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January 2007
Volume 10 / Number 1

Best Practices in Designing a Carrier- Grade Metro Ethernet Network for Triple Play Services

By Ilan Ofer


Carrier Ethernet has established itself as the preferred basic technology in metropolitan networks designed to deliver IPTV and the entire triple play package to residential customers. This is due to the low capital expense per bandwidth, high capacity platforms, and the low operating expenses resulting from the ubiquity of Ethernet, as well as, with a well-designed Carrier Ethernet solution, the ability of the technology to address the many challenges in deploying network infrastructure for triple play services.


Addressing the scalability challenges

Bandwidth scalability. The introduction of rich-media services, such as Broadcast TV (BTV), Video on Demand (VoD) and multiplayer games, gives rise to unprecedented scalability requirements that may result in a need for a revitalization of network infrastructures and operations. A simple calculation of bandwidth requirements shows that each subscriber will need at least 25Mb/s. A typical metro area aggregation network of about 50 thousand subscribers is likely to require at least 25Gb/s of bandwidth.

VoD traffic constitutes the largest component in the overall bandwidth due to its unicast nature and high bandwidth per stream. It is also the most unpredictable in terms of concurrency and total bandwidth requirements since it is dependent on subscriber behavior and is a relatively new service with which carriers have little experience. Moreover, VoD may be used as an umbrella term for a number of unicast services, such as Network-based Personal Video Recorder (NPVR). It is, therefore, likely that VoD services will grow significantly beyond the 10% concurrency rate currently assumed.

The above bandwidth requirements may push carriers to challenge the traditional single-edge approach inherited from early broadband aggregation networks built for the delivery of high speed Internet. In this single-edge approach, all services attributed to one regional aggregation network flow through a single Broadband Remote Access Server (BRAS), making it a potential bottleneck. This concern may be addressed by using new high-capacity systems, thus avoiding re-engineering of the network topology. Alternately, carriers may choose the multiple-edge approach, where the metro core aggregation node connects to service-specific edge routers, each dedicated to a different service.

Both approaches are valid. Nevertheless, the following must be considered: in the single-edge approach, the BRAS performs centralized policy enforcement and QoS, centralized subscriber management, and centralized multicast. The multiple-edge method requires that policy enforcement, including QoS and Security, be distributed to the aggregation switches.

Similarly, multicasting is distributed to the metro and access and not simplistically centralized in one BRAS.

While Carrier Ethernet is the best technology to keep low total cost of ownership, it faces scalability challenges that call for innovative solutions. VLAN scalability. Addressing the limited VLAN space requires a degree of sophistication and flexibility in manipulating the VLAN scheme. The VLAN scheme comprises the mapping of the different subscribers and services to the VLAN tags used to represent them in different segments of the network.

VLAN manipulation is the tool used to create the VLAN scheme and includes:

  • VLAN stacking (or “Q-in-Q”): Addition of an outer VLAN to the inner one (e.g. in accordance with the port or inner VLAN). Stacking increases the VLAN range from 4K to 16M,
  • VLAN translation: Changing the value of an outer and/or inner VLAN,
  • VLAN swapping: Swapping between the outer and the inner VLAN,
  • VLAN stripping: Removing a VLAN tag,
  • Any combinations of the above. The resulting VLAN scheme must allow the network to distinguish between the various services, to perform the forwarding task correctly, and to uniquely identify subscribers enabling proper subscriber management at the BRAS. There are two approaches for connecting broadband users to the aggregation network:
  • VLAN per subscriber: This approach centers on the subscriber. An introduction of a new service would not require a new VLAN configuration.
  • VLAN per service: based on servicespecific VLANs in the aggregation network. The basic goal is to optimize the delivery of different services through the access and aggregation networks.
  • A good alternative is a combination of both which embodies the best of both worlds. In this alternative, services destined for the BRAS maintain the VLAN per subscriber approach in the metro to allow subscriber identification at the BRAS. However, broadcast TV and IPTV control traffic (e.g., IGMP control messages) are transported using a single broadcast TV traffic. In this scheme, a VLAN per service approach can still be used at the access to allow generic configuration of the access devices, easy forwarding, and per subscriber policy enforcement. All these benefits contribute to simplifying the functionality of the most mass deployed and hence most expensive part of the network — the access devices.

