T-Carriers


T- Carriers

Digitalization of voice is the breaking point of the transition from analog to digital carriers. PCM voice made this possible without a large investment and having to run new cables and trunks to double the available capacity over the same copper pairs.

NOTE

Don't forget: The N-carrier of an analog system runs 12 voice channels over the existing copper pairs, and TDM runs 24 channels.


The initial enormous expense for codecs and DSU/CSUs is now offset by relatively inexpensive facility changes and as a result, an increased trunking capacity of the carriers. As previously mentioned, replacing the N-carriers with T-carriers doubles the capacity over the same two pairs, and 24 simultaneous voice calls can be handled from the CO. The digital trunks led to the production of digital switches, and replaced the analog switchboards with its related personnel and problems. The remaining component of the transition process is the last mile, also referred as a local loop. The twisted copper pairs to local residences and businesses make up the vast majority of the existing wires. Copper pairs prevail over digital, fiber, or any other contemporary solution, which creates an opportunity for the emergence of xDSL technologies.

T1/E1 and Primary Rate Interfaces, T1s, and DS

Usage of T1s is still a growing LEC and inter-exchange carrier (IXC) service. The T1 signal can be transmitted one mile before requiring a repeater, which regenerates the signals, recovers the timing, and sends the regenerated version of the coding sequence.

A T1 signal is referred to as a DSX-1 interface (digital signal crossconnection point for DS-1 signals), which is capable of sending/receiving the T1 signal up to 655 feet. The maximum distance between the CSU and the last network repeater should not exceed 3000 feet. Network repeaters are installed every 6000 feet within the T1 carrier network to compensate for signal loss. The signal loss, called attenuation of the signal , requires that a CSU be set to automatically adjust attenuation values depending on the received transmission level. To adjust power levels, Cisco routers support interface configuration commands called cablelength short and cablelength long , with short-haul being the 655 ft run (max of 1310 ft) and long-haul being the more typical CO to CPE run of 6000 ft (repeated every 3000 ft).

T1 carriers support 24 full-duplex voice channels that only use two pairs of unshielded twisted pair (UTP). The control signals are generated within the CPE. T1 circuits can be terminated at the premises in a number of ways including DS3, SONET, or high-data-rate DSL (HDSL), among others. Typically, the local provider installs some type of network interface unit (NIU) or card to deliver the T1 to the CPE. This device could be a smartjack, M13 multiplexor, or HDSL device, and it usually terminates the circuit through a punch-down . The T1s are terminated by using two twisted-pair circuits. CSUs, and possibly DSUs, are required to connect the CPE to the DS1 service.

CSU functions include conditioning and equalization, error control, and the ability to test local and loopback circuits. For a phone company, line conditioning and equalization is the spacing and operation of amplifiers, so the gain provided by the amplifiers for each transmission frequency compensates for the line signal loss at the same frequency. DSUs provide transmit and receive control logic, synchronization, and timing recovery across the T1 and the other digital circuits (when these signals are not implemented in the CPE). A DSU also converts ordinary binary signals that are generated by the CPE to special bipolar signals. These signals are designed specifically to facilitate transmission at 1.544-Mbps rates over UTP cable, which was a media originally intended for 3-kHz voice band signals (see Figure 3-3). A typical DSU has a legacy serial interface (such as a RS232, or V.35, and so on) and a T1 interface.

Figure 3-3. How Data Service Unit/Channel Service Unit (DSU/CSU) Works


The DSU and CSU may or may not be part of the CPE. On the left and right side of Figure 3-3, you can see the same devices, but on the left side, they are separated. Usually, Cisco routers have DSU/CSU devices built-in and external devices are not necessary. Cisco routers that contain a built-in CSU can attach directly to this termination point through external cabling. If the router does not contain a built-in CSU, an external CSU must be purchased to connect to the carrier network.

T1s and DS and TDM Hierarchy

T1 frames consist of 24 8-bit words plus a framing bit. Each time slot of the frame contains 8 bits of binary information and is called DS0, which is sampled 8000 times per second. Because each DS0 contains 64 kbps (8 k samples/sec x 8 bits/sample) of user information and 24 DS0s are in a T1 frame, this 1544-kbps signal is commonly referred to as DS1. (DS-1 refers to digital signal level 1, which is a framing specification.) T1 refers to the digital transmission system that happens to operate at DS1 rates. Unlike DS1, T1 specifically includes physical transport definitions and line-coding schemes, and as with every other technology, there is a T-carrier hierarchy (see Table 3-1). A T3 carries 28 T1s, yielding 672 channels over one coaxial cable, and can be considered an analog E-carrier.

