Router with VSAT link

Unanswered Question
Jan 18th, 2008


I am having problem with cisco routers. I have 2 MB VSAT link between two routers. But I am not able to get the throuhput on VSAT link. When i ping router A from Router B it works fine and i opened 15 telnet session with Router B my far end router. It gave me 1.5 MB or even more througput. But as soon as i attached laptops with both of router lan interface. I was not able to get the throughput. Very very slow transfer rate of file.

Only router to router link is working fine. but laptop to laptop that are connected with both router ethernet interface is not working.

Can anyone tell me what is the problem. Routers are with simple configuration. No QOS, NO policy is there. Only basic E1 configuration and Ip addresses.Is this window size problem or something else.

Please help me out.

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 3 (1 ratings)
Paolo Bevilacqua Fri, 01/18/2008 - 02:55


this depends by the nature of satellite transmission. First of all the bandwidth promised in in reality shared, so a lot depends by the over subscription ratio used by the provider.

Then, TCP suffers of the large delay and the typical result is just that, poor performances.

This is the reason why satellite modem makers, cisco and other vendors do put special code in their device to optimize sessions. For cisco that is a NM that works only with a certain satellite network, other solutions are more independent.

wasiimcisco Fri, 01/18/2008 - 04:56


Thanks for the reply. Please now tell me how to solve this problem. I am using VWIC2-2MFT-T1/E1.

IF router to router ping is successful and i am getting the proper bandwidth. Then why the file transfer is getting the problem. Laptop are attached with giga interface of 3825 router.

Even if the bandwidth is shared, at least on 2 MB link i should get some bandwidth so that i can transfer few day plus the voice.

Please guide me how to get proper throughput on VSAT link.

Paolo Bevilacqua Fri, 01/18/2008 - 05:31


Unfortunately I think you cannot solve the problem without inserting an optimizing device specifically made for satellite. You can try some TCP window tuning on the PCs but honestly I don't think these will change much.

I think the sat provider could suggest to you optimizing/accelerator devices. The cisco VSAT module works only with Gilat SkyEdge.

When you do pings. etc, from the router, you are not really putting any load on the circuit, so it doesn't mean much.

wasiimcisco Fri, 01/18/2008 - 05:47

I telnet the router B from Router A. I open 15 telnet session and from each session i did extended ping with max datagram size 18024.

and while doing this i also check the interface status, it was showing me

5 minute input rate 1531000 bits/sec, 426 packets/sec

5 minute output rate 431000 bits/sec, 359 packets/sec

means i m using the bandwidth. ping itself has some packet size.

kindly guide me if i am wrong. VSAT guy is saying our transmission is okay. you check ur router. They are saying router to router link is fine and getting full bandwidth. Our responsibility is not end to end connectivity.

Please give comments on this.

Paolo Bevilacqua Fri, 01/18/2008 - 06:04


the problem is not the bandwidth, is the delay. Delay has adverse effect against TCP connections, this is called Long Fat Pipe effect. There is ample literature about the problem, see for example:

The router cannot do that to fix that, because it is not a proxy. What you need is sometime called PEP (performance enhancement proxy).

pcameron Fri, 01/18/2008 - 09:39

Sounds like you need to use Rate Based Satellite Control Protocol (RBSCP) which came out in 12.3.7T -

This protocol was designed to get around the issues Paolo has mentioned. PEP can cause problems with some applications as it effectively proxys the TCP session - some TCP based higher layer protocols including IPSec cannot handle this.

The RBSCP uses a tunnel interface so you route your satellite traffic through the Tunnel. The link above has some good examples

yashdedhiya Mon, 01/21/2008 - 00:03


I gone through ur problem.May be u can try to do a throughput test with software called iperf.It is a windows based software which will have server and client connection between both laptops.You can try to test the throughput test either via TCP sessions or UDP sessions.It will give you exact throughput of the WAN link.Since with FTP it doesnt chokes the full bandwidth and u will not be able to get the proper throughput of the WAN link.

Try to search this iperf software on microsoft site.


This Discussion