1 AVIAT NETWORKS FEBRUARY 2011

WHITE PAPER

RAN CAPACITY OPTIMIZATION THROUGH IPoTDM MICROWAVE TRANSPORT

EXECUTIVE SUMMARY Mobile operators are facing the daunting challenge of preserving legacy TDM assets and existing revenue streams while simultaneously accommodating the divergent demands of existing 2G networks and those of emerging 3G/4G technologies. As these operators roll out new IP services as part of their offerings, selecting and deploying the right Radio Access Network (RAN) and backhaul migration approach is becoming an increasingly pivotal part of their overall strategy. Gradual migration towards an all-packet network necessitates cost-effective interim steps that avoid forklift infrastructure upgrades and network disruptions.

This brief paper summarizes how an IP-over-TDM (IPoTDM) approach can facilitate this evolution, consolidate the transport and management of circuit- and packet-based services in a single physical RAN infrastructure, and allow operators to address CAPEX and OPEX cost challenges by leveraging existing TDM transport resources to the maximum extent possible.

THE CASE FOR IPoTDM Generally, the mobile backhaul network can be divided into two segments: from the NodeB or Base Transceiver Station (BTS) to a hub site or aggregation point; and from the aggregation point to the Radio Network Controller (RNC) or Base Station Controller (BSC). The benefits of employing an IP/Ethernet-based approach between the aggregation point and the RNC/BSC are many and well understood, ranging from the inherent efficiencies of statistical multiplexing and the reduction of cost per bit to the scalability and future-proofing of the infrastructure. However, sound business reasons for a different approach in the last RAN segment to the NodeB/BTS may persist. TDM infrastructure is still ubiquitous in rural areas, and the complexity and added CAPEX pressure of replacing all of the RAN equipment may neither be economically viable nor necessary.

Fortunately, there are practicable technical solutions for reaching and augmenting these remote locations with IPoTDM. To support the co-location of a 3G NodeB with a 2G BTS, both IPoTDM and Ethernet-over-TDM (EoTDM) techniques are available today. Both techniques offer the ability to share existing transmission resources with 3G, to utilize spare transmission resources where available, and to flexibly add new circuits as more bandwidth is needed. Leveraging these existing TDM based RAN resources affords a high degree of investment protection for operators, utilizes available TDM capacity, and results in a comparably simpler architecture than one based on IP as the foundation in the RAN and combined with pseudowires to support TDM traffic.

Whereas EoTDM enables the transport of native Ethernet frames over E1/DS-1, IPoTDM avoids the additional overhead induced by Ethernet frame headers, thus making more efficient use of the available bandwidth. Both concepts are usually based on the well-established Multilink Point-to-Point Protocol (ML-PPP) for link layer encapsulation, adding only 6 to 15 Bytes of overhead in case of IPoTDM versus up to 37 Bytes for EoTDM. As a result, IPoTDM provides significant efficiency gains and reduction in overhead tax, especially for applications that entail short Protocol Data Unit (PDU) sizes.

2 AVIAT NETWORKS FEBRUARY 2011

WHITE PAPER

RAN CAPACITY OPTIMIZATION THROUGH IPoTDM MICROWAVE TRANSPORT

ENTER ML-PPP ML-PPP is defined in RFC 1990, and constructed as a PPP extension to allow for the combining of multiple individual PPP links (dubbed “bonding”) into a single logical bundle. Each individual physical link is controlled by the PPP protocol, with ML-PPP residing between the data link and the network protocol layer and serving as the aggregation entity. As a result, multiple physical circuits or channels can be bonded to form one higher capacity virtual circuit, and bandwidth can be flexibly scaled in E1/DS-1 increments.

ML-PPP bundle membership negotiation offers an important tactical tool in that it can be used to change the aggregate capacity via addition or removal of individual PPP links and for purposes of designing resilience into the network. Additionally, operators can manage converged transport of legacy and adjunct IP traffic in a way that allows the allocated ML-PPP capacity to be adjusted as IP bandwidth demands rise. Lastly, ML-PPP can be employed in conjunction with RFC 2686 multi-class extensions, providing the ability to prioritize traffic. Packets waiting to be transmitted over the ML-PPP tunnel are sorted based on priority, with suspension of transmission of low-priority fragments until all high-priority fragments have been first serviced.