Table 3-1. TDM Hierarchy

Digital Signal Level n

North America

Japan

Europe

DS-1

1.544 Mbps24 channels

1.544 Mbps24 channels

2.048 Mbps30 channels

DS-2

6.312 Mbps96 channels

6.312 Mbps96 channels

8.448 Mbps120 channels

DS-3

44.736 Mbps672 channels

32.064 Mbps480 channels

34.368 Mbps480 channels

DS-4

274.176 Mbps4032 channels

97.728 Mbps1440 channels

139.264 Mbps1820 channels

DS-5

Not available

400.352 Mbps5760 channels

560.000 Mbps7680 channels


E1

As you can see from Table 3-1, Europe uses another format at the DS-1 level. This format is in compliance with the European Postal and Telecommunications administration, known as the Computer Emergency Response Team (CERT). In the frame format known as E1, CERT defines 32 time slots that are multiplexed in a way to yield a frame of 256 bits in 125 microseconds. One of the time slots is used for signaling, another one is used for alignment and synchronization, and the remaining 30 time slots are used for data, yielding a user data rate of 1.920 Mbps, and an overall data rate of 2.048 Mbps.

NOTE

A key standard for managing digital access lines is RFC 1406 "Definitions of Managed Objects for DS1 and ES1 Interface Types." It defines a set of Management Information Bases (MIBs) and can be used for Transmission Control Protocol/Internet Protocol (TCP/IP) Simple Network Management Protocol (SNMP). The MIBs track the error conditions, performance defects and parameters, and failure states. When a carrier provides proactive monitoring of circuits for enterprises , they basically implement MIBs for reporting.


Network Signaling Systems and SS7

Although totally hidden from the end user, the carrier's signaling systems are similar to a central nervous system in the human body. Signaling systems provide essential information between switches to exchange link status, connection control (signaling), and routing information. Robbed bit signaling (RBS), which was typical in the early 1970s, was replaced with out-of-band signaling, which eventually became a de facto standard for the exchange of signaling information.

NOTE

Out-of-band signaling uses frequencies outside of the normal frequency band for signaling, and it is the core of SS7. In contrast, in-band signaling relies on using certain bits out of each frame in the frequency band, which is why it is called RBS.


Common channel signaling (CCS) was another type of signaling that historically dominated the way carriers exchanged supervisory , addressing, and call information. One of the benefits of the new system (SS7) was that it reduced the time for call setup from between 15 to 20 seconds, to 1 to 3 seconds. In 1976, the system was called Common Channel Interoffice System No. 6, as it was based on signaling system No. 6 (SS6) from the International Telecommunication Union Telecommunication Standardization Sector (ITU-T). It offered a wide range of services for end users, such as callback and 800 (INWATS) capabilities. Because SS6 was not flexible enough for the emerging ISDN standards and services of the mid 1970s, a new standard called SS7 was developed.

The Q.700 recommendations from ITU-T and ANSI's T1.110-Series standards define the SS7 specification. The architectural model is composed of four layers . The lower three levels are part of the message transfer part (MTP), and provide reliable connectionless routing for user data through the network. MTP includes the following:

  • Signaling data link, which corresponds to Open System Interconnection (OSI) Layer 1

  • Signaling link, which corresponds to OSI Layer 2

  • Signaling network

MTP does not provide all the functions of the first three layers of OSI, especially when it comes to connection-oriented services. In 1984, a module called the Signaling Connection Control Part (SCCP) was added and today these four layers are called the Network Service Part (NSP), and perform functions typical of the first three layers of the OSI model. An important element of the model is the ISDN User Part (ISUP), which was developed to provide control signaling for ISDN and related subscriber lines, calls and functions, and which covers layers from SCCP all the way to the top of the OSI model. The higher layers of the SS7 architecture include the transaction capabilities applications part (TCAP), for transaction-oriented as opposed to connection-oriented applications and functions. Also, operations, administration, management, and provisioning (OAM&P) and a set of application service elements (ASEs) defined the remaining layers and defined support for other applications.

In SS7, the control messages are routed through the network to perform call management, such as call setup, call maintenance, and call termination. These messages are standardized blocks of data that are routed throughout the network, and consequently the circuit-switched networks are running packet-switched messages, which transform legacy functions to a new set of features.

SS7 defines control and information planes of operation, where the control plane is responsible for the call setup and for managing the connection, and the information plane is responsible for phases after the call setup, to route additional control information between communicating parties. The latter deals with local exchanges and transit centers, where the core of the control plane signals elements of the network.

From a troubleshooting point of view, the signaling network elements are the most important parts , and include the following:

  • Signaling point (SP) The remote user connects to the SP, obeying the rules of User-Network Interface (UNI) and signaling conventions such as Q.931. SP is capable of handling the control messages.

  • Signal transfer point (STP) A SP connects to one or more STPs. This is an element that is capable of routing signal messages that are received from one signaling link to another signaling link.

  • Signaling links This is a data link that connects SPs.

When a connection is requested by a remote user, it occurs over the D channel, uses Q.931 messaging, and is between the user and the LE. For this purpose, the LE acts as a SP. The Q.931 message is converted to SS7 and the user-requested action establishes and maintains the connection. This process can involve one or more signaling points and STPs. After the connection is set up, the information flows from one end to another, under the control of the information plane.




Troubleshooting Remote Access Networks CCIE Professional Development
Troubleshooting Remote Access Networks (CCIE Professional Development)
ISBN: 1587050765
EAN: 2147483647
Year: 2002
Pages: 235

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