ailable within a limited number of hops away (how many hops depends on the timing budget and the accuracy of the intervening boundary clocks). This does not mean that putting one PRTC+GM in the capital city and another in the second largest city of a country is a good design. For a large, complex network, there might be tens or even hun- dreds of these devices deployed further out in the network in the aggregation or pre-aggregation layers. These PRTC+GM devices can be standalone devices from a vendor dedicated to producing them, or they can be embedded in a network router or other piece of equipment. Sometimes they are no lar- ger than small form-factor pluggable (SFP) that can be plugged into a router's Ethernet port to provide SyncE and PTP to the router. The best performance for carrying timing signals across a transport network comes from combining PTP with a physical frequency signal such as that from SyncE. SyncE has one other great advant- age, which is to provide an extended holdover should the PTP signal disappear (see the following section "Holdover"). For this reason, the SyncE flow throughout the network should be designed to provide the maximum level of redundancy. In the case where a small cell is deployed in a location that is served with only a single network link, not much can be done because the single back-haul link is the single point of failure. In fact, many small cells might not be able to use a SyncE signal at all and might only function with a valid PTP feed, SO redundancy of a source of PTP is more important in this case. If a larger macro cell is connected by multiple network links, however, then it is normally no extra cost to run SyncE on each of those links, or at least on enough of them to survive multiple link fail- ures. Normally, having SyncE is just a matter of configuring it to be enabled, SO it is a very good idea to have it on and not need it rather than need it and not configure it. 11.1.1 Holdover The first thing to understand about holdover is just how hard it is to do fo