Privately Owned Switches

In a corporate environment, where large numbers of staff need access to each other and the outside, individual telephone lines are not economically viable. A PBX is a smaller, privately owned version of the CO switches used by telephone companies, as illustrated in Figure 1-4.


Most businesses have a PBX telephone system, a key telephone system, or a Centrex service. Large offices with more than 50 telephones or handsets choose a PBX to connect users, both in-house and to the PSTN.

PBXs come in a variety of sizes, from 20 to 20,000 stations. The selection of a PBX is important to most companies, because a PBX has a typical life span of seven to ten years.

All PBXs offer a standard, basic set of calling features. Optional software provides additional capabilities.

A PBX connects to telephone handsets using line cards and to the local exchange using trunk cards.

A PBX has three major components:

• Terminal interface The terminal interface provides the connection between terminals and PBX features that reside in the control complex. Terminals can include telephone handsets, trunks, and lines. Common PBX features include dial tone and ringing.

• Switching network The switching network provides the transmission path between two or more terminals in a conversation. For example, two telephones within an office communicate over the switching network.

• Control complex The control complex provides the logic, memory, and processing for call setup, call supervision, and call disconnection.

PBX Installations

PBX switches are installed in large business campuses to relieve the public telephone company switches from having to switch local calls. When you call a coworker locally in your office campus, the PBX switches the call locally instead of having to rely on the public CO switch. The existence of PBX switches also limits the number of trunks needed to connect to the telephone company's CO switch. With a PBX installed, not every office desktop telephone needs its own trunk to the CO switch. Rather, the trunks are shared among all users.

Small organizations and branch offices often use a key telephone system, as shown in Figure 1-5, because a PBX offers functionality and extra features that they may not require. A key system offers small businesses distributed answering from any telephone, unlike the central answering position required for a PBX. Notice in Figure 1-5 that telephones interconnect to a key system via connector blocks, while trunks coming in from the local exchange interconnect to the key system via termination blocks.


Today, key telephone systems are either analog or digital and are microprocessor based. Key systems are typically used in offices with 30 to 40 users, but can be scaled to support over 100 users.

A key system has three major components:

• Key service unit A key service unit (KSU) holds the system switching components, power supply, intercom, line and station cards, and the system logic.

• System software System software provides the operating system and calling-feature software.

• Telephones (instruments or handsets) Telephones allow the user to choose a free line and dial out, usually by pressing a button on the telephone.

Larger companies use proprietary telephone networks with PBXs. In a key telephone system, each telephone has multiple lines that allow users to access outside lines to their CO. When a call comes into the company, a line or a key lights up on the telephone and indicates that a particular line is in use. Users can call another extension or let another person know where to pick up a call by using an intercom function, such as an overhead paging system or speakerphone.

Key telephone system functionality has evolved over time to include a class called hybrid telephone systems. The hybrid system adds many features that were previously available only in PBXs. There is no single definition of the functions and features that are classified as a hybrid system because all vendors provide a mix that they believe gives them a competitive advantage.

The main difference between a key telephone system and a hybrid telephone system is whether a single-line telephone can access a single CO local loop or trunk only (key telephone system) or whether the single-line telephone can access a pool of CO local loops or trunks (hybrid telephone system).

Introduction to Voice Technologies

Voice over IP (VoIP) is experiencing explosive growth. Many corporate environments have migrated, are actively migrating, or are researching the process of migrating to VoIP. Some long-distance providers are using VoIP to carry voice traffic, particularly on international calls. Companies, such as Vonage, offer VoIP service as a replacement for traditional telephony service in the home.

Migration is a process that involves gradually phasing out old components and replacing them with new ones. Many terms have been used to describe the technologies and applications for transporting voice in a converged packet network environment. When designing a converged network, it is necessary to clearly define all requirements and understand the various options that are available.

An important first step in designing a converged network is to understand the traditional telephony network and how it interfaces with voice components. You must know, from the start, how legacy voice equipment is connected and its possible migration paths.

The next step toward a good design is being knowledgeable about the components available for VoIP networks. You should be aware of the difference between voice and data flows within the network and the tools for controlling voice calls. Network requirements vary according to the size of the location. Knowing the difference between campus, enterprise, and service provider environments is crucial for choosing the right components and technologies.

