We’ve seen an increase in smartphones and other mobile devices in the last couple of years that has dramatically increased the amount of bandwidth each individual consumes. This surge in mobile bandwidth is not only here to stay, it will grow at least at the rate of Moore’s Law for the forseeable future. The challenge that carriers face is that mobile subscribers are already paying what they are prepared to pay and the broadband surge necessitates a network infrastructure refresh every 18-24 months.
Carriers have turned to Ethernet as the medium to lessen the impact of this continual demand for mobile broadband. In the US and in a good part of Asia-Pacific legacy TDM networks have been getting “Etherized” since 2007. However the implementations in Europe have been very limited which has led to the European Marketplace relinquishing their leadership position in Mobile Broadband Access.
How did this happen?
The main reason is a little known but very important issue of network timing. In order for mobile towers to hand-off a mobile call or data session from one tower to the next while you are driving they need to be “in-sync”. This synchronization is enabled by having reliable timing at each mobile tower. This timing has traditionally come from the TDM circuits (E1/T1, STM-1/OC-3, etc.) and the mobile tower would get their timing and thus their synchronization by assuming the timing from the TDM circuit.
Ethernet, on the other hand, has no inherent timing mechanism; In fact, Ethernet was specifically designed to be asynchronous because of it’s “any-to-any” communications capabilities. To compound the challenge, the tolerances for error and timing slippage are moving downwards. LTE standards are demanding timing accuracy that is, in some cases, a tenfold increase over where it was expected to be in TDM based networks.
There has been a number of efforts to add synchronization to Ethernet with the main aim of providing timing to the mobile towers. Here is a list and a simple explanation of the methods and technologies that are being considered by mobile providers in Europe.
1588v2 or PTP is a technology that aims to inject timing/synchronization information into each packet that can be retrieved at each element in the network that is configured to use it. The main strength of 1588v2 is that it can be transmitted as a normal Ethernet packet across any switch/router that handles Ethernet natively. The main weakness is that 1588v2 isn’t accurate enough considering the decreased margin of error demanded by these LTE high-speed mobile networks. Furthermore, accuracy decreases with each “hop” in the network.
Another method of having timing natively on Ethernet is to use Synchronous Ethernet (Sync-E). Sync-E is a method of transmitting the timing information directly on the physical wire (or fiber as it were) as another signal. Nothing in the Ethernet frame or packet is modified. The strength of this approach is extremely accurate timing, even more accurate than the most stringent LTE standards. The main weakness is that new hardware is necessary to support this technology. Most networks today are built on switches and routers that cannot support Sync-E without a hardware upgrade.
Network Timing Protocol (NTP) is one of the oldest protocols in use and was invented to synchronize the clocks of computer systems. NTP’s strength is that it’s an inherent IP layer protocol that is widely implemented on LANs and WANs across the globe. The overriding weakness of NTP is it’s accuracy. NTP has an acceptable tolerance of +/- 20 milliseconds which is significantly more than the LTE standards allow for.
Global Positioning Satellites (GPS) are being used in parts of the world currently as a clock source for the Mobile towers. This is how the United States, Canada and parts of Asia-Pacific have recently been able to build very fast mobile broadband networks. With a GPS antenna on each tower, the mobile network can synchronize without ever having to carry timing information on the backhaul Ethernet links. The strength of this approach is the ability to use existing Ethernet infrastructure without having to worry about retrofitting it to support timing. The weakness is cost; GPS antennae aren’t cheap and if we multiply this by the amount of mobile towers in a network (sometimes on upwards of 30,000) the business case quickly turns sour. In addition to the cost issue there is no disaster recovery capability with GPS, however, if satellites start falling out of the sky then network synchronization will be the least of our worries.
So what is the answer? Most pundits have suggested that the mobile networks will solve the synchronization issue using one of two options;
1. Networks that have GPS already (such as CDMA based networks) will continue to use it despite it’s disaster recovery limitations and will enjoy a head start on their non-GPS enabled competitors.
2. Sync-E and 1588v2 will be used simultaneously in non-GPS networks. 1588 will be implemented in the core and distribution layers where there is already a tremendous amount of investment in Ethernet that isn’t Sync-E capable. And Sync-E at the access-to-tower portion of the network.
Considering that GPS is considered anathema in Europe (no one want’s to have their networks rely on US based satellites) I believe we will see almost exclusively option #2 in Europe.
Are you in this business? Have you been working on this issue? Let us know your experiences by commenting below!
