Handover in GSM

Measurements

GSM is mobile-assisted: the network could never measure what the mobile hears, so the mobile measures for it.

In each 26-multiframe the mobile has an idle frame ([GSM Logical Channels and the Frame Hierarchy]) in which it retunes to a neighbouring BCCH carrier and measures it. Over several multiframes it builds a picture of up to six neighbours, identified by their base station identity codes, and reports their levels, along with the level and quality of its own serving cell, on the SACCH, whose frame comes round every 120 ms, giving a report about twice a second.

The BTS measures the uplink at the same time: level and quality of what it receives, and the timing advance, which is a crude measure of distance.

All of it goes to the BSC, which is where the decision is taken, because the BSC owns the radio resources ([GSM Protocols]).

The decision, and why it is hard

The naive rule, hand over whenever a neighbour is stronger, fails badly, because fading makes the comparison flicker: two cells whose signals differ by a decibel will swap places many times a minute. Each swap costs signalling, a short interruption, and a risk of failure. The symptom has a name: ping-pong.

Two mechanisms fix it:

• Hysteresis. The neighbour must be better by a margin, typically a few decibels, before a handover is considered, and the margin means the reverse handover needs the same advantage in the other direction.
• A dwell requirement. The condition must hold for several consecutive reports, so that a single deep fade does not trigger anything.

Both delay the handover, and delay is dangerous: a mobile that clings to a failing cell loses the call. The program measures exactly that trade.

The message sequence

TS 23.009's Figure 7 gives the basic external intra-MSC handover, and the program lists it. The shape is worth learning:

1. BSS-A decides and asks: "When the BSS (BSS-A), currently supporting the MS, determines that the MS requires to be handed over it will send an A-HANDOVER-REQUIRED message to the MSC (MSC-A)." That message "shall contain a list of cells, or a single cell, to which the MS can be handed over ... in order of preference".
2. MSC-A asks the target to allocate a channel, and the target acknowledges with the details.
3. MSC-A tells BSS-A, which tells the mobile to go to the new channel.
4. The mobile accesses the new cell; the target detects it and reports, and when the mobile completes, the target tells MSC-A.
5. MSC-A clears the old resources.

The mobile is off the air only between leaving the old channel and being detected on the new one, which is why GSM's handover is heard, at most, as a click.

Inter-MSC handover and the anchor

When the target cell belongs to another MSC, the call could in principle be rerouted from the caller, but that would involve the whole fixed network in a local radio event. GSM instead keeps MSC-A as the anchor: "In the Inter-MSC handover case, MSC-A is the MSC which controls the call and the mobility management of the Mobile during the call, before, during and after a basic or subsequent handover."

A circuit is established from MSC-A to MSC-B, and the call now travels to MSC-A and onward to MSC-B. If the mobile moves again, that is a subsequent handover, and it may go back to MSC-A, in which case the extra leg is released, or on to a third MSC, MSC-B', in which case MSC-A sets up a new leg and releases the old one. The anchor never changes during the call, so a mobile that crosses many MSC areas can end up with a long path, which is the price of not disturbing the caller.

Handover, computed

The program walks a mobile at 15 m/s between two cells 1 km apart, 200 times, with correlated shadowing and a measurement report every 480 ms, and counts the handovers and the dropped calls under five decision rules; lists the four handover types with the entities each involves; and prints the message sequence of an intra-MSC handover.

# Handover: the decision, and why hysteresis and a dwell timer are needed.
import math
import random

# A mobile walks from cell A to cell B. Each cell's signal falls with distance
# (path-loss exponent 3.5) and fades; the mobile reports every 480 ms on the
# SACCH. Three decision rules are compared.
rnd = random.Random(93)
STEP_S, SPEED, SEP = 0.48, 15.0, 1000.0            # a report every 480 ms, 15 m/s, cells 1 km apart

def level(d, shadow):
    return -15 - 35 * math.log10(max(d, 1.0)) + shadow      # dBm, 35 dB a decade

def walk(rule, hysteresis=0.0, dwell=1, trials=200):
    swaps, dropped = 0, 0
    for _ in range(trials):
        serving, count, x, s_a, s_b = 'A', 0, 0.0, 0.0, 0.0
        for _ in range(int(SEP / (SPEED * STEP_S))):
            x += SPEED * STEP_S
            s_a = 0.7 * s_a + rnd.gauss(0, 4)      # slow fading, correlated
            s_b = 0.7 * s_b + rnd.gauss(0, 4)
            a, b = level(x, s_a), level(SEP - x, s_b)
            here, there = (a, b) if serving == 'A' else (b, a)
            if rule == 'none':
                better = there > here
            else:
                better = there > here + hysteresis
            count = count + 1 if better else 0
            if count >= dwell:
                serving = 'B' if serving == 'A' else 'A'
                swaps += 1
                count = 0
            if here < -125:                        # too weak: the call drops
                dropped += 1
                break
    return swaps / trials, 100 * dropped / trials

