As data rates and mobile Internet use is increasing one of the big challenges of mobile operators is how to keep pace with adding capacity in the backhaul network, i.e. the network that connects the base station to the rest of the network. Currently, many operators still use slow and expensive 2 MBit/s E-1 and T-1 links that don't scale well at all with the rising wireless data rates that a single base station can provide.
This article in the 3/08 edition of the Ericsson review gives a great overview of next generation IP based backhaul and how to get there. Despite only being a few pages long, it touches a lot of different topics. Here are some examples:
In practice, the radio access network is split in two parts: The last mile to the base station and the aggregation network to the border node to the core network. For both parts, the article explains the different technology choices.
For the aggregation layer, different architectures are described from VDSL, microwave Ethernet and fiber.
An introduction to Quality of Service, latency, jitter and circuit emulation are also not missing.
And finally, a migration path from current E-1 centered access networks to hybrid networks in which a base station has both E-1 and IP connectivity to full IP connectivity with pseudo wire capabilities is also inside.
Almost two years ago, I posted an article about how downlink traffic over my DSL line is severely impacted when at the same time I am sending a large amount of data in the uplink. This is due to the fact that acknowledgments are held up by other uplink data which slows down the traffic in downlink direction. I also mentioned then that some DSL routers are capable of prioritizing traffic such as TCP acknowledgments and VoIP packets to reduce this impact. Now two years later, I bought myself a Fritzbox DSL router and could finally put it to the test myself. Seeing is believing!
And indeed, the difference to a standard DSL router is quite amazing. The first picture below shows how the speed of an ongoing data download is severely reduced while I sent an e-mail with a large file attachment. Once the e-mail was sent, the speed returns again to what my DSL line is capable of, about 6 MBit/s. The same test with the Fritzbox shows quite a different behavior as shown in picture 2 below. While one can see a slight impact once the e-mail transfer starts, but the overall data rate remains pretty much the same as during times without the uplink being fully loaded (600 kbit/s).
Next on the test list was a VoIP call while both uplink and downlink were fully used. To my surprise both the standard DSL router I have and the Fritzbox managed to handle the SIP call both from my Nokia N95 and via a VoIP soft-client on the PC I used for the download without a glitch. Voice quality in both uplink and downlink direction to a PSTN line via a media gateway in the Internet was flawless, no packet loss and also no perceptible increase in delay. Quite a surprise indeed, I was expecting some problems with my standard DSL router in uplink direction. However, there were none which means those VoIP UDP packets must have sneaked through well despite the high load.
Heise News reported recently about O3b Networks, a new satellite operator who’s mission statement is to connect “The Other 3 Billion” to the Internet. Hence, their name O3b. It was probably worth a post as one of the investors is a certain Google Inc. The company’s web site doesn’t yet contain a lot of material but what they’ve already put there makes some interesting reading. Heise reports that the company aims to put 16 satellites built by Thales Alenia aerospace into orbit with a total of 2300 transponders. Each transponder has an uplink/downlink bandwidth of 216 MHz, which delivers a throughput of 600 MBit/s in each direction. The company targets fixed line and mobile networks in the developing world for backhaul services and says there system is designed for significant savings over previous backhaul transport in regions where laying a fiber is no option. It would be interesting to get some hard numbers in terms of dollars per month. Latency of the system is given at 65 milliseconds, quite important for real time services such as voice and interactive services such as web browsing. The company also positions itself for emergency scenarios and says they can get bandwidth to any place around the globe +/- 45 degrees of the equator within 10 minutes. The satellites have yet to be brought into orbit which is foreseen around 2010. An ambitious and exciting project!
This report on Telecom: Italy fits to my recent blog post on T-Mobile upgrading their wireless backhaul to meet the rising demand for higher data rates. The report says that Telecom Italia will connect about 1700 ‘antennas’ of their wireless base stations to new fiber infrastructure which will also be used to bring faster fixed line Internet connections to homes and businesses. The wording is a bit strange since base stations are connected to a backhaul fiber and not individual ‘antennas’. I wonder how many antennas they count per base station!? There are usually 3 sectors per base station, each having its own antenna. That would make it around 600 base stations. But that’s just a guess on my side.
Unstrung recently reported that T-Mobile has started to deploy RAD’s Ethernet over DSL solution for backhauling 3.5G traffic from their UMTS / HSPA base stations. I wondered in the past how soon we would see something like this happening since current 2 MBit/s E-1 line rental costs are prohibitively high and several are required for the bandwidth requirements of a 3.5G base station. The article says deployment starts in Germany, where T-Mobile is the incumbent and has surely made a favorable deal with their fixed line branch, T-Com, who is in the process of deploying a VDSL overlay network besides the already existing ADSL/ADSL2+ network. As VDSL only works over short distances, T-COM deploys curbside VDSL cabinets every several hundred meters. With 52 MBit/s in downlink and 11 MBit/s in uplink, a VDSL link offers more than enough bandwidth for a base station with multiple sectors. Backhaul from the cabinet is also not a problem since they are connected to the core network by fiber. The article doesn’t say if the base station continues to use E-1 links for voice traffic or if all data is backhauled via the DSL link.
