Pings

Hi Everybody,

I'm not complaining just asking

I need some information on why the ping times on GPRS and 3G are the following:
on GPRS on average: 600 ms on smtp.vodacom.co.za
on 3G on average: 210 ms on smtp.vodacom.co.za

IBurst, also being wireless and similar technology to 3G( I think?), obtain pings in the low 100 ms. ADSL is in the low 50 ms which I can understand it not being wireless.

Anybody out there want to explain in 'not to technical' terms why the ping times come out the way the do and what influences ping times etc?

Latency, i.e. the time it takes a packet to move from one end of the network to the other, is affected by two parameters.

1) The number of units the packets must pass through (switches, routers and gateways). Every one of these hops requires the packet to be clocked into and then out of the unit. This takes time.

2) The interface speed that interconnect these. This determines how fast the packet can be clocked in or our of the above unit. Also if these links are congested, the unit must wait before clocking the packet out.

The above two parameters add together to produce the total one-direction latency.

In addition when you look at the application level, you have the added delay of handshaking, i.e. where a response must first be received before a new transmission is started. Any protocol that send one packet and then wait for a response suffers the worts from this. Ping and Telnet being two good examples as is any 'interactive' data such as gaming.

Data transfer protocols such as FTP overcome this by using a 'sliding window', sending many packets in one direction before stopping and waiting for a response.

Use a train as an analogy. If you send one locomotive to Jhb from Cape Town and then have to wait for it to return, the average number of units over a week will be small. This is like ping, the 'locomotive latency' is high.

If you now send a locomotive with a 100 cars in tow, get one locomotive back and send another 100 cars up, the 'data throughput' will be much higher. You'll still have the latency on the first locomotive but the cars following it effectively have no latency.

So, latency is only important if you have a time sensitive application such as ping, telnet, VoIP, etc.

For most other applications, data throughput is more important.

In summary, there are either more 'store-and-forward' elements or lower link speeds in the GPRS/3G data path, or probably a combination, although your IP packet changes bearer more and thus will have to cross more elements.

OK Thanks Vodacom3G

It makes more sense.
I can understand that GPRS suffers more from the delayed handshaking effect due to having to wait for a timeslot and having to share the channel with voice traffic that is prioritised at the BTS. Were 3G is a more data oriented network, is the lower 210 ms seen as the norm when it comes to packet roundtrip times? I know that I'm using 210 ms as a refernece point but it seems fair. I have no idea what type of ping times the other 3G users see on smtp.vodacom.co.za.

I have noticed that the latency is affected by the idle time on the connection. I'm refering to 3G here: If the browsing activity is fast and the time delays between page reloads are small the latency is low but if the connection has been idle, far say a hour or two, there is quite a delay before data starts to flow (2 - 4 seconds). I'm sure this is due to the idle state of the connection.

I mostly use my connection for work related http, vpn, research and sometimes casual browsing and ping times don't really influence my 3G experience.

Will HSDPA bring us lower ping times?

Thanks for the info.

grubs,

It should as the link speed is higher.

The 'slow first ping' effect you see is part of the design of the 3G network.

When you don't transmit data for a while, you get put into an idle mode to free up radio bandwidth for other users. When you then start a data stream again the first packet will get delayed by a few seconds. After that the normal latency should be seen.

This IDLE concept sometimes causes unforseen side effects, like a DNS timeout.

grubsner,
I ping smtp.vodaco.co.za every day and late at night. Fastest times I found was also 210ms. Average time is around 250ms. GPRS is slow,very slow even on a sunny day with no traffic.

The 210 ms is early in the morning (before 6am) on 3G. The ping times for smtp on GPRS is anywere between 500 ms to 1200 ms, depending on the time of day. To be expected.