isted-pair (1000BASE-T) has a symbol rate of 125 MHz. But remember that every link is negotiating its own frequency based on the free-running oscillator of the port selected as master for that link. And so, without a central overriding clock as a source, a multi-port switch could end up in a topology where every port is a slave and is running at a slightly different clock frequency (although within +100 ppm of each other). Effectively, every port on the switch becomes a point-to-point synchronous link. Conceptually, this is what happens with an Ethernet switch that is not using synchronous Ethernet (SyncE)-every port negotiates its own transmission and reception speed with the port on the neighboring switch. This means that all the oscillators used for transmission are free running (not aligned with any reference frequency signal). SyncE corrects this by connecting the transmitter on every port to a common oscillator for the device and allows that oscillator to be frequency synchronized to a reference clock. The reference signal comes either over a SyncE link or via an external source of frequency, such as a sine wave signal fed into the switch's building integrated timing supply (BITS) port. See the section on syn- chronous Ethernet in "Chapter 6. Physical Frequency Synchronization", for more details on SyncE. 3.1.1 Synchronous Networks In synchronous networks, all clocks are synchronized to the common clock reference. There are dif- ferent approaches to synchronize the clocks in the network; they are broadly classified into two cat- egories-centralized and decentralized control methods. As shown in Figure 3-2 part a, in the centralized control method, the engineer assembles the clocks into a hierarchy based on a master-slave relationship between the network clock and the remote master clock. In this approach, the ultimate master clock (known in some cases as the grandmaster) is the definitive reference clock for every node in the network. Alternatively, in a decentralized control method (Figure 3-2 part b), eac