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Glosario de TCS

BCCH - Broadcast Control CHannel: Broadcast, difusión en español, es un modo de transmisiónde información donde un nodo emisor envía información a una multitud de nodos receptoresde manera simultánea, sin necesidad de reproducir la misma transmisión nodo por nodo.

• GSM: The Global System for Mobile communications (GSM), es un standardinalambrico de telecomunicaciones para serivicios digitales de telefonia celular.

• GPRS

The General Packet Radio Service (GPRS) is a technology for the support of packet switching traffic ina GSM network. GPRS enables high-speed wireless Internet and other data communications in GSM. Thedata speed of GPRS is more than four times greater speed than conventional GSM systems. Using a packetdata service, subscribers are always connected and always on line so services will be easy and quick toaccess.

In GPRS, the GSM time slots are dynamically allocated to various users according to their instantaneousdemand. Users can alternately transmit data in the same slot. All data is packetized and sendinstantaneously through available resources. This gives the user the experience of being permanently

linked to the network.

Data rates

In GSM the maximum data rate is 9.6 kbps per time slot. In GPRS the data is packetized which gives inprinciple an even lower data rate of 9.05 kbps of which 8 kbps is available for the user. However, in GPRSthere are two technologies introduced to increase this data rate. Firstly, the error correction that is usedcan be adapted to the quality of the radio channel. Secondly, it is possible to use more than one time slot.In theory all 8 time slots can be used.

In GPRS there are four different channel codes introduced. These are given in the table below.

CodingScheme Grossbit rate(kbps)

Userbit rate(kbps)

CS-1 9.05 8

CS-2 13.4 12

CS-3 15.6 14.4

CS-4 21.4 20

In theory GPRS will allow a maximum of 160 kbps netto. In practice, typical values are currently slightlyover 40 kbit/s.

New network elements

GPRS requires the introducton of two new network elements in the GSM network:

• Serving GPRS Support Node (SGSN),

• Gateway GPRS Support Node (GGSN).

The SGSN handles all packet switched data within the network and is respnosible for the authenticationand tracking of the users. The SGSN performs the same functions as the MSC for voice traffic. The SGSNand the MSC are often co-located.

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The GGSN is the interface from the GSM/GPRS network to external networks. The GGSN is also responsiblefor the allocation of IP-addresses.

Mobile terminal classes

In GPRS there are three different classes of mobile terminals defined: Class A: Capable of GPRS and circuitswitched services (voice) simultaneously; Class B: Can operate on only GPRS services or voice service at a

time. However it is capable of listening to incoming calls of both GPRS and voice; Claas C: Only capable of GPRS or circuit switched services. The terminal is either capable of only voice or GPRS or it can only listento one of these services when it is on.

• 2G: 2nd Generation system for mobile communications. The 2nd generation is based ondigital technology. 2G systems offer increased voice quality and capacity over 1G systems.Historically, 2G systems provided voice and a limited data capacity of 9.6 to 14.4 Kbps.

The 2G systems are regional standardized offering the possibility of regional (not worldwide)roaming. The terminals are smaller, have a longer standby time, and are cheaper. 2G systems

have made mobile communications a commodity.

The most important 2G standards are:

1. GSM (Global System for Mobile communications),2. D-AMPS (Digital AMPS)/IS-136,3. PDC (Personal Digital Cellular) and4. cdmaOne/IS-95.

2G systems are circuit switched. Later enhancements made higher speed packet switched datapossible. These possibilities, like GPRS for GSM, are often referred to as 2.5G.

• 2.5G: 2,5th Generation system for mobile communications. 2.5G refers to technology

that made 2G systems capable of packet-data services and data rates that range from 20 to 40Kbps, such as GPRS in GSM and the data capabilities in cdmaOne. The capabilities are not highenough to meet the requirements for 3G systems.

• 3G: 3G is short for 3rd Generation system for mobile communications. The 3rdgenerationconsists of a family of standards under the framework of  IMT-2000. Under thisframework, a number of standards is developed. The European version is knownas UMTS. The main other standards areCDMA2000 and Mobile WiMax.

The third generation is typified by the convergence of voice and data with mobile Internetaccess, multimedia applications and high data transmission rates. The 3rd generation mustmake worldwide roaming possible.

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Tx and Rx Levels

The Tx level is the power in decibels per milliwatt (dBm) at which a modem transmits its signal.The Rx level is the power in dBm of the received signal. The server modems normally transmit at-13 dBm by default. Ideally, the Rx level should be in the range of -18 to -25 dBm. If the Rxlevel is under -25 dBm, the Signal-to-Noise Ratio (SNR) is likely to decrease, meaning that the

speed also decreases. If the Rx level is too high, you may see signal distortion or the receiver'sDigital Signal Processor (DSP) being overdriven, and erratic connections are possible.

