diff --git a/src/client.py b/src/client.py
--- a/src/client.py
+++ b/src/client.py
@@ -105,8 +105,7 @@ class Client(NetNode):
 
 	def sendData(self,blocksToTransfer):
 		log.info(blocksToTransfer)
-		dataFile=open(self._filename, mode="rb")
-		i1=-1
+		dataFile=DataFile.open(self._filename)
 
 		print(datetime.now(), "sending data:")
 		progress=Progress(len(blocksToTransfer))
@@ -116,16 +115,13 @@ class Client(NetNode):
 			blocks=[]
 			for j in range(conf.batchSize.data):
 				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)
+				i=blocksToTransfer[k+j]
+				block=dataFile.readFrom(i)
 
-				indices.append(i2)
+				indices.append(i)
 				blocks.append(block)
-				log.info("block #{0}: {1}...{2}".format(i2,block[:5],block[-5:]))
+				log.info("block #{0}: {1}...{2}".format(i,block[:5],block[-5:]))
 
-				i1=i2
 				progress.p(k+j)
 			if indices: self._sendData(indices,blocks)
 		progress.done()