Second-generation, digital, wireless systems in .NET Create Code 128A in .NET Second-generation, digital, wireless systems

Second-generation, digital, wireless systems using vs .net tocompose code 128 code set c in web,windows application iPhone Measure Order ACK Channel Quality Measurement Stop Measurement Order ACK Handoff Order ACK Figure 8.34 Mobile-assist ANSI/AIM Code 128 for .NET ed handoff, IS-136.

handoff. We then move on to location management and paging, concluding with a simple consideration of tradeoffs involved in dealing with these two con icting functions. (Fewer boundary crossings by individual mobile terminals are incurred as the location area size is increased, reducing the number of registrations, but more paging messages must be broadcast to nd a given mobile.

) Consider a communicating mobile traveling within a given cell. As it reaches the boundary (generally ill-de ned) of that cell, the power received from the cell base station with which the mobile has been in communication across the air interface between them will drop below a pre-de ned threshold. In contrast, assume the power received from a neighboring base station exceeds a threshold.

A decision to handoff to the neighboring base station and enter the new cell associated with that base station would then be made by the MSC controlling both base stations. Second-generation wireless systems support mobileassisted handoffs (MAHO) in which power measurements are carried out by a mobile under the command of the MSC and base station, with the results of the measurements transmitted to the base station as uplink signaling messages. We focus on IS-136 as a speci c example.

GSM uses a similar handoff strategy. Figure 8.34 diagrams the messages transmitted between the MSC, base station, and mobile as mobile-assisted handoff is carried out.

The MSC noti es the base station that channel quality measurements are to be carried out. The base station responds by transmitting to the mobile a Measurement Order message identifying neighboring forward traf c channels, as well as the forward traf c channel over which it is currently receiving messages from the base station, for which channel quality is to be measured. Channel quality measurements consist of received signal strength measurements for the current and neighboring traf c channels, and bit error rate measurements for the current traf c channel.

The results of these measurements are reported back to the base station by the mobile, when they are completed, in a Channel Quality Measurement message carried on the SACCH (see Section 8.2). The base station, in turn, forwards the measurement results to the MSC, which then issues a stop measurements command, sent on to the mobile by the base station.

Mobile Wireless Communications as a Stop Measurement Ord er message. If, on analysis of the measurements, handoff is deemed necessary, the MSC so orders, with the base station then signaling the mobile to which new channel to tune. (Figure 8.

34 also indicates acknowledgement messages tranmitted uplink by the mobile, acknowledging correct receipt of the corresponding command messages.) Handoff of a mobile to a new base station thus results in the immediate need to allocate to the mobile a channel within the new cell. Should a channel not be available, the ongoing call would have to be dropped.

The dropping probability of a handoff is clearly the same as that of blocking a newly generated call if new calls and handoffs are treated alike in the allocation of channels. The probability of blocking a call was discussed in 3 (see equation (3.7)).

It is generally agreed that mobile system users nd the dropping of ongoing calls much more onerous than receiving a busy signal on attempting to initiate a call, i.e., having a new call attempt blocked.

After all, it is distinctly unpleasant to be cut off in the middle of a conversation! A number of proposals have thus been made to reduce the probability of dropping a call. These include, among others, giving handoff calls priority over new call attempts or queueing handoff calls for a brief time while waiting for a channel to become available. With a xed number of channels available, giving priority to handoff calls obviously increases the probability of blocking new calls.

There is thus a tradeoff to be considered. We shall return to some of these proposals, comparing them where possible, in the next chapter, in studying call admission control quantitatively. Mention was made in 6 that the CDMA-based IS-95 uses soft handoff rather than the hard handoff we have been implicitly assuming to this point.

It was noted there that soft handoff results in an improvement in signal-to-interference ratio, and hence an increase in system capacity. In the soft handoff procedure, a mobile makes a connection to two or more base stations before choosing the one with which to communicate. It is thus an example of a make before break operation.

Hard handoff implies the connection with the current base station is broken before connecting to the new one. IS-95 systems, in carrying out handoffs, use mobile-assisted handoffs, as do IS-136 and GSM. But, unlike the handoff procedure adopted for these other systems, in IS-95 it is the mobile that initiates the handoff procedure.

It is the mobile that determines handoff may be necessary based on measurements it carries out. It is left to the MSC, however, to actually make the decision to hand off. A handoff is completed when the MSC commands the mobile, by sending messages to the base stations involved, to drop the old base station connection and continue with the new base station connection.

But connections to the old and new base stations are maintained until the decision to connect to the new base station only is made. This is the essence of soft handoff. We now move on to inter-system handoffs.

This type of handoff is encountered when a mobile roams, moving from an area controlled by one MSC to that controlled by another. A number of possible handoff scenarios have been de ned by IS-41, the North American inter-system mobile communications standard. In Fig.

8.35 we diagram the inter-system procedure in the simplest possible case, that of a mobile m moving from one region controlled by an MSC, labeled MSC-A here, to a region under the control of MSC-B. This procedure is called handoff-forward.

As the mobile moves through region A, as shown in Fig. 8.35(a), it eventually comes into an overlap region between A and B.

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