In some modulation standards, such as V.34, a receiver can tell its peer that the signal level istoo high and the transmitter then reduces the level at which it transmits. (If this behavior iswidespread, try configuring the transmitter to transmit at a lower level.) Modems that use othermodulation standards (such as K56 Flex) may not be able to do this, resulting in problems.

Therefore, an effective Rx level is a function of the peer's initial Tx level, the negotiated dBmreduction (if any), and the attenuation in the voice circuit. The voice circuit attenuation is, inturn, a function of link attenuation and of analog or digital pads, which are telephone companycircuitry designed to insert attenuation into the voice circuits.

If you need to reduce or increase your Tx level, this is attainable with the following modems andmodulation standards:

• Microcom through T51???For details, refer to the AT Command Set and Register Summaryfor V.34, 56K, and V.90 12-Port Module.

• Modem ISDN Channel Aggregation (MICA) through S39 or S59???For details, refer to the ATCommand Set and Register Summary for Cisco MICA Six-Port Modules.

• NextPort through S39 or S59???For details, refer to the NextPort AT Commands and SRegisters Reference.

If you need to reduce or increase your Rx level, you need to do this either at the peer transmitter(although this is not feasible if there are thousands of peers) or within the telephone company

(more likely), by increasing or decreasing the padding.

On a live connection, you can see or infer the Rx and Tx levels as follows:

• Microcom modems???Initiate a reverse telnet session and issue the AT@E1 command.

• MICA modems???Issue the show modem operational-status command.

• NextPort modems???Issue the show port operational-status command.

Some MICA modem examples are as follows:

router# show modem operational-status 1/0 Parameter #8 Connected Standard: V.34+Parameter #20 TX,RX Xmit Level Reduction: 0, 0 dBmParameter #22 Receive Level: -22 dBm

In this case the Rx level is -22, which is fine. The peer has not requested that the modemattenuate its Tx, so you can infer that it is transmitting at the default output level of -13 dBm.You can also infer that the signal level is not too high for the peer's receiver, because the peerhas not requested a reduction in signal strength (though it could still possibly be too high???youcannot be certain without directly interrogating the peer).

Another example is as follows:

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router# show modem operational-status 2/14 Parameter #8 Connected Standard: V.34Parameter #20 TX,RX Xmit Level Reduction: 0, 3 dBmParameter #22 Receive Level: -19 dBm

In this case there is a good Rx level of -19, but the peer has asked this modem to reduce its Tx

level by 3 dBm. Therefore, it starts to transmit at -16 dBm instead. This modem's signal is arrivingwith excessive strength at the peer. If this occurrence is widespread, you might want to cut backon your configured Tx level globally through S39. In this case, the problem appears to be an issuewith this particular peer, so there is no need to do so.

You can also check the output of the show modem operational-status command for otherpotential issues and fixes with theOutput Interpreter (registered customers only) tool.

Padding

Telephone companies can insert a digital or analog pad, which is circuitry designed to addattenuation on a per-channel basis. Padding ensures that end-to-end circuits that take variouspaths through the Public Switched Telephone Network (PSTN) end up with comparable signal

levels. For instance, if a modem transmits at -13 dBm, the receivers see a signal at the rightlevel.

For purely analog carriers (V.34 and earlier standards), pads are useful if they result in thedesired levels being received. If the Rx levels being observed are too high on a widespread basis,then pad insertion can make analog carriers perform better.

However, the effect of pads on a digital (Pulse Code Modulation (PCM)) carrier (K56 Flex andV.90) can be problematic. An analog pad (line pad), which merely attenuates the signal, is not aproblem for a PCM carrier. However, a pad in the Network Access Server's (NAS's) T1 line totrunk, or within the telephone company's trunk-to-trunk connection, can have implications forPCM connects.

Digital pads remap the PCM data, which can disrupt communication. The general rule is thatzero-dB digital pads are optimal for PCM connects. However, zero-level padding is less thanoptimal in other cases; for example, K56 Flex modems are less tolerant of Rx levels that are toohigh.

Different kinds of PCM modems can adapt to different flavors of digital pads. Rockwell K56 Flexmodems (as well as Microcom and MICA modems) can handle zero-, three-, or six-dB pads. Lucentmodems have a finer granularity of pad handling, and can cope with one-, four-, five-, and seven-dB pads as well. V.90 modems can handle zero to seven dB of padding in one-dB increments. If you see good V.34 connections, but poor or no K56 Flex connections, and if you know that thereis no extra A-to-D conversion in the circuit path, then you may have a digital padding issue. Inthat case you need to contact your telephone company to resolve the problem. In such a case it

may be helpful to conduct circuit traces of the suboptimal connections.