“Getting to the Next G”- Chris Anderton for The CCA Voice (November 7,2012)
Getting to the Next Generation end user means ensuring carrier networks are upgraded to “Next Gen” to handle the explosive growth from data services on smart mobile devices. These devices have fueled the demand for digital content by mobile end users, resulting in signiﬁcantly higher bandwidth requirements on the back-haul services.
Currently the majority of wireless cellular backhaul is carried over leased private lines. Voice traﬃc typically uses 8-12 T1s on a standard multi-user cell tower, but as we move to Next Gen networks the data bandwidth requirement will grow by over 500% and the number of towers being connected will also grow by almost 50% over the next several years. This growth is being fueled almost exclusively by data traﬃc with voice being carried almost as a second thought. Integrating the voice circuits on to the packet based backbone, while reducing the bandwidth that has to be allocated to voice circuits will make cellular networks simpler to install as well as increasing net-work eﬃciency and reducing costs. This can be achieved by grooming T1s from separate remote cell sites onto a single network and through the development of business models that are based on the advantages of Ethernet as a converged packet network. TDMoE presents wireless service providers with an opportunity to deploy new data centric services while continuing to support the traditional, legacy voice customers. However there are technical issues that need to be considered deploying TDMoE. The major issues result from trying to replicate a Constant Bit Rate Service (CBR) over a Variable Bit Rate (VBR) Ethernet net-work. Ethernet is a “best eﬀort” technology and the inherent latency, errant, and lost packets have a detrimental eﬀect on the synchronization of TDM services.
- Clock synchronization is key to maintaining the high quality of service required to support TDM based services. In a TDM network all the equipment is synchronized using a speciﬁed clock rate either from an internal or external clock source, and all devices maintain synchronization with this clock. Out of sync clocks result in buffer overﬂow and lost frames. TDMoE network devices require synchronization that is able to cope with latency, packet delay variation and packet loss, and still maintain the strict clocking requirements.
- Packetization takes TDM traﬃc and puts it into Ethernet packets. The delay has to be minimal and constant.
- Latency affects voice quality (results in echo) on the backhaul. Quality of Service (QoS) capabilities in to today’s Ethernet networks allow latency mitigation.
- Ethernet’s way of dealing with congestion or over subscription is to discard packets and have the lost packets resent. Voice and TDM services have no concept of resending missing packets and frames are lost if they do not arrive within the window of the jitter buﬀer. Resequencing of frames arriving within the window are handled by the receiving device reading and sorting sequence numbers within the headers. Careful design of the back-bone network with particular regard to QoS and bandwidth control set-tings minimize packet loss situations.
- Fixed, known delays are relatively easy to compensate for, however variable delays can cause unexpected degradation of services. Packet Delay Variation due to a combination of the variable packet sizes and Ethernet’s asynchronous nature, especially when carrying a range of network traffic can be signiﬁcant and have an adverse eﬀect on TDM services being carried. Adjusting the amount of jitter buﬀering can minimize, or in some cases neutralize the eﬀects of Fixed Delay Variation. However increasing jitter buﬀering increases the latency.
Working with a partner to provide a wide range of solutions that oﬀer support for TDMoE services is essential to accomplish the Next Gen network. All of these solutions must address the timing/synchronization issues detailed above, providing cost eﬀective choices to transport E1/T1 TDM signals over the Ethernet network, with ﬂexible and expandable solutions supporting 4, 8, 12, or 16 T1 channels, 10/100/1000 BaseT connections handle the Ethernet traﬃc. The WAN connection consists of two Gigabit Ethernet (GbE) uplinks. Solutions must also preserve timing and both jitter and wander must adhere to G.823 Traﬃc and G.823 Synchronous speciﬁcations.
Chris Anderton works at Tempest Telecom Solutions, Engineered Solutions team, and specializes in architecting solutions to help carriers migrate from TDM to IP based networks. Tempest is a leading provider of infrastructure equipment and related services, helping carriers build, expand and maintain their networks faster and more cost-eﬀectively.