diff --git a/owlps-positioner/src/inputcsv.cc b/owlps-positioner/src/inputcsv.cc
index 1f4b705..56304c9 100644
--- a/owlps-positioner/src/inputcsv.cc
+++ b/owlps-positioner/src/inputcsv.cc
@@ -56,8 +56,8 @@ bool InputCSV::fill_current_request()
   current_request->set_mobile(&mobile) ;
 
   // Read request type
-  uint8_t type ;
-  uint16_t type_r ;
+  uint_fast8_t type ;
+  uint_fast16_t type_r ;
   if (! file.read_field(type_r))
     {
       // Wrong number of fields
@@ -68,6 +68,17 @@ bool InputCSV::fill_current_request()
   type = type_r ;
   current_request->set_type(type) ;
 
+  // Read the number of packets
+  uint_fast16_t nb_packets ;
+  if (! file.read_field(nb_packets))
+    {
+      // Wrong number of fields
+      if (Configuration::is_configured("verbose"))
+        cerr << "InputCSV: cannot read the number of packets.\n" ;
+      return false ;
+    }
+  current_request->set_nb_packets(nb_packets) ;
+
   // Read Timestamp field
   Timestamp timestamp ;
   if (! file.read_timestamp(timestamp))
@@ -108,12 +119,21 @@ bool InputCSV::fill_current_request()
   string mac_ap ;
   while (file.read_field(mac_ap))
     {
-      int ss ;
+      uint_fast16_t packet_id ;
+      if (! file.read_field(packet_id))
+        {
+          // Wrong number of fields
+          if (Configuration::is_configured("verbose"))
+            cerr << "InputCSV: cannot read the packet ID.\n" ;
+          return false ;
+        }
+
+      int_fast16_t ss ;
       if (! file.read_field(ss))
         {
           // Wrong number of fields
           if (Configuration::is_configured("verbose"))
-            cerr << "InputCSV: cannot read mac_ap.\n" ;
+            cerr << "InputCSV: cannot read the signal strength.\n" ;
           return false ;
         }
       PosUtil::to_upper(mac_ap) ;
@@ -124,7 +144,7 @@ bool InputCSV::fill_current_request()
 
       const AccessPoint &ap = Stock::find_create_ap(mac_ap) ;
       measurements[mac_ap].set_ap(&ap) ;
-      measurements[mac_ap].add_ss(ss) ;
+      measurements[mac_ap].add_ss(packet_id, ss) ;
     }
   if (measurements.empty())
     return false ;
diff --git a/owlps-positioner/src/inputcsv.hh b/owlps-positioner/src/inputcsv.hh
index 0688144..33733c4 100644
--- a/owlps-positioner/src/inputcsv.hh
+++ b/owlps-positioner/src/inputcsv.hh
@@ -16,7 +16,8 @@
 /// Reads \link Request requests \endlink from a CSV file
 /**
  * CSV format is:
- *   Mobile_MAC;Timestamp;X;Y;Z;Direction;AP_MAC_1;SS_1;…;AP_MAC_n;SS_n
+ *   Mobile_MAC;Request_type;Number_of_packets;Timestamp;X;Y;Z;Direction;
+ *     AP_MAC_1;Packet_ID_1;SS_1;…;AP_MAC_n;Packet_ID_n;SS_n
  */
 class InputCSV: public InputMedium
 {
diff --git a/owlps-positioner/src/inputlogcsv.cc b/owlps-positioner/src/inputlogcsv.cc
index 84ceed5..df2e87f 100644
--- a/owlps-positioner/src/inputlogcsv.cc
+++ b/owlps-positioner/src/inputlogcsv.cc
@@ -28,6 +28,7 @@ const string InputLogCSV::request_to_csv(const Request &request) const
     csv_line << request.get_mobile()->get_mac_addr() ;
   csv_line
       << ';' << static_cast<uint_fast16_t>(request.get_type())
+      << ';' << request.get_nb_packets()
       << ';' << request.get_time_sent()
       << ';' ;
 
