Triple play over SDH unites the traditional with the modern

Volker Bendzuweit

 

To be able to provide an infrastructure for high-revenue broadband services like triple play, operators are working feverishly to deploy modern networks capable of supporting TV and video streaming. But “pure-bred” next-generation networks with end-to-end IP infrastructure will remain the exception in the foreseeable future. Even in modern carrier networks, traditional lines will continue to play a role, and technologies like Ethernet over SDH are making it possible to build pragmatic solutions.

 

With next generation networks (NGNs), carriers are deploying an Ethernet IP platform capable of supporting not just services like Layer-2 VPNs, VLANs, and voice over IP, but also video and TV transmission. These networks consist of IP/MPLS-based transport networks and Ethernet-based Metro networks. To reach the greatest possible number of customers, these networks support different types of access – on DSL copper lines, cable TV lines and optical Ethernet in the First Mile (EFM).

 

Triple Play services – a combination of Internet access, telephony, TV and video – are expected to offer considerable sales potential in the consumer segment. Triple Play has plenty of appeal for customers: they only need to pay for one access service and can place phone calls affordably over the Internet, surf the Web, watch TV, and access videos, games and music for their home entertainment platforms over the network. These services can be offered on both DSL lines and back channel-enabled broadband TV cable, so competition for consumer business will get tougher. DSL providers and TV cable network operators are suddenly finding that they are rivals in the same market.

 

There are, however, opportunities amid the competitive risks: Triple Play is seen as a means of increasing the average revenue per user (ARPU), attracting new customers, and offsetting shrinking margins in former cash-cow segments, like call charges and DSL access, with revenue streams from value-added services. So it’s hardly surprising that Internet service providers (ISPs) are working hard to begin offering attractive Triple Play services and to enrich these services with value-added communication capabilities and entertainment content.

 

But to deliver these services, ISPs need more than just a simple infrastructure for data transportation. They need modern networks that are optimized for real-time applications, that can sustain the transmission of TV broadcasts and video on demand and enable guaranteed Quality of Service (QoS). This is good for carriers because, with the decline in prices in recent years, they can hardly turn a profit worth mentioning by simply providing data transport infrastructure.

To support Triple Play services, the broadband infrastructure needs to be expanded. TV and video streams require much higher bit rates than, say, Internet access and VoIP applications. Because an individual video stream today needs a transmission rate of between 1 Mbps and 4 Mbps (and this will increase with the advent of tomorrow’s new high-definition TV formats), vendors need infrastructures capable of supporting rates of between 5 Mbps and 25 Mbps.

 

Furthermore, high Quality of Service is crucial to Triple Play services because no one, for example, would be willing to put up with a jerky motion on TV. So network operators are currently upgrading their networks to support more demanding services like IPTV (TV and video streaming over Internet Protocol) and deliver them to the customer on access technologies like ADSL2+. This means that the local loop needs to be modernized.

 

One key component in the new access networks are Internet Protocol DSL access multiplexers (IP DSLAMs). These provide residential users with the services on DSL lines and essentially form the backbone of the Local Loop. The multiplexers are connected to the network operator’s IP, MPLS or Ethernet transport infrastructures. Known as backhaul networks, these segments generally use packet-switched lines, with Ethernet being the connection of choice because of its simplicity. Unlike traditional DSLAMs, IP DSLAMs use IP or Ethernet rather than ATM. Besides the greater data throughput and the need to carry video streams smoothly, they also have to support voice traffic, in keeping with the principle of Triple Play. Equipment has to be capable of delivering the requisite high Quality of Service to sustain voice over IP and video real-time applications.

To connect an IP DSLAM, network operators generally use dark fiber and attach the equipment to the fiber over Gigabit Ethernet. Optical fiber is not available at every location, but instead of installing new fiber in such cases, DSL network operators can rent conventional fixed SDH lines to connect their multiplexers. This pragmatic approach is not just more economical, because there is no need to deploy an all-fiber infrastructure, but also creates valuable competitive advantages: Triple Play vendors are eager to get their services out into the marketplace and they need to roll out a suitable network infrastructure as quickly as they can.

