Cisco Design Essentials: Enterprise Voice Over Data Design
1. PBX Integration with Data Networks
Objectives
Upon completion of this lesson, you will be able to:
Lesson Map
PBX Review
This topic will review the PBX and its place in the Voice over Data plan. At the end of this topic, you will be able to:
PBX Functions
PBX technology helps to control costs by allowing many phones in an office to share fewer outside lines. Without PBX, each phone would require its own outside line.
Support features include:
Additionally, PBX provides for detailed use records and sophisticated selection of route, thereby saving on toll costs.
Inside a PBX
A private branch exchange (PBX) cabinet is a metal housing designed to hold the electronic components that make the PBX work. Each cabinet contains one or more shelves or carriers with slots. Each shelf in the PBX accepts a certain number of circuit boards. There are different types of circuit boards within a PBX, including the following. Click each type in the graphic above to learn more about the that subject.
When two PBXs communicate over non-ISDN tie trunks, they can use a number of signaling types including E&M signaling.
Question
A major benefit of a PBX is its reliance on industry-wide standards for all its communication and network.
Correct
While there are some industry wide standards, most PBXs use vendor specific standards.
Wide-Area PBX Networking
This graphic illustrates typical legacy PBX-to-PBX connectivity that connects two PBXs in different local access and transport areas (LATAs).
The actual connections are from the PBX to the local exchange carrier (LEC), for example, the central office (CO) switch to a LEC. From there, the connection is made to an inter-exchange carrier's (IXC) point-of-presence (POP), then into the IXC network cloud. The connections at the other end follow the reverse path.
Trunks are generally cross-connected instead of switched at the LEC and IXC level. If they are configured to go into a LEC/IXC switch, they are configured as dedicated circuits.
PBX-LATA-IXC Connections
This is a simplified Diagram of PBX-LATA-IXC connections
Question
When connecting two PBXs in different LATAs, a typical connection goes from the PBX to the LEC, then to a LEC tandem switch, then to an IXC's POP, then to the IXC's cloud. From the IXC's cloud, it follows a path that is the reverse of that into the cloud.
Correct
This correctly describes the connection chain that occurs when two PBXs in different LATAs connect.
QoS Data Network
This graphic shows the data network that will be able to support PBX-to-PBX communications only if the quality of service (QOS) issues are handled.
Delay and throughput characteristics:
PBX Connectivity
This graphic shows a typical generic first step in migrating PBX-to-PBX connectivity to a Voice over Data network. At this point of design, the enterprise still has its legacy connections.
PBX Circuit Types
There are two PBX circuit types.
Analog
Digital
The PBX Stack
The PBX stack is a representation of the various layers of functionality supported by a PBX. The lists of items for each layer are illustrative and not complete.
Physical Line: Loop-Type Analog, T1, E1, n-wire, PRI, BRI
PBX Line Services: Tie Trunks, CO Trunks, DID Trunks, FX Trunks, RAN Trunks, Subcriber Lines
PBX Signaling: CCS, CAS, QSIG, Digital (PBX-to-PBX), E&M, Propietary Digital (PBX-Phone)
PBX Services: Hold, Fordward, Park, Conference, Building Page, Networking, Local Extensions.
PBX Supported Applications: IVR, ACD, Voice Mail, Msg on Hold, Call Center.
PBX Analog Signaling
There are two types of PBX circuits or signaling:analog and digital. This topic will look at analog signaling.
At the end of this topic, you will be able to:
Signaling - Types and Roles
There are two types of PBX signaling: analog and digital. The analog lines are associated with loop-starting signaling. Typically, digital lines are associated with out-of-band signaling.
Additionally, The three roles of telephony signaling are:
Supervisory : Monitors the status of a line or circuit to determine if it is busy, idle, or requesting service (flash-hook). Supervision is the term derived from the function of monitoring manual circuits on a switchboard. At that time, a switchboard lamp for the line conveyed signal. Now voltage levels, signaling tones, or bits (digital signaling) provide this function.
Addressing: The process of transmitting routing and destination signals over the network. Addressing signals include dial pulses, tone pulses, data pulses over loops, trunks, and signaling networks.
Call Progress: Signals that inform the user of the status of the call setup, for example busy tones.
Key Point
Types of Analog Signaling
There are three traditional types of analog signaling: E&M, station-side loop-start signaling (FXO interface side), and CO-side loop-start signaling. Stations connected to PBXs or central offices use the FXS interface, while CO switches use the FXO interface.
Consider the various interfaces available from the PBX when interconnecting Cisco components. Understand that the connection needs to be supervised. Supervision handles such things as call setup and teardown and monitors the actions of the connected entities.