@@ -48,18 +49,7 @@ const string InputLogCSV::request_to_csv(const Request &request) const
     request.get_measurements() ;
   for (unordered_map<string, Measurement>::const_iterator i
        = measurements.begin() ; i != measurements.end() ; ++i)
-    {
-      const vector<ss_t> &ss_list = i->second.get_ss_list() ;
-      const string &ap_mac = i->second.get_ap() != NULL
-                             ? i->second.get_ap()->get_mac_addr()
-                             : "" ;
-      for (vector<ss_t>::const_iterator i = ss_list.begin() ;
-           i != ss_list.end() ; ++i)
-        {
-          csv_line << ';' << ap_mac ;
-          csv_line << ';' << *i ;
-        }
-    }
+    csv_line << i->second.to_csv() ;
 
   csv_line << '\n' ;
   return csv_line.str() ;
diff --git a/owlps-positioner/src/inputudpsocket.cc b/owlps-positioner/src/inputudpsocket.cc
index 803a804..ff8ff06 100644
--- a/owlps-positioner/src/inputudpsocket.cc
+++ b/owlps-positioner/src/inputudpsocket.cc
@@ -104,6 +104,7 @@ bool InputUDPSocket::fill_current_request()
     }
 
   // Endianess conversions
+  request.nb_packets = ntohs(request.nb_packets) ;
   owl_ntoh_timestamp(&request.request_time) ;
   request.nb_info = ntohs(request.nb_info) ;
   request.x_position = owl_ntohf(request.x_position) ;
@@ -120,6 +121,7 @@ bool InputUDPSocket::fill_current_request()
     cout
         << "*** Request received from the aggregator ***"
         << "\n\tType: " << static_cast<uint_fast16_t>(request.type)
+        << "\n\tNumber of packets: " << request.nb_packets
         << "\n\tMobile MAC: " << mac_mobile
         << "\n\tRequest timestamp: " << Timestamp(request.request_time)
         << "\n\tPosition X: " << request.x_position
@@ -146,6 +148,9 @@ bool InputUDPSocket::fill_current_request()
   // Request type
   current_request->set_type(request.type) ;
 
+  // Number of packets
+  current_request->set_nb_packets(request.nb_packets) ;
+
   // Timestamp
   current_request->set_time_sent(Timestamp(request.request_time)) ;
 
@@ -167,6 +172,8 @@ bool InputUDPSocket::fill_current_request()
           return false ;
         }
 
+      uint_fast16_t packet_id = ntohs(request_info.packet_id) ;
+
       string mac_ap(
         owl_mac_bytes_to_string(request_info.ap_mac_addr_bytes)) ;
       PosUtil::to_upper(mac_ap) ;
@@ -179,6 +186,7 @@ bool InputUDPSocket::fill_current_request()
       if (Configuration::is_configured("verbose"))
         cout
             << "\t* Packet received from the aggregator:"
+            << "\n\t\tPacket number: " << packet_id
             << "\n\t\tAP MAC: " << mac_ap
             << "\n\t\tSignal: "
             << ss << " dBm"
@@ -195,7 +203,7 @@ bool InputUDPSocket::fill_current_request()
 
       const AccessPoint &ap = Stock::find_create_ap(mac_ap) ;
       measurements[mac_ap].set_ap(&ap) ;
-      measurements[mac_ap].add_ss(ss) ;
+      measurements[mac_ap].add_ss(packet_id, ss) ;
     }
   if (measurements.empty())
     return false ;
diff --git a/owlps-positioner/src/measurement.cc b/owlps-positioner/src/measurement.cc
index c74cb3c..8e7c809 100644
--- a/owlps-positioner/src/measurement.cc
+++ b/owlps-positioner/src/measurement.cc
@@ -8,6 +8,9 @@
 #include "measurement.hh"
 #include "posexcept.hh"
 
+#include <iostream>
+#include <sstream>
+
 using namespace std ;
 