As with many point-to-point applications, simple conversion at the protocol level is the most affordable means of carrying Ethernet traffic from the IP DSLAM across an SDH network. The IP DSLAM’s Gigabit Ethernet interface is connected using a protocol converter. This converts the Ethernet data into virtual containers (VC-4) and transfers them across the STM-1 interface directly to the network operator’s add-drop multiplexer (ADM).

 

One known issue associated with Ethernet over SDH is avoided here: the transportation of signals of variable length like Ethernet frames on fixed-size units like virtual containers (VC) in SDH is anything but efficient. And the scale of Ethernet traffic (100 Mbps, 1,000 Mbps) does not fit the hierarchy levels in SDH (around 150 Mbps with STM-1 and 450 Mbps with STM-3). Thus, the transmission of traffic on a fully loaded Fast Ethernet interface over an STM-1 line would waste one-third of the available bandwidth.

 

Here, however, the traffic from an only partially loaded Gigabit Ethernet interface is transported on the STM-1 line, and the size of converter can be chosen to best suit the actual data throughput. In the application example in the sidebar, an intelligent RIC-155GE Ethernet-over-SDH converter carries Gigabit Ethernet traffic with a maximum rate of around 150 Mbps from the IP DSLAMs across an STM-1 line. This prevents bandwidth loss and eliminates the need for complex and costly solutions like virtual concatenation technology. If additional bandwidth is needed, equipment with an STM-4 interface can be deployed.

 

The name of RAD Data Communications’ product reveals plenty about the device: it is a rate and interface converter with STM-1 (155 Mbps) and Gigabit Ethernet interfaces. The converter has a number of additional functions, which make it especially well suited for use in Triple Play-enabled networks. It supports a large number of MAC addresses and, therefore, a large number of DSL users. In addition, when traffic is high the converter can buffer data when it peaks. Using VLAN priority tagging, the device can also distinguish between applications – an important capability, particularly for the efficient transmission of TV channels (multicasts) on a network.

The requisite IP connectivity is not always on hand to transport Ethernet data streams. One way around this problem is to convert IP traffic into SDH containers and to carry it on an SDH transport network. Bandwidth can be adjusted as necessary, depending on the data rate.

Telefónica Germany is expanding its broadband infrastructure for business customers and Internet Service Providers (ISPs). Technology from RAD Data Communications is being deployed to connect the Ethernet infrastructure transparently using conventional leased lines: the RIC-155GE Ethernet-over-SDH converter enables Gigabit Ethernet interfaces to connect up over SDH lines.

 

Telefónica Germany has a comprehensive, nationwide MPLS-based backbone. The company is currently building an unbundled Local Loop (LLP) platform. By the middle of this year, up to 40 percent of all households in Germany will be connected. With this highly advanced, Ethernet-based DSL infrastructure, service providers will be able to offer broadband services like ADSL2+ and, later, multimedia services like Triple Play.

Around 1,600 IP-DSLAMs will be connected over Gigabit Ethernet lines to form ring-structured Metro networks, mostly on dark fiber. However, fiber is not available at every DSLAM location. Telefónica soon found a solution to close the gap: It leases SDH lines from other network operators to carry its Ethernet traffic. To be equipped to meet tomorrow’s demands, the company was looking for an affordable product that would transparently convert the data traffic and support multicasts.

 

“When new services are rolled out, the issue is not rapid time to market. If a company does not have its own Ethernet or IP lines for DSLAM backhaul and if no dark fiber is available, SDH networks, which are still widely available, can provide an answer,” explains Ramon Horkany, Product Line Manager at RAD Data Communications. “Leasing lines is often more economical, particularly as the current price trend is very favorable.” Says Ingo Stampe, Director Networks with Telefónica in Germany, “Of all the solutions we looked at, RAD’s RIC-155GE converter proved to be the most affordable option for carrying Gigabit Ethernet interfaces over SDH lines.”

 

Volker Bendzuweit heads RAD Data Communications in Ottobrunn-Riemerling