Specifications

Engineering Guidelines
246
Block size is not user configurable on either the 3300 or the phone, however TFTP block size
could be user configurable on some 3'rd party external TFTP servers.
In situations where phones are accessing an external TFTP server over a very slow connection
reduce, if possible, the transmitted block size from 4096 to a smaller number; 512 or 1024. This
will increase the number of ack/nack messages, but will ensure that the four second inter-packet
timer is less likely to be exceeded, especially when multiple phones share the same restricted
link.
For best performance the TFTP server should be connected to the network with a minimum
bandwidth of 100Mbits/s. Lower bandwidth will reduce the throughput and result in increased
delays to bring the phones into service.
For a WAN link, the minimum bandwidth to ensure timely startup with minimum retries at the
phone is 15 kbits/s per phone. Higher bandwidths will result in phones returning to service
quicker, and a practical value to consider might be 100kbits/s/phone. Less available bandwidth
may result in phones retrying to complete the download and hence take additional time.
Depending on the total number of phones that require access to the common TFTP server and
the time to have these in service the following WAN minimum bandwidths per phone are
recommended:
Although lower bandwidths may be used, this may result in a number of phones failing to
complete the download in the expected time, resulting in subsequent retries and time to come
into service.
Table 75: TFTP Server Recommended Bandwidth
Total number of phones
on TFTP server
Recommended Minimum WAN bandwidth per phone
500 20kbits/s
1000 35kbits/s
1500 50kbits/s
2000 70kbits/s
2500 85kbits/s
3000 100kbits/s
3500 120kbits/s
4000 135kbits/s
4500 150kbits/s
5000 170kbits/s