 5 switch provides dial tone to the subscriber. This is a very distinguishing feature because providing the ringing signal as well as the dial tone are among the few telephony requirements which still require the need for high voltage analog technology. Zarlink is one of few companies today providing such analog chipsets.
As mentioned, today only the term Class 4 and Class 5 are used. Class 1, 2, and 3 exchanges have been consolidated in the Class 4 exchange. And this consolidation is continuing as increasingly, Class 4 and Class 5 exchanges are combined.
Connecting the various telecommunication networks, including the public switch telephone network (PSTN), is the media gateway. Media gateways enable multimedia communications across next generation networks, over multiple transport protocols, such as ATM and IP. Because the media gateway can connect different types of networks, one of its main functions is to convert between transmission and coding mechanisms. Additionally, most key functions, including media streaming, echo cancellation, dual tone multi frequency (DTMF) and tone generation are located in the media gateway.
There are various protocols used in communications between media gateways and call agents, including Media Gateway Control Protocol (MGCP), also known as H.248 or Megaco, and Session Initiation Protocol (SIP). SIP has been the protocol of choice in recent years. It is the signaling protocol most widely used for setting up and tearing down multimedia communication sessions, such as voice and video calls over the internet. In voice over Internet Protocol (VoIP) applications, media gateways use SIP to perform the conversion between TDM and voice over internet protocol. SIP is also at the heart of the IMS (IP Multimedia Subsystem).
IMS is an architectural framework for delivering internet protocol (IP) multimedia to mobile users. It was originally designed by the wireless standards body 3rd Generation Partnership Project (3GPP), and is part of the vision for evolving mobile networks beyond GSM. Its original formulation (3GPP R5) represented an approach to delivering "Internet services" over GPRS. This vision was later updated by 3GPP, 3GPP2 and TISPAN by requiring support of networks other than GPRS, such as Wireless LAN, CDMA2000 and fixed line. For the evolution of the line card to be completed, IMS must be established at the foundation of the next generation network. IMS also lends itself to the end goal which is FMC (fixed mobile convergence), a clear trend emerging in the form of fixed and mobile telephony convergence.
The aim is to provide both services with a single phone, which could switch between networks ad-hoc. While we will not discuss FMC here, it is worth noting that the evolution of line card is a stepping stone to reach FMC, which is the Holy Grail for the service providers to connect wireless and wire line networks through the same backhaul infrastructure.
Today in the next generation networks and multi-service access networks (MSANs) nearly all of the media gateway functionality has been integrated in the PSTN line card.
MSANs are used to connect the customer's telephone line to the core of the network. MSANs provide telephony, broadband (DSL) services from a single platform and typically backhaul all the data stream over IP or Asynchronous Transfer Mode (ATM). MSANs are the next step in the evolution of line card, rolling-up all the functionality found in the previous generations of line cards.
The need to bring higher-speed broadband to the home has forced service providers to deploy remote boxes to neighborhoods. These cabinets, with their space constraints, require multiple communications-enabling functions to be consolidated on a single platform. A typical outdoor MSAN cabinet consists of narrowBand (POTS), broadband (xDSL) services, batteries with rectifiers, an optical transmission unit and copper distribution frame. The functions that were once performed by the media gateway and a media controller are now achieved by a single multi-core DSP-micro controller. Thanks to the standardization of IP, several companies have been able to cost effectively develop line cards witch such advanced capabilities. Companies such as Mindspeed, TI, Freescale, and Broadcom have established themselves as leaders in this industry, with standard off-the-shelve products that can not only reduce the end product cost, but also significantly reduce the time to market, and lower development cost.
In the US, an example of this type of deployment closer to the customer is the U-verse offering from AT&T, which uses IP to offer television service, Internet access and voice telephone service. The new services are carried on phone lines (or over fiber) to the customer's premises, and are enabled by AT&T's initiative to push fiber-optic lines closer to customers' premises. Inside the customer's home the new services are carried over Ethernet or a HomePNA home network.
Unlike traditional offerings from U.S. cable companies, with telecom carriers, such as AT&T, video is delivered over IP from the head-end to the consumer's set-top box. Broadcast channels are distributed via IP multicast, allowing a single stream (channel) to be sent to any number of recipients. U-verse uses H.264 (MPEG-4) encoding, which compresses video better than MPEG-2, the standard used in traditional media, including DVD. Better compression means simply that less bandwidth is required for a given level of video quality.
In this model, the set-top box does not have a conventional tuner, but is an IP multicast client, which joins the IP multicast group corresponding to the stream ("channel") desired, so that only that stream is sent over the customer's connection. This contrasts greatly with conventional cable television service, in which all channels (streams) are sent to all customers over very-high-bandwidth connections, regardless of which channel(s) a customer is viewing or recording at any given time, causing customer lines to carry mostly unused data, and allowing each channel to use only a small portion of the capacity of a customer's connection. By contrast, in the IP multicast model, only the streams the customer uses are sent, so the customer's connection need not have the capacity to carry all available channels simultaneously. While an improvement, even with this method of communication the system is limited to the bandwidth of the copper wire (in the case of the VDSL deployment). To overcome this, an alternate approach, fiber to the home, was developed.
Fiber-optic communication is a method of transmitting information from one place to another by sending light through an optical fiber. The light forms an electromagnetic carrier wave that is modulated to carry information. First developed in the 1970s, fiber-optic communication systems have revolutionized the telecommunications industry and played a major role in the advent of the Information Age. Because of its advantages over electrical transmission, the use of optical fiber has largely replaced copper wire communications in core networks in the developed world.