This chapter provides an overview of the basic telephony functions and devices, including private branch exchanges (PBXs), switching functions, call signaling, and multiplexing techniques. It also reviews the basic components of the VoIP network and identifies the different requirements in campus, enterprise, and service provider environments. Together, these concepts and techniques provide a solid introduction to the VoIP arena.


Fundamentals of Telephony Networks

In traditional telephony networks, many components and processes are transparent to the customer. As you move from traditional telephony networks to converged voice and data networks, you must manage new components and processes to ensure seamless end-to-end call handling. To maintain acceptable service levels, you need to understand which devices you must now support and the processes that are necessary to ensure end-to-end call functionality.

Basic Components of Telephony Networks

A number of components must be in place for an end-to-end call to succeed. These components are listed here and shown in Figure 1-1:

• Edge devices

• Local loops

• Private or central office (CO) switches

• Trunks

Edge Devices

The two types of edge devices used in a telephony network include:

• Analog telephones Analog telephones are most common in home, small office/home office (SOHO), and small business environments. A direct connection to the public switched telephone network (PSTN) is usually made by using analog telephones. Proprietary analog telephones are occasionally used in conjunction with a PBX. These telephones provide additional functions such as speakerphone, volume control, PBX message-waiting indicator, call on hold, and personalized ringing.

• Digital telephones Digital telephones contain hardware to convert analog voice into a digitized stream. Larger corporate environments with PBXs generally use digital telephones. Digital telephones are typically proprietary, meaning that they work with the PBX or key system of that vendor only.

Local Loops

A local loop is the interface to the telephone company network. Typically, it is a single pair of wires that carry a single conversation. A home or small business may have multiple local loops.


Private or CO Switches

The CO switch terminates the local loop and handles signaling, digit collection, call routing, call setup, and call teardown.

A PBX switch is a privately owned switch located at the customer site. A PBX typically interfaces with other components to provide additional services, such as voice mail.


Trunks

The primary function of a trunk is to provide the path between two switches. There are several common trunk types, as shown in Figure 1-2, including the following:

• Tie trunk A dedicated circuit that connects PBXs directly

• CO trunk A direct connection between a local CO and a PBX

• Interoffice trunk A circuit that connects two local telephone company COs

The telephone installed in your home is considered an edge device because it terminates the service provided by your local telephone company. The local loop is the pair of wires that come to your house and provide residential telephone service. Trunks are the interconnections between telephone switches. They can be between private switches or telephone company switches.

CO Switches and Switching Systems

Figure 1-3 shows a typical CO switch environment. The CO switch terminates the local loop and makes the initial call-routing decision.


The call-routing function forwards the call to one of the following:

• Another end-user telephone, if it is connected to the same CO

• Another CO switch

• A tandem switch (that is, an intermediary switch between the source and destination switch)

The CO switch makes the telephone work with the following components:

• Battery The battery is the source of power to both the circuit and the telephone. It determines the status of the circuit. When the handset is lifted to let current flow, the telephone company provides the source that powers the circuit and the telephone. Because the telephone company powers the telephone from the CO, electrical power outages should not affect the basic telephone, also known as a POTS (plain old telephone service) phone.

• Current detector The current detector monitors the status of a circuit by detecting whether it is open or closed. Table 1-1 describes current flow in a typical telephone.

• Dial-tone generator When the digit register is ready, the dial-tone generator produces a dial tone to acknowledge the request for service.

• Dial register The digit register receives the dialed digits.

• Ring generator When the switch detects a call for a specific subscriber, the ring generator alerts the called party by sending a ring signal to that subscriber.

Some telephones on the market offer additional features that require a supplementary power source that the subscriber supplies; for example, cordless telephones. Some cordless telephones may lose functionality during a power outage.

When configuring a PBX connection to a CO switch, the signaling should match that of the CO switch. This configuration ensures that the switch and the PBX can detect on hook, off hook, and dialed digits coming from either direction.

