diff --git a/smpplib/client.py b/smpplib/client.py
index bcd3b91..fe82ad9 100644
--- a/smpplib/client.py
+++ b/smpplib/client.py
@@ -28,6 +28,7 @@
 import socket
 import struct
 
+from time import sleep
 from smpplib import consts, exceptions, smpp
 
 
@@ -61,12 +62,14 @@ class Client(object):
     vendor = None
     _socket = None
     sequence_generator = None
-
-    def __init__(self, host, port, timeout=5, sequence_generator=None, logger_name=None):
+    
+    def __init__(self, host, port, timeout=5, sequence_generator=None, max_outstanding_operations=None, logger_name=None):
         self.host = host
         self.port = int(port)
         self._socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
         self.timeout = timeout
+        self.outstanding_operations = set()
+        self.max_outstanding_operations = max_outstanding_operations
         self.logger = logging.getLogger(logger_name or 'smpp.Client.{}'.format(id(self)))
         if sequence_generator is None:
             sequence_generator = SimpleSequenceGenerator()
@@ -167,18 +170,41 @@ def unbind(self):
         except socket.timeout:
             raise exceptions.ConnectionError()
 
-    def send_pdu(self, p):
+    def manage_outstanding_operations(self, sending, pdu):
+        if not self.max_outstanding_operations:
+            return
+
+        if pdu.is_response():
+            operation = '{}{}'.format('S' if sending else 'C', pdu.sequence)
+            try:
+                self.outstanding_operations.remove(operation)
+                self.logger.debug('Unregistering outstanding operation {}'.format(operation))
+            except KeyError:
+                self.logger.warning('Failed to unregistered outstanding operation {}'.format(operation))
+        else:
+            operation = '{}{}'.format('C' if sending else 'S', pdu.sequence)
+            if pdu.sequence in self.outstanding_operations:
+                self.logger.warning('Outstanding operation {} already registered'.format(operation))
+            else:
+                self.outstanding_operations.add(operation)
+                self.logger.debug('Registering outstanding operation {}'.format(operation))
+
+    def send_pdu(self, pdu):
         """Send PDU to the SMSC"""
 
-        if self.state not in consts.COMMAND_STATES[p.command]:
+        if self.state not in consts.COMMAND_STATES[pdu.command]:
             raise exceptions.PDUError("Command %s failed: %s" % (
-                p.command,
+                pdu.command,
                 consts.DESCRIPTIONS[consts.SMPP_ESME_RINVBNDSTS],
             ))
 
-        self.logger.debug('Sending %s PDU', p.command)
+        if self.max_outstanding_operations and pdu.is_request():
+            while len(self.outstanding_operations) >= self.max_outstanding_operations:
+                sleep(.1)
+
+        self.logger.debug('Sending %s PDU', pdu.command)
 
-        generated = p.generate()
+        generated = pdu.generate()
 
         self.logger.debug('>>%s (%d bytes)', binascii.b2a_hex(generated), len(generated))
 
@@ -194,6 +220,8 @@ def send_pdu(self, p):
                 raise exceptions.ConnectionError()
             sent += sent_last
 
+        self.manage_outstanding_operations(sending=True, pdu=pdu)
+
         return True
 
     def read_pdu(self):
@@ -227,6 +255,8 @@ def read_pdu(self):
 
         self.logger.debug('Read %s PDU', pdu.command)
 
+        self.manage_outstanding_operations(sending=False, pdu=pdu)
+
         if pdu.is_error():
             return pdu