CONSIDERATIONS FOR IMPLEMENTATION OVER MICROWAVE Once a multilink session has been successfully negotiated by the Link Control Protocol (LCP), ML-PPP can fragment incoming PDUs according to the available capacity and transport these fragments in parallel across the physical links or circuits. On the receiving end, the fragments are properly sequenced based on their PPP headers, and reconstructed to the original PDU format. The latency is thus reduced to the amount of time required for the last PDU fragment to reach the far side of the link, resulting in substantially decreased latency relative to the sequential transmission of PDUs over any of the individual links.

The use of fragmentation, Ethernet framing overhead removal, and IPv4/v6 header compression techniques as defined in RFC 2507 and RFC 2509 are crucial considerations for microwave transport, both from the perspective of latency and capacity optimization. A small unfragmented PDU is encapsulated with a PPP header and a ML-PPP header, whereas when fragmentation is enabled, all but the first fragment require only the PPP header for transmission. When properly optimized, operators can realize dramatic improvements in effective microwave capacity of up to 40%.

As an example, a GSM codec may generate 33 Bytes of payload, encapsulated in 64 Byte Ethernet frames at the data link layer, and with the IPv4 header consuming 20 Bytes. As a result of the overhead reduction achieved through the IPoTDM approach, a TDM microwave with an actual transport capacity of 64 Mbps (32 x E1) can now carry the equivalent of approximately 90 Mbps of uncompressed voice traffic load.

At the same time, the latency reduction achieved through fragmentation of longer PDUs will benefit delay-sensitive voice and synchronization packets.

If packet-based synchronization (Precision Time Protocol/IEEE 1588v2) is neither desired nor required by the NodeB, operators can continue to utilize the traditional E1/DS-1 line synchronization method, as timing is maintained underneath the PPP/ML-PPP layers. As a result, the IPoTDM architecture is not subject to synchronization discontinuities, and its introduction can be decoupled from the challenges in migrating to new synchronization technologies.

3 AVIAT NETWORKS FEBRUARY 2011

WHITE PAPER

RAN CAPACITY OPTIMIZATION THROUGH IPoTDM MICROWAVE TRANSPORT

TYPICAL MIGRATION SCENARIO To support transmission over E1/DS-1 circuits using ML-PPP, the ML-PPP bundle is typically terminated at the cell site via a Cell Site Router (CSR) or NodeB, and ML-PPP tunnels are established between the CSR or NodeB and hub site, where the tunnels are terminated and the IP packets subsequently extracted. If packet microwave is used for backhaul from the aggregation point, a modular system equipped with IPoTDM-capable interface cards can natively merge the bonded E1/DS-1 tributaries, thereby eliminating the need for a separate aggregation router. Upon reconstruction of the received IP packets, traffic is then forwarded to the RNC/BSC across packet microwave, in the original format.

Figure 1: Bonded E1/DS1 tributaries, between CSR/NodeB and hub site

CONCLUSIONS Microwave will continue to dominate mobile backhaul deployments for years to come; however, the optimal phasing of the TDM migration strategy is not universally expressed, but rather a function of mobile operator objectives, existing assets, available capacity, scalability, cost of operation, and other factors. Aviat Networks offers multiple technology options to serve these varied needs, including IPoTDM as described in this paper. In summary, operators stand to realize the following benefits from it:

• Because IPoTDM leverages existing TDM technology, equipment already in service as well as domain knowledge can continue to be exploited

• Where TDM based microwave is predominant, operators can significantly improve the effective RAN capacity as an interim step, while still allowing for the benefits of packet microwave to be employed at aggregation sites.

• The IPoTDM approach enables faster and less disruptive capacity expansion than a forklift upgrade of the TDM infrastructure.

• Fewer routers are deployed in the RAN, resulting in a simplified and less costly network architecture.

• Mature and standards-based technology minimizes interoperability risks between multiple equipment vendors.

WWW.AVIATNETWORKS.COM Aviat, Aviat Networks, and Aviat logo are trademarks or registered trademarks of Aviat Networks, Inc. © Aviat Networks, Inc. 2011. All Rights Reserved