Convergence—Technical Advances Leading to Improvements in IP Networks

The definition of convergence varies throughout the telecommunications industry. For purposes of this chapter, convergence is the capability of one public network to carry all types of traffic—voice, data and video—as packets. These networks either use Internet protocol (IP)-based routers or asynchronous transfer mode (ATM) switches, which send information in fixed-sized packets called cells. (See Chapter 6 for ATM.) Internet backbone networks are generally based on IP, a protocol used for routing packets in the Internet and in private networks. Wholesale carriers' networks that carry a mix of voice, data and video tend to be based on ATM- and IP-based routers.

Technical advances are improving the quality of voice and video carried on packet networks. They also are lowering the cost of building high-capacity networks capable of carrying voice along with data over backbone networks. The vast majority of voice traffic is still carried in circuit switched networks. According to the article, “VOIP—Still Only a Drop in the Bucket,” published in Business Communications Review, Voice 2001, February 2001, page 78, only 1% of traffic in the United States is carried as voice over IP (VOIP).

Packet networks are those that use ATM equipment and IP-based routers. IP is not a connection-oriented protocol. A path is not guaranteed for the entire length of the voice, data or video transmission. If capacity is not available for some of the variable-length packets, they are dropped or retransmitted on another route. This is fine for data such as email, but dropped packets can result in choppy voice or video, called “clipping.” Thus, any technology that increases the capacity of networks improves its quality of service and decreases dropped cells or packets resulting from congestion.

The technical advances include:

• Improvements in routers

• Faster digital signal processors (DSPs)

• Dense wavelength division multiplexing (DWDM)

• High-capacity optical switches

• Lower cost, programmable switches, called softswitches

• Protocols that improve the quality of voice and video over packet networks

Improvements in Routers

Faster routers enhance the quality of voice and video carried on Internet Protocol networks. In traditional networks, carriers use both routers and asynchronous transfer mode (ATM) switches for their data traffic. ATM switches carry voice, data, video and modem traffic. Routers often take the customer data from the “edge” of the network and send it to ATM switches located in the backbone or core of the network.

The ATM switches provide circuit-like services. They have the capability to establish a path through the network and reserve capacity for high-priority services such as voice and video. They guarantee a particular level of service, referred to as quality of service (QoS).

Older routers keep lists of addresses of devices on the network in tables in the router's memory. When packets are transmitted through the network, each router looks up the destination address in the router's table. These lookups result in delays. In high-speed terabit routers, addresses are stored in the silicon on cards in the router. They use multi-protocol label switching (MPLS), which speeds up routers.

When ATM switches and routers work together in a network, the ATM switch, if it uses quality of service, must be able to read the quality of service bits in packets sent by routers. This is called mapping bits from the router to the ATM switch.

Routers:

• Directly connect with customers' IP data without converting the data to another format

• Carry voice and video without degrading the quality of either

• Use the ATM portion of the system to read the IP header and know what priority to give it

Some new pure IP networks are eliminating ATM equipment and just using high-speed routers.

Routers in Cellular Networks for IP Backbone Traffic

Many cellular providers are planning to upgrade their backbone networks by installing routers at the edge of their networks. The edge of carriers' networks is the point where traffic from mobile central offices is aggregated before being carried between central offices and to other cellular carriers. The routers will carry cellular voice and mobile data traffic in cellular networks that have been upgraded for high-speed data service.

Digital Signal Processors (DSPs)

Digital signal processors (DSPs) (see Figure 5.18) are special purpose microprocessors on pieces of silicon that execute instructions. These powerful processors are good at performing a small number of repetitive tasks such as compressing voice, packetizing voice and converting analog voice into digital. They also compensate for delay and bit losses in networks. They are used in voice mail, wireless telephone handsets and for handling voice in data networks. DSP vendors include Texas Instruments, Motorola, Lucent Microelectronics and Analog Devices.

Transporting voice over packet networks requires extremely fast processing to replicate the quality in the public switched network. In data communications, email can be delayed a few seconds without a problem. Choppy sound, pieces of conversation received out of sequence, clipped conversations and long silences generally are unacceptable for television and voice. Voice compression done on DSPs has improved voice quality in IP networks.