 
@@ -15,23 +18,6 @@ using namespace std ;
 /* *** Constructors *** */
 
 
-Measurement::Measurement(const AccessPoint *_ap):
-  ap(const_cast<AccessPoint*>(_ap)), average_ss(0)
-{
-  ss_list.reserve(10) ;
-}
-
-
-Measurement::Measurement(const AccessPoint *_ap,
-                         const vector<ss_t> &_ss_list):
-  ap(const_cast<AccessPoint*>(_ap)), ss_list(_ss_list)
-{
-  ss_list.reserve(10) ;
-  update_average_ss() ;
-}
-
-
-
 /**
  * Note that values pointed by #ap are not deleted.
  */
@@ -46,31 +32,48 @@ Measurement::~Measurement()
 
 
 /**
+ * @param packet_id The sequence number of the packet in the request,
+ * as sent by the mobile.
  * @param ss_dbm The signal strength to add to #ss_list (in dBm).
  */
-void Measurement::add_ss(const ss_t &ss_dbm)
+void Measurement::
+add_ss(const uint_fast16_t packet_id, const ss_t ss_dbm)
 {
+  unsigned int ss_list_size = ss_list.size() ;
+
+  // Add the new value (in dBm) along with the packet identifier:
+  pair<uint_fast16_t, ss_t> packet(make_pair(packet_id, ss_dbm)) ;
+  if (! ss_list.insert(packet).second)
+    {
+      cerr
+          << "Cannot insert the packet (#" << packet_id << ", "
+          << static_cast<int_fast16_t>(ss_dbm) << " dBm)\n" ;
+      return ;
+    }
+
+  // Update the average with the new value
   double total_ss_mwatts =
     pow(10, static_cast<double>(ss_dbm) / 10.0) + // New SS in mW
-    (ss_list.size() * pow(10, average_ss / 10.0)) ; // Other values in mW
-
-  ss_list.push_back(ss_dbm) ; // Add new value (dBm)
-
-  // Update average converting back in dBm
+    (ss_list_size * pow(10, average_ss / 10.0)) ; // Other values in mW
+  // Convert the average back in dBm:
   average_ss = 10.0 * log10(total_ss_mwatts / ss_list.size()) ;
 }
 
 
-void Measurement::set_ss_list(const std::vector<ss_t> &_ss_list)
+/**
+ * Add an SS list to the current Measurement's SS list.
+ * If the new SS list contains a packet with the same ID, the old one
+ * is overwritten.
+ */
+void Measurement::
+add_ss_list(const map<uint_fast16_t, ss_t> &_ss_list)
 {
-  ss_list = _ss_list ;
-  update_average_ss() ;
-}
+  // We cannot use insert() here because we want to overwrite the
+  // previous values with the same ID, if any.
+  for (map<uint_fast16_t, ss_t>::const_iterator
+       i = _ss_list.begin() ; i != _ss_list.end() ; ++i)
+    ss_list[i->first] = i->second ;
 
-
-void Measurement::add_ss_list(const std::vector<ss_t> &_ss_list)
-{
-  ss_list.insert(ss_list.end(), _ss_list.begin(), _ss_list.end()) ;
   update_average_ss() ;
 }
 
@@ -117,10 +120,10 @@ void Measurement::update_average_ss()
 
   double total_ss_mwatts = 0 ;
 
-  for (vector<ss_t>::const_iterator i = ss_list.begin() ;
-       i != ss_list.end() ; ++i)
+  for (map<uint_fast16_t, ss_t>::const_iterator
+       i = ss_list.begin() ; i != ss_list.end() ; ++i)
     // Add the current value in mW to the total
-    total_ss_mwatts += pow(10, static_cast<double>(*i) / 10.0) ;
+    total_ss_mwatts += pow(10, static_cast<double>(i->second) / 10.0) ;
 
   // Compute the average in mW and convert it to dBm
   average_ss = 10 * log10(total_ss_mwatts / ss_list.size()) ;
@@ -157,23 +160,51 @@ bool Measurement::operator==(const Measurement &m) const
 
 
 
