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Demands of LTE backhaul push chip companies to create more powerful processors

Mobile data usage is growing at a clip and this will continue with the rollout of Long Term Evolution (LTE) networks. Globally, smartphone mobile data is expected to grow by a factor of 19 from 2012 to 2017, according to the latest mobile traffic forecast from Cisco Systems.

This means new demands are being placed on the mobile backhaul network.

Mobile equipment makers use various technologies to backhaul mobile traffic, depending on the location and the infrastructure available. The networking media include microwave links, digital subscriber line (DSL) and fibre. "If you put together the 2G, 3G and LTE protocols in all these types of equipment, you end up with a complex list of features and protocols to be supported," said Liviu Pinchas, PMC Sierra's director of technical marketing.

Manufacturers would like one device that addresses the requirements and protocols across the various equipment types. The market for wireless equipment is also highly competitive such that any merchant silicon should aid product differentiation.

According to market research firm The Linley Group, two main approaches are appearing to deal with mobile backhaul: PMC's WinPath4, a 40Gbit/s network processor tailored for mobile backhaul for the LTE cellular standard; and its main competitor, Broadcom's StrataXGS BCM56450, also known as Katana 2.

"A fundamental difference is that Katana 2 is a configurable Ethernet switch, whereas WinPath4 is a programmable chip," said Jag Bolaria, a senior analyst at The Linley Group. Accordingly, WinPath4 is more flexible and can be customised, while Katana 2 has a greater throughput of 100Gbit/s, making it more suited to Carrier Ethernet switch router platforms, he says.

LTE has more advanced networking requirements, with its need for internet protocol (IP) Layer 3 addressing and routing, rather than traditional Layer 2 Ethernet. This is because LTE base stations, known as eNodeBs, must be linked for call handover between radio cells. "With the direct connection between eNodeB and its neighbours, it is no longer possible to stay at Layer 2 [networking]," said Pinchas. "The only way is to have a flat IP network, with each eNodeB having its own IPv6 address." Supporting routing using IPv6, rather than IPv4, is more complex and expensive, says Pinchas, but that is what is required.

WinPath4 is designed with these challenges in mind. The Carrier Ethernet router on a chip also supports existing 2G and 3G backhaul requirements, as well as small cells used to complement LTE eNodeB signal coverage. "The cell side routers that used to serve one macrocell will now have to serve one macrocell plus up to 10 small cells," said Pinchas.

"This means [supporting] a larger routing table, more users, more services; everything scales up." Like the Broadcom device, WinPath4 is also being aimed at Carrier Ethernet switch router traffic aggregation platforms.

The device is the latest wireline processor based on the decade old WinPath architecture. Wintegra developed the earlier WinPath designs before being acquired by PMC in 2010 (see NE, 23 November 2009).

To support LTE and its enhancement, LTE Advanced, WinPath4 has added programmable packet processors – WinGines – and hardware accelerators to meet new protocol requirements and the greater data throughput.

The 10Gbit/s WinPath3 features 6, 9 or 12 WinGines, clocked at 450MHz, and two 650MHz MIPS 34K processors. With up to 12 engines, the device can act as a uniprocessor with a hardware scheduler allocating tasks to the engines, called via an application programming interface running on one of the MIPS. WinGines are multithreaded, with each thread involving packet processing. Tasks performed include receiving, classifying, modifying, shaping and transmitting a packet.

The WinPath4 has 48 WinGines, clocked at 500MHz, and microprogrammable hardware accelerators for such tasks as packet parsing, packet header extraction and traffic matching: tasks too processing intensive for the WinGines.

The WinPath4 also support tables with up to 2million IP destination addresses, up to 48,000 queues with four levels of hierarchical traffic shaping, encryption engines to implement the IP Security (IPsec) protocol and supports the IEEE1588v2 timing protocol.

For control tasks, such as setting up and removing connections, PMC has retained the two MIPS 34k cores, now clocked at 825MHz. "The reason we didn't go for a more powerful core is that things are pretty static in the mobile network, while all the packet processing is done in the [WinGine/ accelerator] data path," said Pinchas.

WinPath4 also supports the emerging software defined networking (SDN) standard that aims to enhance network flexibility by making underlying switches and routers appear as virtual resources.

For OpenFlow, the open standard implementation of SDN, the processor acts as a switching element, with the MIPS core used to decode the OpenFlow commands. Such commands dictate how certain traffic flows are handled and require the creation of a look up table that is accessed using the device's programmable packet parsing hardware.

As for mobile networks, backhaul links are typically 1Gbit/s. "Each LTE [cell] sector is about 400Mbit/s so, with a few sectors, a 1Gbit link is OK," said Pinchas. "LTE Advanced is coming and its data rate will be 3x higher; each sector will be more than 1Gbit/s and there will be a need to migrate the backhaul to 10Gbit."

More than enough capacity
At 40Gbit/s, WinPath4 has more than enough capacity to support a 10Gbit/s Ethernet ring linking cell sites, whilst adding and dropping 10Gbit/s traffic at each cell. The device's full capacity is used for line card designs with two 10Gbit/s network connections and two 10Gbit/s backplane interfaces.

WinPath4 supports networking protocols such as Automatic Protection Switch (APS) and Bidirectional Fault Detection (BFD).

APS is a Carrier Ethernet defined standard that switches traffic to another link in less than 50ms following a networking fault. The protection needs to be implemented at several levels. This can be a single Ethernet pseudowire – the packet based emulation of a point to point connections – carrying data, or several combined in a bundle, known as a section. Sections can further be combined to form a port, says Pinchas.

The protection must work for all these levels, depending on whether the fault is local, involving a single psuedowire, or at the aggregated level. "Protection needs to either move the traffic, or duplicate the traffic," said Pinchas. "If you need to duplicate it at each level, you can end up duplicating it three times [eight times the traffic]."

This is complex and demanding, but is what is needed to achieve the quality of communications which operators are used to with legacy Sonet/SDH networking, he says. Meanwhile, the BFD standard, used to detect faults, is a Layer 3 equivalent to a similar Ethernet scheme.

PMC says it will provide WinPath4 customers with software updates every few months, with added features as various standards evolve. "Some customers also have the [WinPath4] software development kit to add proprietary features," Pinchas concluded. "They do not need to code standard protocols but, rather, focus on coding that differentiates them from their competitors."

Roy Rubenstein

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