diff --git a/src/client.py b/src/client.py
--- a/src/client.py
+++ b/src/client.py
@@ -109,20 +109,24 @@ class Client(NetNode):
 
 		print(datetime.now(), "sending data:")
 		progress=Progress(len(blocksToTransfer))
-		for (k,i2) in enumerate(blocksToTransfer):
-			jsonData={"command":"send", "index":i2, "dataType":"data"}
-			if i1+1!=i2:
-				dataFile.seek(i2*HashTree.BLOCK_SIZE)
-			binData=dataFile.read(HashTree.BLOCK_SIZE)
 
-			log.info("block #{0}: {1}...{2}".format(i2,binData[:5],binData[-5:]))
+		for k in range(0,len(blocksToTransfer),conf.batchSize):
+			indices=[]
+			blocks=[]
+			for j in range(conf.batchSize):
+				if k+j>=len(blocksToTransfer): break
+				i2=blocksToTransfer[k+j]
+				if i1+1!=i2:
+					dataFile.seek(i2*HashTree.BLOCK_SIZE)
+				block=dataFile.read(HashTree.BLOCK_SIZE)
 
-			self._outcoming.writeMsg(jsonData,binData)
-			stats.logTransferredBlock()
-			jsonData,binData=self._incoming.readMsg()
-			assert jsonData["command"]=="ack" and jsonData["index"]==i2, jsonData
-			i1=i2
-			progress.p(k)
+				indices.append(i2)
+				blocks.append(block)
+				log.info("block #{0}: {1}...{2}".format(i2,block[:5],block[-5:]))
+
+				i1=i2
+				progress.p(k+j)
+			if indices: self._sendData(indices,blocks)
 		progress.done()
 
 		self._outcoming.writeMsg({"command":"end","action":"push"})
@@ -171,5 +175,13 @@ class Client(NetNode):
 		if self._treeFile:
 			self._updateTree()
 
+	def _sendData(self,indices,blocks):
+		jsonData={"command":"send", "index":indices, "dataType":"data"}
+		binData=b"".join(blocks)
+		self._outcoming.writeMsg(jsonData,binData)
+		stats.logTransferredBlock(len(indices))
+		jsonData,binData=self._incoming.readMsg()
+		assert jsonData["command"]=="ack" and jsonData["index"]==indices, jsonData
+
 	def setConnection(self,connection):
 		(self._incoming,self._outcoming)=connection