Note
Analog is not the best approach for more than a few trunks. Today, it is best to connect PBXs via an ISDN PRI over a T1 circuit. The signaling used must support call supervision. Without supervision, if a caller hangs up the phone after receiving no answer, the called, remote phone will continue to ring.
Key Point
PBX Analog Signaling
FXS and FXO Interfaces
The above diagram reviews the FXS interface and FXO interface connections.
Key Point
E&M Interface
Like digital signaling, analog trunk circuits connect automated systems (such as a PBX) and the network (such as a central office or CO). The most common form of analog tie trunk is the E&M interface. Like a serial port, E&M has a DTE/DCE type of reference. In the telecom world, the trunking side is similar to the DCE, and is usually associated with CO functionality. The Cisco 3600 acts as this side of the interface.
The other side is referred to as the signaling side, like a DTE, and is usually a device such as a PBX. With analog signaling there are five distinct physical configurations for the signaling part of the interface (types 1 to 5), and two distinct flavors of audio interface (2-wire or 4-wire). Note that even though it may be called a 4-wire E&M circuit, it is likely to have 6 to 8 physical wires! The difference between a 2-wire and 4-wire circuit is whether the audio path is full-duplex on one or two pairs of wires.
What is a Trunk?
A trunk is a circuit which connects two pieces of a switching environment together. A trunk, DIDs excepted, is not assigned a publicly dialable directory number. A line is a circuit that connects a piece of the switching environment to a subscriber. A line always has a directory number assigned to it.
When a subscriber places a call to another subscriber on the same switch, the call is a line-to-line call. When a subscriber places a call to a subscriber on another switch, it is a line-to-trunk to trunk-to-line call.
What is a Line?
The terms line and trunk sometimes exist only in the eye of the beholder. Circuits that PBXs use to connect to a CO switch and to place or receive calls are viewed by the PBX as trunks. The same circuits may be considered, however, as "lines" to the CO switch. The same CO POTS circuit which has been assigned a directory number (for example, 573-555-1212) can be used equally well by a residential subscriber with a single line phone as it can by a PBX. The circuit is connected on the CO's line side, and on the PBX's trunk side. To the CO, the circuit is a line. To the PBX, the circuit is a trunk.
Question
Which of the following is not a traditional type of analog signaling?
Correct
E.164 does define the international numbering plan for ISDN
Answer Supervision
Answer supervision indicates that the called party has answered the phone. Answer supervision may take several forms depending on the type of trunk and so on as follows:
For non-PBX loop-start lines, the CO does not provide answer supervision to the originating end. For loop or ground-start lines terminating on PBXs, the CO may provide battery reversal answer supervision if the customer requests it.
Dual-tone, multifrequency (DTMF) is most commonly used for PBX connections. Multifrenquency (MF) can be used between the PBX and a CO for feature-group D trunks (in which case the PBX appears to the CO as an IXC switch) or for centralized automatic message accounting (CAMA) trunks. CAMA trunks were once used widely in the CO environment (and PBX hotel/motel environment) for customer billing. Today, their use is mostly limited to 911 connectivity, where it is frequently a requirement over ISDN.
PBX Digital Signaling
The last topic reviewed PBX analog signaling. This topic covers PBX digitial signaling.
At the end of this topic, you will be able to:
The User and the Network
There is a clear relationship between the user and the network.
A central office trunk (COT) is a both-way trunk that generally can be used to place outgoing calls from the PBX to the PSTN. Incoming calls are generally sent to the operator console or a predefined station. This is called direct In-line (DIL), private lines, or auto-terminate.
In a PBX environment, ground start is the preferred signaling type for both-way trunks. Ground start prevents a condition known as glare, collision, or head-on. This is where an incoming call arrives on a trunk at the same time the PBX is selecting the trunk for an outgoing call.
There are two primary digital signaling protocols: channel associated signaling (CAS) and common channel signaling (CCS). ISDN is a method of common channel signaling. Q Signaling (QSIG) is an internationally standardized implementation of ISDN that emphasizes feature functionality between different brands of PBXs. We will be covering these in depth later in this lesson.
Digital Trunk Signaling
This graphic is an overview of PBX-to-CO and PBX-to-PBX legacy connectivity. Typically, the connection is a T1 in North America, Japan, Taiwan, and South Korea; or an E1 in Europe and the rest of the world.
DASS#2
DASS#2, also know as ISDN 30-DASS#2 and ADS 2, is British Telecom's primary rate offering which does not conform to European standard I.421. ISDN 30 is the equivalent of up to 30 exchange lines (known as channels) delivered to the customers' premises and connected to an Integrated services private branch exchange (ISPBX) or for data applications to a PC or multiplexer. It is available from only eight channels upward and is therefore applicable to all industry sectors. ISDN 30 is provided normally over fiber cable, but can be provided over transverse screen, microwave, or copper twisted pairs. Each system is a 2-Mbps digital connection from the telephone exchange presented as an RJ-45 connector.