Voice Compression

Voice compression (see Chapter 1) is a key capability of DSPs. Compression uses mathematical algorithms to make voice smaller. Voice is sent through circuit switched networks at 64,000 bits per second. Digital signal processors, using mathematical algorithms, are capable of compressing voice, making it smaller, so that it can be sent intelligibly at lower speeds of from 8,000 to 12,000 bips per second. Thus, the capacity needed to transmit voice over packet networks is lowered. (In packet networks, headers are added to packetized voice, increasing the 8000 bits per second to 12,000 bits per second.)

Digital signal processors have become so fast that they compress voice and video and digitize and packetize it so that voice and video are acceptable over IP networks.

Once the voice or video signal leaves the DSP, delays in the network can impair voice quality. For example, congestion can cause pieces of the voice conversation to be dropped. Slow routers or switches can lead to delays and choppy voice conversations.

Higher Capacity Networks—Optical Technologies

Optical technologies have had a tremendous impact on capacities in the core, backbone sections of long-haul networks. The backbone is the “interstate highway” portion of the network used for routes that carry the highest percentage of traffic. They are starting to have an impact on metropolitan areas as well.

• Dense wavelength division multiplexing (DWDM) is a fiber optic multiplexing technique. New DWDM equipment is capable of carrying 160 channels of data over a single pair of fibers. Each channel has a speed of OC-192 (10 gigabits). The single pair thus carries 1.6 terabits (a trillion bits) per second (10 x 160). (See Chapter 2.)

  Digital Signal Processing Impact on Modems, Fax Cards and IP Brough Turner, one of the founders and now Chief Technology Officer at NMS Communications (formerly called Natural Microsystems), has been at NMS since 1980. Brough has seen the impact that DSPs have had on modems, voice mail, fax cards and voice over IP. In 1980, analog circuitry limited modems to a top speed of 1200 bits per second. Modems converted digital signals to analog and analog signals back to digital using analog circuitry. Once digital signal processors were used, they digitally encoded signals using mathematical algorithms. The digital algorithms were performed consistently and with increasing speeds. Thus, modems increased in speed from 1200 bits per second to 14,400, 19,000, 28,000, 33,600 and eventually 56,000 bits per second. Digital signal processing systems made by NMS Communications and other vendors digitize speech and convert analog fax signals to digital bits, at real-time speeds in voice mail, speech recognition, videoconferencing, fax modems, and fax cards within voice mail systems. DSPs are installed on chips in routers and switches within the public network. They also are present in wireless handsets, routers and enterprise-based telephone systems. In addition to digitizing signals, they also perform the digital-to-analog conversions.

  
  

• Optical switches, previously discussed, increase the capacity and routing flexibility of backbone networks by switching thousands of light waves simultaneously.

• Passive optical networking (see above) is bringing the capacity of fiber closer to end-user customers, in the last mile access network.

Softswitches—Programmable Switches

Softswitches are central office switches built on standard computer platforms for sending voice over packet networks. Softswitches are made using standard protocols so that they can easily interface with network-based applications such as unified messaging and billing systems. They also interface with SS7 services for sending traffic to proprietary central office switches. Because softswitch functions are built-in layers, they are low in cost to modify and upgrade. Each layer can be changed without changing the other layers. The following are the three layers of softswitches:

• Service selection— Interfaces with databases in the network to determine which carrier to send the call to and which features such as 800-number translation to apply. This layer determines what to do with the call.

• Call control— Sets up and tears down calls and interfaces with tones in the network. This layer interfaces with protocols used in packet networks.

• Transport, signaling and processing calls— Announcements are played, voice is packetized, and calls are transported.

Because they are built on standard platforms, programmable switches cost less than traditional central office switches. Moreover, switches made on these platforms are easier to interface with network-based applications such as unified messaging and network-based systems for call centers when these systems are also based on compatible platforms. Vendors of softswitches include Cisco Systems, Sonus Networks, Telcordia Technologies, and Tellabs. Companies such as Lucent and Nortel sell the largest share of proprietary central office switches in the United States. Siemens and Alcatel have a large international presence. Manufacturers of proprietary switches are in various stages of development for next generation softswitches.

Calls coming into an IP network from the circuit switched network are converted into packets by digital signal processors housed in the programmable switch. The call is then transferred to a router, where it is transmitted through the network in a packet format. In this scenario, the voice travels “free” along with the data.