print("A mobile crossing between two cells 1 km apart at 15 m/s, 200 walks:")
print("  rule                                  handovers per crossing   calls dropped")
for label, rule, hyst, dwell in (("no hysteresis, act at once", 'none', 0.0, 1),
                                 ("hysteresis 3 dB", 'h', 3.0, 1),
                                 ("hysteresis 6 dB", 'h', 6.0, 1),
                                 ("hysteresis 6 dB, 3 reports", 'h', 6.0, 3),
                                 ("hysteresis 12 dB, 3 reports", 'h', 12.0, 3)):
    swaps, drops = walk(rule, hyst, dwell)
    print("  %-38s %18.2f %15.1f%%" % (label, swaps, drops))
print("  one handover is wanted. Without hysteresis the mobile ping-pongs; too much, and it")
print("  clings to a cell until the call drops.")

# 2. The four types of handover, and who is involved in each.
print("\nThe four handover types, and who must take part:")
for name, what, involved in (("intra-cell", "another channel in the same cell (interference)", "BSC"),
                             ("inter-cell, intra-BSC", "another cell of the same BSC", "BSC"),
                             ("inter-BSC, intra-MSC", "a cell of another BSC under the same MSC", "BSC, MSC"),
                             ("inter-MSC", "a cell under another MSC", "BSC, MSC-A, MSC-B")):
    print("  %-22s %-46s %s" % (name, what, involved))
print("  MSC-A anchors the call through an inter-MSC handover: the route from the caller never moves")

# 3. The message sequence of an intra-MSC handover (TS 23.009, figure 7).
print("\nAn external intra-MSC handover, message by message:")
for i, (frm, to, msg) in enumerate([
        ("BSS-A", "MSC-A", "A-HANDOVER-REQUIRED (a list of target cells, in order of preference)"),
        ("MSC-A", "BSS-B", "A-HANDOVER-REQUEST (allocate a channel)"),
        ("BSS-B", "MSC-A", "A-HANDOVER-REQUEST-ACK (here it is)"),
        ("MSC-A", "BSS-A", "A-HANDOVER-COMMAND"),
        ("BSS-A", "MS", "RI-HO-COMMAND (go to that channel)"),
        ("MS", "BSS-B", "RI-HO-ACCESS (access bursts on the new channel)"),
        ("BSS-B", "MSC-A", "A-HANDOVER-DETECT"),
        ("MS", "BSS-B", "RI-HO-COMPLETE"),
        ("BSS-B", "MSC-A", "A-HANDOVER-COMPLETE"),
        ("MSC-A", "BSS-A", "A-CLEAR-COMMAND, then A-CLEAR-COMPLETE")], 1):
    print("  %2d. %-6s -> %-6s %s" % (i, frm, to, msg))

Ping-pong. With no hysteresis, one crossing caused 10.24 handovers on average: the mobile swapped cells every time fading moved the balance, which is ten times the signalling and ten chances to fail, for one crossing that needed one handover.

Hysteresis. 3 dB halves it to 5.32; 6 dB gives 3.00; 6 dB with a requirement of three consecutive reports gives 1.18, which is about right: one handover per crossing, with the occasional extra where the shadowing genuinely reversed.

Too much. At 12 dB with three reports, handovers fall to 0.93, but 14.5 per cent of calls drop, against 5.5 per cent at 6 dB: the mobile holds on to a cell that has become too weak. That is the whole design problem in one table: too little hysteresis wastes signalling, too much loses calls, and the operator tunes the margin for each cell.

The types. Intra-cell and inter-cell within a BSC are the BSC's business alone, which is why they are fast and common. An inter-BSC handover adds the MSC; an inter-MSC handover adds a second MSC and a circuit between them, with MSC-A anchoring the call.

The sequence. Ten messages, of which only two cross the air: the command telling the mobile to move, and the access and completion on the new channel. Everything else is the network preparing and cleaning up.

Distinctions

What it does not mean

The mobile does not decide. It measures and reports; the BSC decides. That is what mobile-assisted means.

A handover is not always caused by movement. Interference and load cause handovers in a stationary mobile, and an intra-cell handover does not change cell at all.

The anchor MSC is not a detour that can be avoided. Keeping MSC-A in the path is what spares the caller's network any knowledge of the mobile's movement.