Replacement of Private Network
This graphic shows the replacement of the private network that was providing a virtual tie line between the two PBXs. Now, there is a Frame Relay/ATM network connecting the two PBXs. The connections to the PSTN are for off-net calls.
Typically the configuration relies on co-existence in the initial phases. Savings are incurred by the reduction of trunk lines to the CO. The connection between the PBX and the router would normally be E&M and the router will transport voice over the existing WAN protocol which may be Frame Relay or ATM.
Digital Signaling Options
A review of digital signaling options includes:
Key Point
Note
The use of proprietary protocols was provided for networking of services before international standards were agreed upon and implemented, therefore many enterprise networks use them as a legacy or in preference to the use of international standards.
Signaling Methods
In North America, the de facto standard for digital transmission is T1. T1 uses 24 time slots for a total speed of 1.544 Mbps (line rate, but 1.536-Mbps payload rate).
The two different methods of signaling that can be employed are:
CAS
In Channel Associated Signaling (CAS), the signaling information is conveyed within the voice channel. In every sixth frame a bit is stolen from the voice channel to signal information, such as on-hook or off-hook.
CCS
The second method is called Common Channel Signaling or CCS. In North America, a signaling channel is designated on the T1, and the signaling bits or protocol messages for all the other T1 channels are transmitted across the single CCS channel.
Channel Associated Signaling (CAS)
The new functionality of T1 CAS for Voice over IP (VoIP) includes all the T1 CAS and E1/R2 signaling already supported for the Cisco AS5300 in data applications, with the addition of dialed-number and calling-party number capture whenever available.
The implementation of this feature supports the following T1 CAS signaling systems for VoIP applications:
E&M
E&M Wink Start-Feature - Group B
E&M Wink Start-Feature - Group D
E&M Immediate Start
Ground Start/Station-Side Loop Start Line/FXS Interface
E&M
Typically used for trunks. It is normally the only way that a CO switch can provide two-way dialing with direct inward dialing. In all the E&M protocols, off-hook is indicated by A=B=1, and on-hook is indicated by A=B=0. If dial pulse dialing is used, the A and B bits are pulsed to indicate the addressing digits.
E&M Wink Start Feature - Group B
In the original wink start protocol, the terminating side responds to an off-hook from the originating side with a short wink (transition from on-hook to off-hook and back again). This wink tells the originating side that the terminating side is ready to receive addressing digits. After receiving addressing digits, the terminating side then goes off-hook for the duration of the call. The originating endpoint maintains off-hook for the duration of the call.
E&M Wink Start Feature - Group D
In Feature Group D, wink starts with the wink acknowledge protocol. The terminating side responds to an off-hook from the originating side with a short wink (transition from on-hook to off-hook and back again) just as in the original wink start. This wink tells the originating side that the terminating side is ready to receive addressing digits. After receiving addressing digits, the terminating side then provides another wink (called an acknowledgment wink) that tells the originating side that the terminating side has received the dialed digits. The terminating side then goes off-hook to indicate connection when the ultimate called endpoint has answered. The originating endpoint maintains off-hook for the duration of the call.
E&M Immediate Start
In the immediate start protocol, the originating side does not wait for a wink before sending addressing information. After receiving addressing digits, the terminating side then goes off-hook for the duration of the call. The originating endpoint maintains off-hook for the duration of the call.
Ground Start/Station-Side Loop Start Line/FXS Interface
Ground start signaling was developed to aid in resolving glare when two sides of the connection tried to go off-hook at the same time. This is a problem with loop start because the only way to indicate an incoming call from the network to the CPE using loop start was to ring the phone. The 6-second ring cycle left a lot of time for glare to occur. Ground start signaling eliminates this problem by providing an immediate seizure indication from the network to the CPE. This indication tells the CPE that a particular channel has an incoming call on it. Ground start is different than E&M in that the A and B bits do not track each other (that is, A is not necessarily equal to B). When the CO delivers a call, it seizes a channel (goes off-hook) by setting the A bit to 0. The central office equipment also simulates ringing by toggling the B bit. The terminating equipment goes off-hook when it is ready to answer the call. Digits are usually not delivered for incoming calls.
Key Point
Question
ISDN is a method of common channel signaling (CCS), which is one of two primary digital signaling protocols. Common associated signaling (CAS) is the other.
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False | |||
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True |
Incorrect
ISDN is a method of CAS not CCS. (OJO: Esto está mal ver: CCS más adelante).
