Network Bandwidth Considerations

^{IBM-AUSTRIA - PC-HW-Support 30 Aug 1999}

Network Bandwidth Considerations

Network Bandwidth Considerations

When implementing a backup solution that backs up data to a backup server over the network, an important factor is network bandwidth. The reason for this is that all the data must go over the network. This becomes even more important when different machines are trying to back up to one server at the same time, since the amount of data increases. That is why network bandwidth will be one of the factors when deciding how many machines will be backed up to one backup server, and which backup window will be needed.

To calculate the time needed for a backup, the following points must be considered:

The amount of data that will be backed up
Unfortunately, this number can differ from backup system to backup system. Let's say you have a file server with 20 GB of data. When you do a full backup of this system, it will indeed send 20 GB. But most backup programs also work with incremental or differential backup algorithms, which only back up changed data. So, to figure out the amount of data that is backed up in such an operation, we will have to consider the following points:
- How much data changes between two backups?
- What does 'changed' mean to my backup program?
Backup programs will normally also compress data. Unfortunately, the compression rate is strongly dependent on the type of file you are backing up, and therefore hard to define. For initial calculations, you could take the worst case scenario, where no compression would take place.
Nominal network speed (commonly expressed in Mbps) This is the published speed of the network. Token-ring for example will have a nominal network speed of 4 or 16 Mbps.
The practical network speed
Since a communication protocol typically adds headers, control data and acknowledgments into network frames, not all of it will be available for our backup data. As a rule of thumb, the practical capacity is 50-60% for token-ring, FDDI or ATM networks, and 35% for Ethernet networks.
Note: The above values are only used as indicative values. Actual figures might differ, since they are influenced by numerous factors. These factors include network topology, number of concurrent network connections and the protocol that is used. If more exact values are necessary, we suggest that you trace the network, and get average values for data throughput.
Bandwidth available to the protocol we are using. For simplicity, we will suppose that we have the entire bandwidth for the protocol.

This brings us to the following formulas:

For token-ring, FDDI or ATM:
For Ethernet:

Note: When calculating backup throughput, factors other than network speed will matter. For example, processing on a backup server and tape mounting will also play an important factor. See performance reports from specific products to get an idea of application capabilities.

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