MAC scalability. Connection-oriented Ethernet is a technology that allows carriers to fully serve any number of subscribers in the metropolitan network, while enjoying the traffic engineering and high availability attributes typical to deterministic connection oriented technologies. This concept can be considered an extrapolation of the time slot switching concept of TDM, or VPI/VCI switching in ATM.

VLAN Cross-connect is a leading connection oriented Ethernet technologies (alongside PBT and T-MPLS). Each connection is identified by its inner and outer VLAN tags and its port. MAC learning is disabled and thus the forwarding decision is based on the port and VLAN tags. Connection-oriented Ethernet is fully interoperable with connectionless Ethernet; the two schemes can be deployed over the same node and even on a single port. This type of cross connect is most appropriate for point-to-point services and may be used for High-Speed Internet (HSI) or point-to-point VPNs.


Addressing both efficient multicasting AND quick zapping time

Clearly, there is a trade-off between bandwidth efficiency and zapping time, which is an important element of user satisfaction. Delivering all channels to all subscribers would indiscriminately consume scarce bandwidth. Conversely, distributing channels only to interested subscribers requires time consuming intelligence, and induces delays.

We believe that an ideal compromise is the Mixed approach: The carrier assigns a subset of the TV channels, consisting of the most popular channels, over a static configuration. This way, zapping time for those channels is minimized. Other channels, which are either intended for special interests, or more dynamic by nature (e.g., a pay-per-view program), can be provided over a dynamically assigned multicast stream in order to minimize bandwidth utilization. IGMP is the protocol typically used for controlling the multicast delivery tree. The most recent version of IGMP, version 3, includes inherent mechanisms for further reducing the zapping time.


Guaranteeing a high level of user satisfaction

End customers will not tolerate degradation in quality compared with the satellite or cable TV service they have become accustomed to. It is, therefore, of paramount importance to meet the stringent delay and jitter requirements of video and voice services.

Traditionally, Ethernet is perceived as problematic in providing a differentiated and guaranteed QoS. Ethernet was designed as a connectionless technology. The difficulty in predicting the path taken by a given packet stream makes traffic engineering, aimed at guaranteeing QoS parameters such as bandwidth, delay, and jitter challenging. One possible solution is employing connection oriented Ethernet. Alternatively enhanced queuing schemes combined with a network management-based Connection Admission Control (CAC) mechanism which predicts network behaviour and regulates network provisioning, can ensure that network resources (such as bandwidth) are reserved and service quality is guaranteed.

Hierarchical QoS is a further enhancement which supports both port and subscriber level scheduling. In addition it allows shaping on a per-port, per-subscriber and per-service level. In some cases this may be the responsibility of the BRAS, but in scenarios where the BRAS is offloaded, it is up to the aggregation switch to perform this task. Both shaping and scheduling can be fine tuned according to the actual bandwidth available to each subscriber and according to the service bundle the subscriber purchased. Since this bandwidth may change with time, depending on physical conditions, an accurate QoS mechanism should be aware of this parameter. Access Node Control Protocol (ANCP) is typically used by the DSLAM to notify the BRAS of DSL link parameters. By snooping or teminating such messages, the aggregation switch can retrieve bandwidth information pertaining to the DSL link and adjusts scheduling and shaping accordingly.



The main challenges encountered in the design of an IPTV networks in the IPTV context can be effectively addressed by Carrier Ethernet

  • The difficulties in significantly scaling up networks originally designed for best-effort high-speed internet are overcome by advanced Ethernet tools such as advanced VLAN manipulation, VLAN Cross-Connect, QoS and multicasting techniques, combined with an intelligent and cost-efficient hybrid Ethernet-MPLS network design. These tools do away with the scalability limitations inherent in Ethernet.
  • Multicasting can be performed efficiently while reducing channel switching (zapping) time through the right balance between static and dynamic multicasting, sa well as implementing IGMPv3.
  • Guaranteed and differentiated Quality of Service can be provided with the Advanced Ethernet Package, enhancing each specific customer’s user experience, in accordance with the product bundle paid for by that customer.

Ilan Ofer is Senior Product Marketing Manager, Access Networks, Siemens Communications (news - alert) (


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