Channelized T1 Robbed Bit Features
Internet service providers (ISPs) can provide switched 56-kbps access to their customers using the Cisco AS5300. The subset of T1 CAS (robbed bit) supported features include:
Supervisory: Line side
Supervisory: Trunk Side
Informational: Line-side
Informational: Trunk-side
Supervisory: Line side
Supervisory: Trunk Side
Informational: Line-side
Informational: Trunk-side
Common Channel Signaling (CCS) - T1
In North America, the standard for digital transmission is T1 which uses 24 time slots for a total speed of 1.544 Mbps. In North America CCS, a signaling channel is designated on the T1, and the signaling bits for all the other T1 channels are transmitted across the single CCS channel.
In Europe and in most parts of the world, the standard for digital transmission is E1 which uses 32 time slots for a total speed of 2.048 Mbps. The first time slot is used to maintain synchronization and pass control information, while the 16th time slot is used to pass signaling information.
Key Point
Common Channel Signaling (CCS) - E1
While E-1 uses time slot 16 for signaling, CCS uses slot 24 and CAS uses robbed bit signaling.
In CAS, there is a fixed relationship between the bits and the other time slots. For example, the first "x" number of bits are associated with time slot 1 and so on.
CCS eliminates the requirement for a fixed relationship. In CCS, signaling is passed in messages between processors that control the terminating switches. For example, a signaling message sent between two PBXs may indicate that time slots 2, 8, and 15 are off-hook.
ISDN Digital Signaling
nB+D PRI has the following features:
Key Point
QSIG Digital Signaling
In
this standard arrangement, each digital carrier (T1/E1) has an associated D
channel. The first 23 (or 30 for E1) channels of the carrier are used for clear
channel (64K) voice, while the 24th channel (T1) or 16th channel (E1) is used
for clear channel (64K) signaling data. The 23B + D configuration can connect
to any other device offering a standard ISDN interface.
QSIG maintains compatibility with existing analog (FXS, FXO, and E&M) interfaces, as well as T-1 (CAS) and non-Q.SIG PRI PBX system
QSIG Digital Signaling 2
QSIG
recognizes and manages the network characteristics from point-to-point leased
line to satellite and radio.
An important aspect of QSIG is its support of supplementary services defined by ITU-T in the Q.95x series. With full implementation of QSIG the widest range of supplementary services typically supported by PBXs within their proprietary envelope can now be supported across vendors and between the public and private networks.
Key Point
QSIG
QSIG Benefits
QSIG provides an extremely powerful method of connecting Private Integrated Services Network Exchange (PINX) equipment in a corporate network. The flexibility of QSIG is best demonstrated by reviewing the benefits that QSIG brings to the user. These benefits are numerous and are listed below:
Vendor independence
Guaranteed inter-operability
Free-form topology
Unlimited number of nodes
Flexible numbering plan
Public ISDN synergy
Supplementary services for corporate users
Feature transparency
Innovation
Multi-application domains
Vendor Independence
QSIG is not a proprietary standard. It is an open, international standard and the world's leading PBX suppliers support it. Customers can purchase equipment from more than one supplier.
Guaranteed Inter-operability
Twelve of the world's leading PBX manufacturers signed a memorandum of understanding concerning the development and support of QSIG. The MoU became effective in February 1994 and it commits the signatories to facilitate the performance of interoperability tests. The multivendor commitment to QSIG means that users can confidently operate corporate networks that comprise of PINX equipment supplied by a variety of suppliers.
Free-form Topology
The use of QSIG does not impose the use of a specific network topology and it can be used with any network configuration: meshed, star, main, satellite, etc. Existing networks can be upgraded to QSIG, regardless of their topology. New networks can be installed using the most effective and economical topology.
Unlimited Number of Nodes
There is no limitation on the number of nodes that can be used in a QSIG network. New nodes can be added to the network as business needs dictate.
Flexible Numbering Plan
QSIG does not impose any restriction on the numbering plan for the network. The network designer/communications manager is free to adopt the most suitable numbering plan.
Public ISDN Synergy
Supplementary Services for Corporate Users
In addition to the supplementary services supported by DSS1, QSIG provides additional business features specifically designed for corporate users. Such features include:
Feature Transparency
QSIG is an intelligent and powerful signaling system, providing great flexibility in terms of network architecture.
Innovation
Manufacturers supporting QSIG are still free to develop custom, innovative features for particular customers and/or markets. A special mechanism within QSIG, Generic Functional Procedures (QSIG GF), provides a standardized method of transporting nonstandard features.
Multi-Application Domain
QSIG provides messaging to support which of the following?
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