+const string Measurement::to_csv() const
+{
+  ostringstream csv_line ;
+
+  if (ss_list.empty())
+    return "" ;
+
+  string mac_ap("") ;
+  if (ap)
+    mac_ap = ap->get_mac_addr() ;
+
+  for (map<uint_fast16_t, ss_t>::const_iterator
+       i = ss_list.begin() ; i != ss_list.end() ; ++i)
+    {
+      if (i != ss_list.begin())
+        csv_line << ';' ;
+      csv_line
+          << mac_ap << ';'
+          << i->first << ';'
+          << static_cast<int_fast16_t>(i->second) ;
+    }
+
+  return csv_line.str() ;
+}
+
+
+
 ostream &operator<<(ostream &os, const Measurement &m)
 {
   // MAC address
   os
-      << "AP: " << (m.ap != NULL ? m.ap->get_mac_addr() : "Unknown_AP")
+      << "AP: " << (m.ap ? m.ap->get_mac_addr() : "Unknown_AP")
       << ": " ;
 
   // List of SS
   if (m.ss_list.empty())
     os << "No values" ;
   else
-    for (vector<ss_t>::const_iterator i = m.ss_list.begin() ;
-         i != m.ss_list.end() ; ++i)
+    for (map<uint_fast16_t, ss_t>::const_iterator
+         i = m.ss_list.begin() ; i != m.ss_list.end() ; ++i)
       {
-        os << *i ;
-        if (i != m.ss_list.end() - 1)
-          os << ";" ;
+        if (i != m.ss_list.begin())
+          os << ';' ;
+        os << static_cast<int_fast16_t>(i->second)
+           << '(' << i->first << ')' ;
       }
   os << " [AVG=" << m.average_ss << "]" ;
 
diff --git a/owlps-positioner/src/measurement.hh b/owlps-positioner/src/measurement.hh
index 449fb54..950964c 100644
--- a/owlps-positioner/src/measurement.hh
+++ b/owlps-positioner/src/measurement.hh
@@ -10,7 +10,7 @@
 
 #include "accesspoint.hh"
 
-#include <vector>
+#include <map>
 #include <ostream>
 #include <cmath>
 
@@ -23,7 +23,7 @@ protected:
   /// The AccessPoint that performed the measurement
   AccessPoint *ap ;
   /// List of signal strengths captured (in dBm)
-  std::vector<ss_t> ss_list ;
+  std::map<uint_fast16_t, ss_t> ss_list ;
   /// Average of all signal strength captured (dBm)
   double average_ss ;
 
@@ -35,10 +35,8 @@ protected:
 
 
 public:
-  Measurement(const AccessPoint *_ap = NULL) ;
-
-  Measurement(const AccessPoint *_ap,
-              const std::vector<ss_t> &_ss_list) ;
+  Measurement(const AccessPoint *_ap = NULL):
+    ap(const_cast<AccessPoint*>(_ap)), average_ss(0) {}
 
   Measurement(const Measurement &source):
     ap(source.ap), ss_list(source.ss_list),
@@ -49,7 +47,6 @@ public:
   /** @name Read accessors */
   //@{
   AccessPoint* get_ap() const ;
-  const std::vector<ss_t>& get_ss_list() const ;
   double get_average_ss() const ;
   int get_ss_list_size() const ;
   //@}
@@ -57,11 +54,10 @@ public:
   /** @name Write accessors */
   //@{
   void set_ap(const AccessPoint *_ap) ;
-  void set_ss_list(const std::vector<ss_t> &_ss_list) ;
   /// Adds a signal strength to #ss_list
-  void add_ss(const ss_t &ss_dbm) ;
+  void add_ss(const uint_fast16_t packet_id, const ss_t ss_dbm) ;
   /// Adds several signal strengths to #ss_list
-  void add_ss_list(const std::vector<ss_t> &_ss_list) ;
+  void add_ss_list(const std::map<uint_fast16_t, ss_t> &_ss_list) ;
   /// Merges a given Measurement into the current Measurement
   void merge(const Measurement &source) ;
   void clear(void) ;
@@ -83,6 +79,12 @@ public:
   operator bool(void) const ;
   //@}
 