Hysteresis is not a delay setting. It is a margin in decibels; the dwell requirement is the delay, and the two are tuned separately.

A dropped call at a boundary is not always a coverage fault. It can be a handover decision that waited too long, which is a parameter problem, not a radio one.

Quick revision

• Handover: moves an ongoing call to another channel. Causes: movement, quality, load. GSM's is hard (break before make) and mobile-assisted (mobile measures, network decides).
• Four types: intra-cell (BSC), inter-cell intra-BSC (BSC), inter-BSC intra-MSC (BSC, MSC), inter-MSC (MSC-A anchors, MSC-B serves).
• Measurements: the mobile uses the idle frame to measure up to six neighbours and reports on the SACCH about twice a second; the BTS measures the uplink and the timing advance; the BSC decides.
• Decision: hysteresis (a margin in dB) plus a dwell requirement (several consecutive reports). Program: 10.24 handovers a crossing with none; 1.18 at 6 dB and three reports; 14.5 per cent of calls dropped at 12 dB.
• Sequence (TS 23.009 Fig. 7): HANDOVER-REQUIRED (with a preference-ordered cell list), HANDOVER-REQUEST, ACK, HANDOVER-COMMAND, the mobile's HO-ACCESS, HANDOVER-DETECT, HO-COMPLETE, HANDOVER-COMPLETE, CLEAR-COMMAND, CLEAR-COMPLETE.
• Inter-MSC: MSC-A "controls the call ... before, during and after a basic or subsequent handover"; later moves are subsequent handovers, back to MSC-A or on to MSC-B'.

Test yourself

1. Why is handover needed, and what are its causes? Because a call in progress must survive changes in the radio link. The usual cause is movement out of the serving cell, but a handover may also be triggered by falling quality without movement, for example interference or a fade, in which case even a change of channel within the same cell may help, and by load, when a busy cell hands a mobile to a neighbour that can serve it.

2. Describe the four types of handover in GSM and say who takes part. An intra-cell handover moves the call to another channel of the same cell and is handled by the BSC alone, usually because of interference. An inter-cell handover within one BSC also involves only that BSC. An inter-BSC handover within one MSC involves both BSCs and the MSC, which switches the connection. An inter-MSC handover involves the original MSC, which remains the anchor and keeps control of the call, a second MSC which takes over the radio side, and a circuit between them.

3. How does GSM collect the information a handover decision needs? The handover is mobile-assisted. Each mobile uses the idle frame of its 26-multiframe to retune to neighbouring broadcast carriers, measuring the received level of up to six neighbours and identifying them by their base station identity codes, and it reports these together with the level and quality of its serving cell on the slow associated control channel, about twice a second. The BTS measures the uplink level, quality and timing advance. All these reports go to the BSC, which takes the decision.

4. What are hysteresis and the dwell requirement, and why are both needed? Hysteresis is a margin: a neighbouring cell must be better than the serving cell by several decibels before a handover is considered, so that the reverse handover would require the same advantage in the other direction. The dwell requirement is that the condition must hold for several consecutive measurement reports. Without them, fading makes the comparison flicker and the mobile ping-pongs between cells: in the chapter's simulation, a single crossing produced 10.24 handovers with no hysteresis and 1.18 with 6 dB and three reports. Too much of either is also harmful: at 12 dB with three reports, 14.5 per cent of calls dropped because the mobile clung to a failing cell.

5. Give the message sequence of an intra-MSC handover. BSS-A, which is serving the mobile, decides and sends A-HANDOVER-REQUIRED to MSC-A with a list of candidate cells in order of preference. MSC-A sends A-HANDOVER-REQUEST to BSS-B, which allocates a channel and replies with A-HANDOVER-REQUEST-ACK. MSC-A sends A-HANDOVER-COMMAND to BSS-A, which passes the handover command to the mobile over the air. The mobile accesses the new channel, BSS-B reports A-HANDOVER-DETECT, the mobile completes, and BSS-B sends A-HANDOVER-COMPLETE. MSC-A then clears the old resources with A-CLEAR-COMMAND and receives A-CLEAR-COMPLETE.

6. What is an anchor MSC, and what is a subsequent handover? In an inter-MSC handover the original MSC, MSC-A, keeps control of the call and of mobility management before, during and after the handover, and the connection from the caller continues to reach MSC-A, which extends it over a circuit to the new MSC, MSC-B. MSC-A is therefore the anchor. If the mobile moves again, the result is a subsequent handover: either back to MSC-A, in which case the extra leg is released, or onward to a third MSC, in which case MSC-A establishes a new leg and releases the old one. The anchor does not change during the call, which spares the caller's network any involvement at the cost of a longer path.