Switching systems provide three primary functions:

• Call setup, routing, and teardown

• Call supervision

• Customer ID and telephone numbers

CO switches switch calls between locally terminated telephones. If a call recipient is not locally connected, the CO switch decides where to send the call based on its own call routing information, which is stored in a call-routing table. The call then travels over a trunk to another CO or to an intermediate switch that may belong to an inter-exchange carrier (IXC). Although intermediate switches do not provide dial tone, they act as hubs to connect other switches and provide interswitch call routing.

PSTN calls are traditionally circuit-switched, which guarantees end-to-end path and resources. Therefore, as the PSTN sends a call from one switch to another, the same resource is associated with the call until the call is terminated.

CO switches provide local service to residential telephones. The CO switch provides dial tone, indicating that the switch is ready to receive digits. When you dial your phone, the CO switch receives the digits, then routes your call. The call routing may involve more than one switch as the call progresses through the network.

Cisco Wireless Router and Switch Services

While the preceding devices are the most common that organizations use in constructing and maintaining a WLAN, Cisco offers other devices that can help your organization provide a robust, feature-rich wireless solution.


Cisco 3200 Series Wireless and Mobile Routers

To connect mobile networks to a wireless network, Cisco offers its Cisco 3200 Series wireless and mobile routers, shown in Figure 1-19. Contained in rugged enclosures and offering 802.11g functionality, these small devices (they are about as wide and long as a pen) can fit in vehicles or in outdoor locales. They offer the capability to transfer voice, data, and video across mobile wireless networks.


These routers are targeted at public safety, homeland security, defense agencies, and transportation agencies that need a durable router in a compact design that can be installed in vehicles.


Cisco Catalyst 6500 Series Switches

The Cisco Catalyst 6500 Series switches are a popular line of Cisco switches. In addition to serving wired clients, these switches can also be upgraded with a WLAN Services Module (WLSM). The WLSM is a key component of the Cisco SWAN architecture and enables fast, secure WLAN roaming within and across IP subnets. It also enhances WLAN security and smoothes out WLAN deployment and subsequent management.


Cisco Wireless 7920 IP Phone

Convergence is bandied about in the world of technology. Think of convergence as a techie's Swiss Army knife. We have cellular telephones that can play video games and MP3s players that can take pictures. Who knows what else they will be able to do in the coming years. Cisco is no stranger to the world of convergence. In the realm of wireless networks, one of the more compelling and useful devices is the Cisco Wireless 7920 IP phone. This telephone, shown in Figure 1-20, looks like a cellular telephone; however, it connects via the WLAN infrastructure (through an Aironet AP, for instance) then to the organization's gateway to allow VoIP telephone calls.

The phone uses the 802.11b protocol and Cisco CallManager. The product is ideally suited for environments in which users need telephony, but are constantly on the move and cannot be pinned down to a hardwired telephone. For example, hospitals, warehouses, universities, and retailers are ideally suited for these telephones.


Cisco Compatible Extensions (CCX)

As wireless technology has exploded in popularity, Cisco has seen the necessity for providing a mechanism through which third-party vendors can ensure compatibility among products. As a result, Cisco developed the CCX program.

Through the CCX program, WLAN vendors licensefree of chargeWLAN technology from Cisco. After that technology is implemented into the vendor's product, it is tested at an independent, third-party lab. If the product passes the testing procedures, the vendor is allowed to add a Cisco-compatible logo with the product, indicating that it not only works with Cisco equipment, but also takes advantage of advanced features. Intel is an example of this program in action. The company earned Cisco-compatible status with its Centrino mobile technology. This has been integrated in a number of laptop computers, such as Dell, Hewlett Packard, and Toshiba, among others.

The CCX program has been rolled out in three iterations. The requirements of CCX Version 2 build on the requirements of Version 1. For example, Version 1 of CCX security demands:

• WEP
• IEEE 802.11 and 802.1X
• Wi-Fi compliance
• Windows Hardware Quality Labs (WHQL)

Version 2 also requires WPA compliance.

From connection for two wireless clients working in ad hoc mode to a hospital nurse connected via a wireless IP phone; from an enterprise connecting its clients and office buildings in a MAN to police cars equipped with wireless routers, Cisco has a number of devices that enable a plethora of wireless networking functionality.

Version 3 includes EAP-FAST, wireless multi-media, CCKM for EAP-FAST, and single sign on.