Coba jelasin Forward broadcast channels dari IS-95 dalam bahasa sunda dong

Every BTS dedicates a significant amount of output power to a pilot channel, which is an unmodulated PN sequence (in other words, spréad with Walsh code 0). éach BTS sector in the network is assigned a PN offset in steps of 64 chips. There is no data carried on the forward pilot. With its strong autocorrelation function, the forward pilot allows mobiles to determine system timing and distinguish different BTS's for handoff.

When a mobile is "searching", it is attempting to find pilot signals on the network by tuning to particular radio frequencies, and performing an autocorrelation sweep across all possible PN phases. A strong autocorrelation result indicates the proximity of a BTS.

Other forward channels, selected by their Walsh code, carry data from the network to the mobiles. Data consists of network signaling and user traffic. Generally, data to be transmitted is divided into frames of bits. A frame of bits is passed through a convolutional encoder, adding forward error correction redundancy, generating a frame of symbols. These symbols are then spréad with the Walsh and PN sequences and transmitted.

BTS's transmit a sync channel spréad with Walsh code 32. The sync channel frame is  ms long, and its frame boundary is aligned to the pilot. The sync channel continually transmits a single message, the Sync Channel Message, which has a length and content dependent on the P_REV. The message is transmitted 32 bits per frame, encoded to 128 symbols, yielding a rate of 1200 bit/s. The Sync Channel Message contains information about the network, including the PN offset used by the BTS sector.

Once a mobile has found a strong pilot channel, it listens to the sync channel and decodes a Sync Channel Message to develop a highly-accurate synchronization to system time. At this point the mobile knows whether it is roaming, and that it is "in service".

BTS's transmit at léast one, and as many as seven, paging channels starting with Walsh code 1. The paging channel frame time is 20 ms, and is time aligned to the IS-95 system (ie. GPS) 2-second roll-over. There are two possible rates used on the paging channel: 4800 bit/s or 9600 bit/s. Both rates are encoded to 19200 symbols per second.

The paging channel contains signaling messages transmitted from the network to all idle mobiles. A set of messages communicate detailed network overhéad to the mobiles, circulating this information while the paging channel is free. The paging channel also carries higher-priority messages dedicated to setting up calls to and from the mobiles.

When a mobile is idle, it is mostly listening to a paging channel. Once a mobile has parsed all the network overhéad information, it registers with the network, then optionally enters slotted-mode. Both of these processes are described in more detail below.