Softswitches for End Offices vs. Toll Offices—The Core vs. the Edge

Most softswitches currently are installed in long-haul backbone networks rather than as replacements for end-office switches that connect directly to end users. End-offices are part of the last mile, local access network. Last mile networks have the following complexities not found in toll offices:

• Playing announcements such as “All circuits are busy”

• Setting up the call

• Supplying dial tone

• Providing 411 directory and 911 emergency notification services

Edge networks also have more of the following enhanced services to which switches interface:

• Calling card services

• Debit cards

• Voice messaging

• Voice-activated dialing

• Fax services

• International callback

• Three-way calling

• Follow-me calling (calls reaching people wherever they are)

While the vast majority of central office switches installed are proprietary in nature, softswitches with the capability to handle complex features are starting to be deployed mainly by competitive local exchange carriers in local access, or last mile edge networks.

Backbone networks do not have the complex features found in local access networks. Incumbent telephone companies that have been slower to adopt softswitches, have started to issue requests for proposals for softswitches as toll office replacements in their backbone networks. Toll switches do not have connections to end users. They transmit traffic in the backbone portion of incumbent telephone company networks. Telephone companies are purchasing fewer proprietary central office switches. New developments planned for softswitches are the addition of ports compatible with DSL and cable modem service. These will be tested in labs by various manufacturers by year-end 2001.

Softswitches for Internet Access PBX Trunks

Some carriers are starting to use softswitches to offload heavy Internet dialup traffic and PBX trunks from end central offices. Rather than buy a new central office switch when they need more capacity, they purchase a softswitch to handle dialup Internet traffic and PBX trunks. The Internet traffic is sent directly to the Internet via packet networks without tying up circuit switched central office ports.

The Quality of Service Issue for Voice over IP

While quality for voice in packet networks is improving, it still is not as consistent as that in the public switched network. In addition to capacity to ensure quality, protocols used by routers and switches that attach priority to packets carrying voice and video are critical. The following (plus MPLS discussed in the section on optical switches) are key protocols used for sending mixed media messages in packets networks:

• Session Initiation Protocol (SIP) is used between end points for negotiating features of the transmission.

• Realtime Transport Protocol (RTP) is used to identify packet content.

• Differentiated Services (Diffserve) uses bits in packet headers to identify and prioritize traffic.

• H.323 is a suite of protocols for setting up and sending calls over IP networks.

SS7 in Packet Networks

Carriers such as Qwest Communications, Williams Communications and Level 3 are building networks based on the Internet Protocol and ATM rather than on circuit switching. They feel that packet networks are more suitable for the preponderance of data traffic, which is growing at a faster rate than voice traffic. IP-based networks use SS7 signaling information for billing and advanced features.

Gateways translate and deliver SS7 between signaling system networks and IP switches. Gateways also translate signaling messages between the public switched network's central office circuit switches and IP networks. Note that in Figure 5.19, two signaling gateway systems are pictured. These are mated pairs for redundancy. If one signaling gateway fails, the other one takes over. The links between all systems also are redundant.

International Long Distance and Real-Time Fax over IP Carriers such as iBasis, Inc. and IDT sell wholesale international voice and real-time fax long distance to domestic and international carriers and Internet service providers. They offer savings on international long distance and real-time fax services. They have switches in worldwide locations such as New York City, Los Angeles, Singapore, China, Korea, Taipei, the Middle East and Latin America. Carriers that use iBasis route calls to the iBasis switches, or points of presence (POPs). At these POPs, a Cisco System programmable switch aggregates the traffic from multiple carriers and sends it to a Cisco router that acts as a gateway. See Figure 5.20. Some of iBasis's carrier customers are: Telestra, WorldCom, KDD, NTT, Teleglobe, Communications Authority of Thailand and China Unicom. The gateway translates circuit switched traffic to packets. Digital signal processors (DSPs) in the routers packetize and compress the voice and fax traffic. The calls are then routed over backbone Internet facilities rented from carriers such as AT&T, Cable & Wireless and WorldCom Group. At the other end of the call, the gateways reverse the process sending the calls back to the circuit switched network. Figure 5.20. Wholesale provision of IP telephony for international calls. Courtesy of iBasis, Inc iBasis monitors its traffic from a control center in Burlington, Massachusetts. The network management system measures the time it takes packets to travel “round trip,” for example, from New York to Singapore and back to New York. If delays (latency) exceed preset parameters, traffic is routed over the public switched network instead of over the Internet backbone. Thus, iBasis eliminates delays (latency) that can impair voice and fax traffic carried over IP. iBasis calls this rerouting Assured Quality Routing.