+  /** @name Conversion accessors */
+  //@{
+  /// Converts to a CSV string
+  const std::string to_csv(void) const ;
+  //@}
+
   /// Displays a Measurement
   friend std::ostream &operator<<(std::ostream &os, const Measurement &m) ;
 } ;
@@ -98,12 +100,6 @@ inline AccessPoint* Measurement::get_ap() const
 }
 
 
-inline const std::vector<ss_t>& Measurement::get_ss_list() const
-{
-  return ss_list ;
-}
-
-
 inline double Measurement::get_average_ss() const
 {
   return average_ss ;
diff --git a/owlps-positioner/src/posutil.cc b/owlps-positioner/src/posutil.cc
index ef17518..aeffcf9 100644
--- a/owlps-positioner/src/posutil.cc
+++ b/owlps-positioner/src/posutil.cc
@@ -48,7 +48,7 @@ void PosUtil::complete_with_dummy_measurements(
   assert(! measurements2.empty()) ;
 
   Measurement dummy ;
-  dummy.add_ss(-98) ; // FIXME: should be the smallest possible value
+  dummy.add_ss(1, -98) ; // FIXME: should be the smallest possible value
 
   for (unordered_map<string, Measurement>::const_iterator i =
          measurements1.begin() ; i != measurements1.end() ; ++i)
diff --git a/owlps-positioner/src/request.cc b/owlps-positioner/src/request.cc
index 98d2b57..54d7367 100644
--- a/owlps-positioner/src/request.cc
+++ b/owlps-positioner/src/request.cc
@@ -49,6 +49,8 @@ void Request::set_real_position(const Point3D &_real_position)
 
 
 /**
+ * - #nb_packets is set to 1 (this is the default value when
+ *   constructing a Request).
  * - #mobile is NULLified, but the value it pointed to is not deleted.
  * - The fields of #time_sent are initialised to 0.
  * - #measurements is cleared.
@@ -56,6 +58,7 @@ void Request::set_real_position(const Point3D &_real_position)
 void Request::clear()
 {
   type = OWL_REQUEST_UNDEFINED ;
+  nb_packets = 1 ;
   mobile = NULL ;
   time_sent.clear() ;
   measurements.clear() ;
@@ -91,6 +94,7 @@ Request& Request::operator=(const Request &source)
     return *this ;
 
   type = source.type ;
+  nb_packets = source.nb_packets ;
   mobile = source.mobile ;
   time_sent = source.time_sent ;
   measurements = source.measurements ;
@@ -116,6 +120,7 @@ bool Request::operator==(const Request &source) const
   return
     real_position_equal &&
     type == source.type &&
+    nb_packets == source.nb_packets &&
     mobile == source.mobile &&
     time_sent == source.time_sent &&
     measurements == source.measurements ;
@@ -134,6 +139,7 @@ ostream& operator<<(ostream &os, const Request &r)
   // MAC address
   os
       << "Type: " << static_cast<uint_fast16_t>(r.type)
+      << ", Number of packets sent: " << r.nb_packets
       << ", Mobile: "
       << (r.mobile != NULL ? r.mobile->get_mac_addr() : "Unknown_Mobile")
       << ":" ;
@@ -160,6 +166,7 @@ size_t hash_value(const Request &source)
   size_t seed = 0 ;
 
   boost::hash_combine(seed, source.type) ;
+  boost::hash_combine(seed, source.nb_packets) ;
   boost::hash_combine(seed, source.time_sent) ;
   if (source.mobile)
     boost::hash_combine(seed, source.mobile->get_mac_addr()) ;
diff --git a/owlps-positioner/src/request.hh b/owlps-positioner/src/request.hh
index 20f379c..12c3eef 100644
--- a/owlps-positioner/src/request.hh
+++ b/owlps-positioner/src/request.hh
@@ -25,6 +25,8 @@ class Request
 protected:
   /// Type of the request
   uint_fast8_t type ;
+  /// Number of packets sent by the mobile for this request
+  uint_fast16_t nb_packets ;
   /// The mobile that sent the request
   Mobile *mobile ;
   /// Local date of the request on the mobile
@@ -48,25 +50,25 @@ public:
           const std::tr1::unordered_map<std::string, Measurement>
           &_measurements =
             std::tr1::unordered_map<std::string, Measurement>()):
-    type(OWL_REQUEST_UNDEFINED),
+    type(OWL_REQUEST_UNDEFINED), nb_packets(1),
     mobile(const_cast<Mobile*>(_mobile)), time_sent(_time_sent),
     measurements(_measurements), real_position(NULL) {}
 
