add some random thoughts on hopping and the oml interface

1 files changed, 54 insertions, 0 deletions

diff --git a/openbsc/doc/gsm-hopping.txt b/openbsc/doc/gsm-hopping.txt
new file mode 100644
index 000000000..591093999
--- /dev/null
+++ b/openbsc/doc/gsm-hopping.txt
@@ -0,0 +1,54 @@
+according to GSM 05.02:
+
+general parameters from CCCH:
+* CA cell allocation of ARFCN's (System Information / BCCH)
+* FN: TDMA frame number (t1,t2,t3') in SCH
+
+specific parameters from channel assignment:
+* MA: mobile allocation, defines set of ARFCN's, up to 64
+* MAIO: index
+* HSN: hopping sequence generator number (0..64)
+
+
+hopping sequence generation (6.2.3):
+
+u_int8_t rntable[114] = {
+	 48,  98,  63,   1,  36,  95,  78, 102,  94,  73,
+	  0,  64,  25,  81,  76,  59, 124,  23, 104, 100,
+	101,  47, 118,  85,  18,  56,  96,  86,  54,   2,
+	 80,  34, 127,  13,   6,  89,  57, 103,  12,  74,
+	 55, 111,  75,  38, 109,  71, 112,  29,  11,  88,
+	 87,  19,   3,  68, 110,  26,  33,  31,   8,  45,
+	 82,  58,  40, 107,  32,   5, 106,  92,  62,  67,
+	 77, 108, 122,  37,  60,  66, 121,  42,  51, 126,
+	117, 114,   4,  90,  43,  52,  53, 113, 120,  72,
+	 16,  49,   7,  79, 119,  61,  22,  84,   9,  97,
+	125,  99,  17, 123
+};
+
+/* mai=0 represents lowest ARFCN in the MA */
+
+
+u_int8_t hopping_mai(u_int8_t hsn, u_int32_t fn, u_int8_t maio,
+		     u_int8_t t1, u_int8_t t2, u_int8_t t3_)
+{
+	u_int8_t mai;
+
+	if (hsn == 0) /* cyclic hopping */
+		mai = (fn + maio) % n;
+	else {
+		u_int32_t m, m_, t_, s;
+
+		m = t2 + rntable[(hsn xor (t1 % 64)) + t3];
+		m_ = m % (2^NBIN);
+		t_ = t3 % (2^NBIN);
+		if (m_ < n then)
+			s = m_;
+		else
+			s = (m_ + t_) % n;
+		mai = (s + maio) % n;
+	}
+
+	return mai;
+}
+