FACT SHEET
1588 derived frequency reference challenges.
Packet based timing technologies work on the two way exchange of timing information between a Master
Clock and Slave (or Client) Clock. The 1588 protocol works on the assumption that this two way exchange is
symmetric (i.e. it expects that the Packet Delay from Master to Slave and Slave to Master is the same). How-
ever, in the majority of wide area networks this is not the case and the phenomena of Packet Delay Variation
(PDV) introduces noise into the Client clock. Without severely limiting the scale of the network or introduc-
ing complex management schemes, deriving accurate frequency and phase information from the packet
network is a challenge.
Timing Requirements
Application
CDMA2000
Time/Phase synchronization accuracy
+/- 3μs with respect to UTC* (during normal conditions)
(3GPP2 C.S0010-B, 3GPP2 C.S0002-C) +/- 10μs of UTC (when the time sync reference is disconnected)
W-CDMA
2.5μs phase difference between Base Stations
(TDD mode)
(3GPP TS 25.402)
TD-SCDMA
3μs phase difference between Base Stations
(TDD mode)
(3GPP TR 25.836)
LTE
(TDD)
3μs time diffference between Base Stations (small cell).
10μs time difference between Base Stations (large cell)
(3GPP TS 36.133)
MBSFN
(e.g. over LTE)
< +/- 1μs with respect to a common time reference
(continuous timescale)
WiMAX
(TDD mode)
(IEEE 802.16)
Depends on several parameters.
Ranges from +/-0.5μs to +/-5μs
*See definitions at end of document
1588 Timing Solutions
There are a number of approaches described in the 1588 protocol that can be taken to delivering timing
over a packet network. In broad terms these approached can be divided into the Ordinary Clock approach
and the Transparent Clock approach.
Ordinary Clock approach: In the Ordinary Clock approach the timing information is sent from the Master
clock to the Slave clock without adjustment being made to the time stamp information by intervening
nodes – routers in the path between the master and slave do not tell the slave anything about their behav-
ior. The advantage of this approach is that the disruption to the existing network is minimal and service
providers can deploy 1588 over an existing network – A possible disadvantage is that the Slave needs to be
robust in the presence of PDV.
Transparent Clock approach: In the Transparent Clock approach the timing information is updated as it trav-
els from the master to the slave – routers in the path between the master and slave can make adjustments to
the packet to tell the slave about their behavior. The advantage of the Transparent Clock is that the deploy-
ment can self-correct network disruptions far more easily – a disadvantage is that service providers are faced
with the possibility of expensive ‘forklift upgrades’of existing networks.
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