Cliff Cliff wrote: > > Dear all, > > One of our customer want to know the simply way to calculate > the throughput in their link.
Calculating throughput can be difficult. Your customer should measure actual throughput for a typical case, using a protocol analyzer or one of the many other tools for calculating throughput, including ttcp. > > Right now, they have 64k from us using satellite and both end a > cisco router (normal satellite round trip time is 510ms). Bandwidth = capacity = 64 Kbps in your case. "Capacity and throughput are similar, but not the same. Capacity is the actual amount of resources available across a given path. Throughput is a measure of how much data can be passed across a medium in a stated period of time, and typically this refers to user data." (Source: Darren L. Spohn, "Data Network Design") Note that throughput is a measure. Your customer should make some measurements, after deciding if he/she wants to measure overall throughput or just throughput for user data. Once the customer has some measurements, there are tools available to determine what the measurement would be if bandwidth were increased. NetPredictor is one such tool. Since you are an ISP, you may need a more general answer too, though, and maybe you can't easily measure your customer's throughput... The Sales Answer: You can use as much capacity as we give you and your throughput on large data transfers, assuming well-tuned TCP/IP implementations, can approach the capacity. The Real (Technical) Answer: It depends. Small data transfer throughput results are going to be more affected by the high latency in your satellite network. The bits can be sent at 64 Kbps (your capacity), but the first bit is going to take a long time to get there. By the time it gets there, the sender may no longer be sending. You mentioned using a 1024 KB file. You should use a larger file to get better results. Large data transfer throughput results won't be as affected by the high latency. The first bit takes the same amount of time to get there, but it's immediately followed by numerous additional bits. In addition, the following factors affect throughput: -- Protocol behavior. Is it a request/reply protocol, where each request results in a reply, or does it support a window size that is larger than one packet? -- What is the window size? A host can send up through its send window size worth of data, at which time it must stop and wait for an ACK. During that stopping, no data is sent. -- ACKs. Depending on the protocol and application, some ACKs might just be ACKs and not have any data. They take time and can't be counted if you are measuring throughput of user data. -- TCP slow start. Most implementations of TCP only send a few packets to start and then build up to the window size. -- The TCP 3-way handshake times time, especially in a high-latency network. With FTP, there are many 3-way handshakes, one for control, and one for each data transfer for listing directories, sending files, etc. -- MTU. How much data can be stuffed into each packet? If you are measuring throughput of user data, how much of a packet is user data and how much is overhead? How much overhead is there from packet headers? -- Processing speed at the two hosts involved in the data transfer. -- RAM at the two hosts. RAM affects window sizes. It also affects how much data can be stored at a time before a host has to stop and write to disk. -- Disk access speed -- Packetization and queing time at intermediate-routers and switches -- Errors and how they are handled. Does data have to be retransmitted frequenty? This could be deadly on a satellite link. There's probably many more things too, but that's a start. _______________________________ Priscilla Oppenheimer www.troubleshootingnetworks.com www.priscilla.com > > They just ask me a simple question that how much BW that they > can take in one session for e.g. 1024k file download using this > link. > > I don't think I can answer to them by telling them the answer > is 1024k * 8/64k (throughput), then if they buy more BW from us > (let say 256k), is it just like 1024k *8/256k? > > After I search the info from internet, I see there is some > parameters which make the throughput varies: > > 1. RTT (Round trip time) > 2. Window size > 3. Overhead of HDLC > 4. MTU size (Max Transfer Unit) > 5. link BW > > Does anyone know that how should I tell this customer? OR there > exist any general equipment can show to our customer the > estimated value. Also, I really want to know how the above > parameters correlated which affect the throughput. > > Really thx if someone is spend some time answer my Q. > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=57246&t=57158 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
