Mobile backhaul chips rise to the LTE challenge

The Long Term Evolution (LTE) cellular standard has a demanding set of mobile backhaul requirements. Gazettabyte looks at two different chip designs for LTE mobile backhaul, from PMC-Sierra and from Broadcom.

"Each [LTE Advanced cell] sector will be over 1 Gig and there will be a need to migrate the backhaul to 10 Gig"

Liviu Pinchas, PMC-Sierra

LTE is placing new demands on the mobile backhaul network. The standard, with its use of macro and small cells, increases the number of network end points, while the more efficient bandwidth usage of LTE is driving strong mobile traffic growth. Smartphone mobile data traffic is forecast to grow by a factor of 19 globally from 2012 to 2017, a compound annual growth rate of 81 percent, according to Cisco's visual networking index global mobile data traffic forecast.

Mobile networks backhaul links are typically 1 Gigabit. The advent of LTE does not require an automatic upgrade since each LTE cell sector is about 400Mbps, such that with several sectors, the 1 Gigabit Ethernet (GbE) link is sufficient. But as the standard evolves to LTE Advanced, the data rate will be 3x higher. "Each sector will be over 1 Gig and there will be a need to migrate the backhaul to 10 Gig," says Liviu Pinchas, director of technical marketing at PMC.

One example of LTE's more demanding networking requirements is the need for Layer 3 addressing and routing rather than just Layer 2 Ethernet. LTE base stations, known as eNodeBs, must be linked to their neighbours for call handover between radio cells. To do this efficiently requires IP (IPv6), according to PMC.

The chip makers must also take into account system design considerations.

Equipment manufacturers make several systems for the various backhaul media that are used: microwave, digital subscriber line (DSL) and fibre. The vendors would like common silicon and software that can be used for the various platforms.

Broadcom highlights how reducing the board space used is another important design goal, given that backhaul chips are now being deployed in small cells. An integrated design reduces the total integrated circuits (ICs) needed on a card.  A power-efficient chip is also important due to thermal constraints and the limited power available at certain sites.

"Integration itself improves system-level power efficiency," says Nick Kucharewski, senior director for Broadcom’s infrastructure and networking group. "We have taken several external components and integrated them in one device."

WinPath4

PMC's WinPath4 supports existing 2G and 3G backhaul requirements, as well as LTE small and macro cells.  A cell-side routers that previously served one macrocell will now have to serve one macrocell and up to 10 small cells, says PMC. This means everything is scaled up: a larger routing table, more users and more services.

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

The previous generation 10Gbps WinPath3 has up to 12 WinGines, WinGines are multi-threaded processors, with each thread involving packet processing. Tasks performed include receiving, classifying, modifying, shaping and transmitting a packet.

The 40Gbps WinPath4 uses 48 WinGines and micro-programmable hardware accelerators for such tasks as packet parsing, packet header extraction and traffic matching, tasks too processing-intensive for the WinGines.

WinPath4 also support tables with up to two million 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 IEEE 1588v2 timing protocol.

Two MIPs processor core are used for the control tasks, such as setting up and removing connections.

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 use for SDN, the processor acts as a switching element with the MIPS core used to decode the OpenFlow commands.

StrataXGS BCM56450

Broadcom says its latest device, the BCM56450, will support the transition from 1GbE to 10GbE backhaul links, and the greater number of cells needed for LTE. 

The BCM56450 will be used in what Broadcom calls the pre-aggregation network. This is a first level of aggregation in the wireline network that connects the radio access network's macro and small cells.

Pre-aggregation connects to the aggregation network, defined by  Broadcom as having 10GbE uplinks and 1GbE downlinks. The BCM56450 meets these requirements but is referred as a pre-aggregating device since it also supports slower links such as microwave links or Fast Ethernet.

The BCM56450 is a follow-on to Broadcom's 56440 device announced two years ago. The BCM56450 upgrades the switching capacity to 100 Gigabit and doubles the size of the Layer 2 and Layer 3 forwarding tables.

The BCM56450 is one of a family of devices offering aggregation, from the edge through to 100GbE links deep in the network.

The network edge BCM56240 has 1GbE links and is designed for small cell applications, microwave units and small outdoor units. The 56450 is next in terms of capacity, aggregating the uplinks from the 240 device or linking directly to the backhaul end points.

The uplinks of the 56450 are 10GbE interfaces and these can be interfaced to the third family member, the BCM56540. The 56540, announced half a year ago, supports 10GbE downlinks and up to 40GbE uplinks.

The largest device, the BCM56640, used in large aggregation platforms takes 10GbE and 40GbE inputs and has the option for 100GbE uplinks for subsequent optical transport or routing. The 56640 is classed as a broadband aggregation device rather than just for mobile.

Features of the BCM56450 include support for MPLS (MultiProtocol Label Switching) and Ethernet OAM (operations, administration and maintenance), QoS and hardware protection switching. OAM performs such tasks as checking the link for faults, as well as performing link delay and packet loss measurements. This enables service providers to monitor the network's links quality. The device also supports the 1588 timing protocol used to synchronise the cell sites.

Another chip feature is sub-channelisation over Ethernet that allows the multiplexing of many end points into an Ethernet link. "We can support a higher number of downlinks than we have physical serdes on the device by multiplexing the ports in this way," says Kucharewski.

The on-chip traffic manager can also use additional, external memory if increasing the system's packet buffering size is needed. Additional buffering is typically required when a 10GbE interface's traffic is streamed to lower speed 1GbE or a Fast Ethernet port, or when the traffic manager is shaping multiple queues that are scheduled out of a lower speed port. 

The BCM56450 integrates a dual-core ARM Cortex-A9 processor to configure and control the Ethernet switch and run the control plane software. The chip also has 10GbE serdes enabling the direct interfacing to optical transceivers.

Analysis

The differing nature of the two devices - the WinPath4 is a programmable chip whereas Broadcom's is a configurable Ethernet switch - means that the WinPath4 is more flexible. However, the greater throughput of the BCM56450 - at 100Gbps - makes it more suited to Carrier Ethernet switch router platforms. So says Jag Bolaria, a senior analyst at The Linley Group.

The WinPath4 also supports legacy T1/E1 TDM traffic whereas Broadcom's BCM56450 supports Ethernet backhaul only 

The Linley Group also argues that the WinPath4 is more attractive for backhaul designers needing SDN OpenFlow support, given the chip's programmability and larger forwarding tables. 

The WinPath4 and the BCM56450 are available in sample form. Both devices are expected to be generally available during the first half of 2014.

Further reading:

A more detailed piece on the WinPath4 and its protocol support is in New Electronics. Click here

The Linley Group: Networking Report, "Broadcom focuses on mobile backhaul", July 22nd, 2013. Click here (subscription is required)