Hybrid DSL-LTE access: getting even more out of the copper network

Tuesday 19 January 2016 | 09:20 CET | Background

Swisscom is the latest operator to announce plans to test a broadband service using both the DSL and LTE access networks. Another example of the growing convergence of fixed and mobile networks, hybrid DSL-LTE access, also known as LTE bonding, uses the high bandwidth of the LTE network to boost speeds for DSL customers in areas with limited bandwidth. In addition to helping improve speeds and coverage in rural areas far from DSL exchanges, the technology offers a way for operators to get more out of their existing copper networks and LTE investments, without significant additional costs.

In the past year, Deutsche Telekom has taken the hybrid access technology the furthest. It soft-launched in December 2014 an offer based on a DSL-LTE router developed in-house by its unit T-Labs. The router relies on the DSL connection to ensure a low latency connection and automatically switches on the 4G network when a ‘bandwidth boost’ is needed. In the spring of 2015, the hybrid service was expanded to more areas in Germany and offered under DT’s standard Magenta ZuHause residential plans. The service starts at EUR 29.95 per month for up to 16 Mbps over DSL plus another 16 Mbps from the 4G network, and goes up to EUR 39.95 per month for 100+100 Mbps. Actual speeds depend on the customer’s DSL connection and 4G coverage. The price includes a fixed line with unlimited national calls, but customers must pay an extra EUR 9.95 per month for the special Speedport router (or EUR 400 one-off). By the end of September 2015, DT had 109,000 subscribers for the service, after adding over 50,000 in Q3 alone.

While the subscriber base is still small, DT clearly sees a future for hybrid access. The German operator has made no secret it favours copper upgrades over investment in fibre, and this is another way to exploit its existing assets and combat the higher speeds offered by cable operators. DT sees room to increase hybrid speeds to up to 250 Mbps and also aims to start bundling TV service with the hybrid subscriptions.

Smart followers

DT originally tested the hybrid access technology at its subsidiary in Montenegro and more recently announced plans to roll out the service at its subsidiary Cosmote in Greece. The difficult terrain in Greece, both in terms of geography and the economy, offers a clear opportunity for hybrid access to save on roll-out costs while quickly boosting broadband speeds. Cosmote’s VDSL network offers speeds up to 50 Mbps to only 40 percent of the country, while its LTE network covers 80 percent. Introducing hybrid access technology means only a change in its access servers and replacing customer premises equipment.

Cosmote’s move appears to have prompted Vodafone to also consider hybrid access technology. The two companies announced nearly simultaneously in December 2015 plans to introduce the technology in Greece. Vodafone said DSL-LTE bonding means it can offer customers more than twice the current speeds on VDSL, while also ensuring a constant, reliable connection. The new service is being piloted and will be available in the near future, with Greece the first country in the Vodafone group to introduce the technology commercially. Vodafone said earlier in September 2014 that it was testing DSL-LTE bonding with a Huawei router in Spain, with download speeds possible up to 200 Mbps and upload speeds up to 60 Mbps. The test targeted business customers, focusing on the reliability of the connection thanks to two access methods.

Bandwidth vs reliability

The reliability of the hybrid connection may be more appealing for some operators than the higher speeds. While Switzerland already has some of best infrastructure in the world for high-speed broadband, Swisscom has become the latest operator to announce plans to test DSL-LTE bonding. Swisscom already uses a variety of access technologies, from over 1 million homes passed with FTTH to plans to upgrade its copper network with G.fast. The company has developed a LTE receiver that picks up the mobile data stream and passes it on via WLAN to the DSL router. After successful tests in the laboratory and with Swisscom employees, a pilot trial will be conducted in January with selected residential customers. The technology allows for bandwidths of up to 20 Mbps in its initial phase. Higher bandwidths will also be tested in later phases.

The next company to adopt hybrid access may be Proximus. The Belgian operator is also known for favouring copper over fibre investments and already has near-national coverage with VDSL2. Last year it was one of the first investors in the Belgian start-up Tessares, a spin-off from research labs at the Catholic University in Louvain-la-Neuve. Tessares has developed software that can be deployed on existing home gateways and a hybrid access gateway for the service provider’s data centre. This solution promises minimal investment, using existing CPE, to roll out a hybrid DSL-LTE service.

On existing home gateways (running Broadcom chipset BCM63168), the company achieved an aggregate speed of more than 100Mbps by bonding a DSL link at 70Mbps and a LTE link at 40Mbps. This could help operators such as Proximus better compete with cable in areas where copper upgrade techniques like vectoring and G.fast may not be possible. After developing a commercial product this year with Proximus, Tessares plans to look abroad for new markets where high-throughput services are less common, such as neighbouring France.

New standards: Multipath TCP

Both Tessares and Deutsche Telekom are developing their technology based on Multipath TCP. The Transmission Control Protocol (TCP) is a key networking standard used by all applications to exchange data; it is closely linked to the Internet Protocal (IP) for creating internet connections. However, TCP has one major limitation: to establish a data stream it relies on one communication interface (IP address) on a device at a time. This causes difficulties when a device such as a router or smartphone tries to use two networks simultaneously, such as both Wi-Fi and the cellular network.

Multipath TCP is a recently standardised extension to the TCP protocol that solves this problem. It modifies TCP so that it presents a standard TCP interface to applications, while in fact spreading data across several ‘subflows’ (network paths). By combining several subflows, Multipath TCP makes better use of the available network resources, increases throughput and reduces the chances of connection failure. In addition, Tessares points out that each TCP subflow benefits from TCP’s Congestion Control and can thus adapt in real time to changing network conditions (bandwidth auto-scaling), an important feature when shared networks like cellular are used.

The Internet Engineers Task Force has published the Multipath TCP specification as an experimental standard. This is available as a reference implementation on Linux, for which numerous use cases have been developed (in addition to hybrid network access, e.g. data centres in need of redundant network connections). Available as open source code, the implementation of Multipath TCP does not require any changes to existing TCP-based applications. As the technology is developed and tested further, DSL-LTE bonding appears an inexpensive choice for operators looking to meet the ever-increasing demand for bandwidth and get that bit more out of their copper networks.

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