Sync speed is the speed at which a router/modem connects to the outer edge of an ISP's network (DSLAM that is usually installed at the local telephone network exchange). Internet Service Providers (ISP) in the UK use two methods to initiate sync speed: Rate Adaptive and Fixed Rate; Plusnet, for example, use rate adaptive and state on their website "Your sync speed is not fixed and can change each time you reboot or switch on your router." Fixed rate, as the term suggests, will attempt to sync at a specific speed regardless of the conditions of the line -- the length of the line from the network exchange to the customer premises will dictate what sync speed is set for a fixed rate ISP account. A rate adaptive ISP account will sync at the best possible speed each time a router is switched on; just as with a fixed rate sync speed, this will depend on the length of the line and the condition of the line. Line condition can change hourly, due to: weather conditions; local electrical distortion; and how much capacity the line has in relation to the amount of people using it.
Netgear router attempting to sync to ISP
Examples of fixed rate sync speeds -- that are quoted by ISP's -- are: 128kbps, 258kbps, 512kbps, 1Mb, 2Mb, 3Mb and 8Mb. The actual sync speed of a fixed rate ISP account may differ slightly to the figure quoted. The actual speed users download/upload at is usually lower than the sync speed -- regardless if they have a fixed rate or rate adaptive ISP account -- and this is due to Internet protocols and other technicalities absorbing some of the actual download/upload speed. ADSL 1 has provided the majority of fixed rate sync speed broadband accounts, and ADSL2+ is usually rate adaptive. Sync problems, such as 'no sync' / 'no synchronisation' issues, are caused at five points: router/modem; microfilter/cables; equipment at the network exchange; the actual phone line and cabinets; and the internal wiring of the customers premises.