Hz A.F. digital Available test points on radio Digital (no a.f.) Acoustic Acoustic Need for co-ordination of test and measurement functions For convenience only Essential for successful measurements Signalling: limited, typical accuracy 10ms to 1s Signalling: critical, accuracy 1 us The radio path or Physical Layer (Layer 1) 61/25 OSI Model GSM equivalent Application User tasks (speech) Presentation Session Network tasks Transport Call management Network Mobility management Radio resource management Concatenation/segmentation network Data link tasks Acknowledgement mode Error detection & correction Physical Channel coding Modulation Figure 61.22 The OSI model as applied to GSM Speech Speech User data User data OSI layer 3 Signalling network-functions Signalling Build.frames, request OSI layer 2 Reconstruct acknowledgement frames- data link functions acknowledge Channel coding, error protection, Error correction interleaving deinterleaving Equalization OSI layer 1 modulation physical layer demodulation Transmitter Receiver Figure 61.23 Symbolic representation of GSM transmit and receive functions based on the OSI layer model 61/26 Telecommunication system measurements ventional signal generators are unsuitable for this because there is no direct way of controlling the phase of the carrier. Instead, I/Q Table 61.13 A signal generator for in band GSM modulators will provide this function (Figure 61.24). measurements Parameter Value R.F. carrier frequency range 890 960MHz Frequency resolution < 200kHz modulated Frequency switching speed < 577us (1 time slot) to within r.f. carrier < 4° of final phase Frequency uncertainty 50x10 9 Output level range -115 to 0dBm Level uncertainty < 0.5dB Amplitude response Pulse modulation with rise and fall times of - 20us Figure 61.24 I-Q (vector) modulator Modulation Gaussian minimum shift keying (GMSK) Modulation data rate As far as the magnitude of the frequency deviation and the 270kbit/s stability of the centre frequency (for f.m. d.c. mode) are concerned, Spectral purity, sign