I had to hold back with this blog entry a bit because I wanted to get permission first to write about what I would say was the most interesting demo I’ve been invited to during the 3GSM / Mobile World Congress:
Lots of WiMAX demos where shown at this years congress and it’s good to see that 802.16e mobile devices have now reached PC-card card sizes and are close to general availability. It’s also nice to see that when the antenna is just a couple of meters away you can see data rates beyond 10 MBit/s. However, that tells you only little about how the system performs in practice when the base station antenna is a couple of blocks away on top of a building and there is interference from neighboring base stations. To go the extra step, Intel and Motorola have teamed up to show how their kit works in a real environment during this years show.
Network Setup
In just a few days, Intel has put up four Motorola WiMAX base stations on rooftops in central Barcelona which were connected to the core network via 50 MBit/s microwave backhaul equipment from Dragonwave. Each base station was equipped with 3 sectors, each on its own 10 MHz channel in the 2.5 GHz band. In total they had three channels available for the network so each base station used the same set of frequencies. The distance between the base stations was about 2 kilometers which is a bit more then what you would see in an inner city network deployment. They couldn’t choose the sites themselves and had to be happy with what they got. On the upside, there is less interference from neighboring cells then there would be in a public network since there were only 4 cells and thus there is no interference from cells further away.
Indoor Experience
Sitting comfortably in the lobby of a hotel in Central Barcelona, I first had a chat with the technical project manager responsible for the network setup. Very good to have somebody with a technical background to talk to. During our discussion I got a first impression of the network performance as there were two notebooks connected to the network, one via a WiMAX PC-card adapter and the other via a CPE (Customer Premises Equipment) box the size of a DSL or cable modem. Despite sitting in the ground floor lobby, the base station being a couple of rooftops away on the other side of the hotel, the probably heat insulated and RF absorbing windows and just using the built in antennas of the devices we still got a data rate exceeding 2 MBit/s via both the CPE and the PC-card adapter. Note that both were SISO (Single Input Single Output) devices. As even this speed is far beyond what you can make use of while surfing the web we streamed a couple of video streams being sent live from WiMAX connected vehicles touring the city. The resolution of the stream was around 320×240 pixels and with a frame rate of 30 fps and the video streams were crisp and clear. One of the notebooks also had an engineering monitor software package on it to observe lower layer performance of the PC card and it was interesting to see how the card goes through the different modulation and coding schemes from QPSK to 64-QAM as reception conditions changed.
Outdoor Experience
Later on we went outside and used Segways to speed up and down the streets with a notebook attached to it to see how the network copes with mobility. Again the video stream performance was flawless and we streamed a U.S. TV station over the Internet which is quite bandwidth hungry. But even this does not require a bandwidth beyond 5 MBit/s which was obviously not the limit of the network. When asked what the highest throughput is that can be observed in the network I was told that it is around 13 MBit/s with 64-QAM and about 1.5 MBit/s at the cell edge with QPSK ½ modulation and coding despite the fact that the cells are too far away from each other. Interesting numbers showing the direction in which we are headed once 2×2 MIMO is added and proper cell sizes are used.
Here’s a video taken and produced by Marc Wallis and Michael Ambjorn of Intel/Motorola respectively:
(copyright by M. Wallis / M. Ambjorn of Intel/Motorola)
Conclusion I came away very impressed from the demo as the speeds were amazing. We didn’t loose the connection to the network even once during the one and a half hours sitting in the hotel and touring the city. That says a lot about the software stability of the PC-card and the network. Thanks a lot to Intel for the VIP tour invitation it was definitely the best demo I have seen during the Congress.
With mobile networks getting faster and faster a growing pain for network operators is the backhaul connection between the base station sites and the next element in the network. Today, T-1 or E-1 connections are used with a line rate of 1.5 and 2 MBit/s. With HSDPA being put in place today, backhaul capacity requirements of 3G base stations now reach 10 MBit/s or more. This means putting additional T-1 or E-1 lines in place. While this might still work today for HSDPA speeds despite the associated rising costs it certainly won’t work tomorrow for WiMAX, LTE and other Beyond 3G technologies that require backhaul capacities of 60 MBit/s per base station and more.
So the big question is what comes after T-1/E-1 connections over copper, fiber or microwave!? The common answer these days seems to be:
IP over Ethernet with the capability to carry legacy GSM (TDM) and UMTS/HSDPA (ATM) links in IP pseudo-wires alongside native IP traffic generated by native WiMAX and LTE base stations.
But how do you connect the base station sites to Carrier Ethernet Networks? Can the last mile be done over copper, is fiber required or is next generation microwave an alternative? Questions over Questions 🙂
