6.2 Frequency Synchronization in TDM Networks

There are three TDM (time-division multiplexing) technologies currently in use, PDH (Plesiochronous Digital Hierarchy), SDH (Synchronous Digital Hierarchy) or SONET (Synchronous Optical Network), which is the North-American equivalent of SDH, and OTN (Optical Transport Network). The SONET requirements have been incorporated in the SDH specifications.

6.2.1 Synchronization Architecture in TDM Networks

Figure 6.1 depicts synchronization network architecture. SDH/SONET carries synchronization on the physical layer, i.e. the clock frequency of the physical signal is synchronized to a central reference. In the case of PDH, the different bit rate signals are not synchronized to each other but the client signal, for example E1, 2 Mbit/s, is carried across the network so that the frequency of the signal is preserved. OTN is similar to PDH, where the client TDM signal preserves frequency when carried over the network.

Figure 6.1 Synchronization network architecture.

Usually networks are synchronized centrally by a primary reference clock. The mobile network nodes like base stations and controllers receive synchronization from the transport network. Ring structures can be used for protection purposes. For instance in Figure 6.1 the synchronization chain on the right side ring ends before the bottom node of the ring. However, if a link breaks in the left side of the ring the synchronization chain coming from the right side will be forwarded to the nodes in the left side that would otherwise lose synchronization. PDH and SDH are described below.

6.2.2 PDH

Traditionally the clock of one end of the link is free-running, that is, it is not synchronized by the network. The other end is locked to the first end. However, the maximum allowed frequency error is limited so that the receiver can lock in to the signal for being able to receive the data. For example, the frequency accuracy limit is ± 50 ppm for 2.048 Mbit/s signal and ± 32 ppm for 1.544 Mbit/s signal. Another and more important reason for frequency accuracy limit is the requirements of the multiplexing process where tributaries are combined into larger bit rate signals. The larger rate signal is framed so that there are time slots reserved for the tributaries. For filling almost exactly the reserved times slots and never exceeding the maximum bit rate reserved for each tributary, the frequency error limits need to be accurately defined. Exceeding this limit would cause loss of data. Today, most of the PDH networks obtain the frequency reference from a central reference as in SDH, see below. In cellular networks, this is a requirement. ITU-T recommendations related to PDH synchronization are G.823 and G.824. Especially in the case of 2 Mbit/s PDH hierarchy multiplexing to higher PDH bitrates has disappeared to a large extent. Instead, the signals are multiplexed directly into SDH containers.

6.2.3 SDH/SONET

When designing SDH (SONET in North America) networks, one target was easing up multiplexing and demultiplexing. In each PDH multiplexing level the upper level knows what bits in the lower bit-rate time slots are used for demultiplexing the tributaries at the other end. However, the next multiplexing level no longer knows this. Thus, each node needs a multiplexer mountain to add or drop a small bit rate signal. However, each SDH/SONET multiplexing level knows exactly the used bits by the client signals. By synchronizing all SDH/SONET layers tightly together, the frequency of time slips compared with the nominal frequency is very small. The primary reference clock PRC (North America: primary reference source, PRS) may deviate from the nominal frequency only by ± 0.01 ppb. PRC is defined in ITU-T recommendation G.811.

In the case of 2.048-Mbit/s hierarchy, clocks in SDH nodes are called SECs (SDH equipment clock) and are specified in G.813. There may be maximally 20 in a row. The total number of SECs over a synchronization chain may be 60 if there is an SSU (Synchronization supply unit), G.812 Type I, clock between chains of maximally 20 SECs. The total numbers of SSUs is 10. The SDH equipment clocks in the case of 1.544-Mbit/s hierarchy are SEC option 2 clocks. The jitter and wander network limits are specified in G.823, wander for 2.048-Mbit/s hierarchy, G.824, wander for 1.544-Mbit/s hierarchy, and G.825, SDH jitter specifications. 0.171 and 0.172 describe testing equipment for PDH and SDH, respectively.

The ring protection occurring for synchronization trail, as discussed in relationship with Figure 6.1 requires a messaging mechanism where a clock losing synchronization indicates to the next clock that the quality of the clock is not acceptable. Consequently the next clock will know to select another clock input if available. For indicating clock quality, ITU has defined SSMs (synchronization status messages) and clock selection principles in G.781. Functional blocks are defined in G.783. The mapping of SSMs in SDH frames is specified in G.707/Y.1322.

6.2.4 ATM

ATM (Asynchronous Transfer Mode) was chosen as the transport protocol for WCDMA and it dominated for the first ten years. For transporting ATM cells over transport network they are encapsulated in PDH or SDH/SONET frames and these layers also carry synchronization. In this sense there is no difference compared with SDH or PDH synchronization. Since ATM cells are usually encapsulated directly in SDH/SONET frames at the RNC (radio network controller), there is no end-to-end PDH layer left. This leaves out an option available in PDH where accurate frequency of a mobile operator could be transported asynchronously over a SDH/SONET network of a transport provider even if the SDH/SONET network is not synchronized well.

6.2.5 OTN

Optical Transport Network (OTN) was developed to carry 2.5-Gbit/s and higher data streams over optical wavelength carriers. Other transport protocols are then clients of OTN. Since a synchronization hierarchy was already defined for SDH, instead of defining a new synchronization hierarchy for OTN, the layer was made transparent for synchronized client signals. As a result, SDH clients could pass tens of OTN nodes without accumulating too much jitter and wander.