   Request(const std::tr1::unordered_map<std::string, Measurement>
           &_measurements):
-    type(OWL_REQUEST_UNDEFINED),
+    type(OWL_REQUEST_UNDEFINED), nb_packets(1),
     mobile(NULL), measurements(_measurements), real_position(NULL) {}
 
   Request(const Timestamp &_time_sent,
           const std::tr1::unordered_map<std::string, Measurement>
           &_measurements =
             std::tr1::unordered_map<std::string, Measurement>()):
-    type(OWL_REQUEST_UNDEFINED),
+    type(OWL_REQUEST_UNDEFINED), nb_packets(1),
     mobile(NULL), time_sent(_time_sent),
     measurements(_measurements), real_position(NULL) {}
 
   Request(const Request &source):
-    type(source.type),
+    type(source.type), nb_packets(1),
     mobile(source.mobile), time_sent(source.time_sent),
     measurements(source.measurements), real_position(NULL) {}
 
@@ -74,7 +76,8 @@ public:
 
   /** @name Read accessors */
   //@{
-  uint8_t get_type(void) const ;
+  uint_fast8_t get_type(void) const ;
+  uint_fast16_t get_nb_packets(void) const ;
   Mobile* get_mobile(void) const ;
   const Timestamp& get_time_sent(void) const ;
   const std::tr1::unordered_map<std::string, Measurement>&
@@ -84,7 +87,8 @@ public:
 
   /** @name Write accessors */
   //@{
-  void set_type(const uint8_t _type) ;
+  void set_type(const uint_fast8_t _type) ;
+  void set_nb_packets(const uint_fast16_t _nb_packets) ;
   void set_mobile(const Mobile *_mobile) ;
   void set_time_sent(const Timestamp &_time_sent) ;
   void set_measurements(const std::tr1::unordered_map
@@ -121,12 +125,18 @@ public:
 
 
 
-inline uint8_t Request::get_type() const
+inline uint_fast8_t Request::get_type() const
 {
   return type ;
 }
 
 
+inline uint_fast16_t Request::get_nb_packets() const
+{
+  return nb_packets ;
+}
+
+
 inline Mobile* Request::get_mobile() const
 {
   return mobile ;
@@ -156,12 +166,18 @@ inline const Point3D* Request::get_real_position(void) const
 /* *** Write accessors *** */
 
 
-inline void Request::set_type(const uint8_t _type)
+inline void Request::set_type(const uint_fast8_t _type)
 {
   type = _type ;
 }
 
 
+inline void Request::set_nb_packets(const uint_fast16_t _nb_packets)
+{
+  nb_packets = _nb_packets ;
+}
+
+
 inline void Request::set_mobile(const Mobile *_mobile)
 {
   mobile = const_cast<Mobile*>(_mobile) ;
@@ -194,8 +210,8 @@ inline bool Request::operator!=(const Request &comp) const
 
 /**
  * @return \em false if the Request is empty.
- * @return \em true if at least one attribute (other than #type) is
- * initialised.
+ * @return \em true if at least one attribute (other than #type and
+ * #nb_packets) is initialised.
  */
 inline Request::operator bool() const
 {
diff --git a/owlps-positioner/src/stock.cc b/owlps-positioner/src/stock.cc
index 1d7b89d..2a20dd5 100644
--- a/owlps-positioner/src/stock.cc
+++ b/owlps-positioner/src/stock.cc
@@ -598,7 +598,7 @@ void Stock::generate_reference_point(const Point3D &point)
 
       /* Create the measurement, add it to the list */
       Measurement m(&rx->second) ;
-      m.add_ss(rx_ss) ;
+      m.add_ss(1, rx_ss) ;
       measurements[rx->second.get_mac_addr()] = m ;
     }