Network Evolution Drives the Convergence Trend
Improved access capability of OLTs creates conditions for network flattening. The trunk fiber of a PON has a range of 20 to 40 km, and the point to multipoint architecture of the PON allows the OLT to support large-capacity user access. ZTE's flagship ultra-large capacity OLT, the TITAN, can connect 32,000 users with a single frame and 64,000 users with two frames stacked, capable of replacing multiple OLTs and convergence switches in an existing access office (AO). Integrated with the OTN functionality, the TITAN can vastly cut equipment footprint, reduce device types and power consumption, thereby slashing operators’ Capex and Opex. By integrating transport and access, the TITAN can also deliver the high bandwidth and low latency needed to carry 5G and big video services.
SDN/NFV technologies advance network evolution. A commonly used virtual broadband network gateway (vBNG) solution employs universal servers for control plane virtualization and specialized devices for high-performance forwarding in the forwarding plane. Since the TITAN is based on a high-end distributed router platform, it can provide the capability required by the BNG forwarding plane. While the BNG control plane is cloudified, the BNG forwarding plane can be integrated with the OLT. That eliminates the need to deploy specialized BNG equipment for a forwarding plane, which makes the network flatter and cuts network costs. The integration of the OLT with the BNG also means that the L3 functions are moved to the OLT, meeting the 5G bearing requirement of moving L3 to the edge and laying a foundation for future movement of the CDN into the AO.
Multi-Service Bearing Drives Convergence of Access Network with OTN
The growth of new services, including 5G, big video, VR/AR and high-speed dedicated lines for governments and enterprises, spurs the demand for bandwidth. This trend compels operators to build an end-to-end OTN that covers the access layer, convergence layer and trunk lines. That is where the ZTE TITAN comes in. The TITAN supports built-in OTN uplink to simplify the network layers, as well as optical pass-through to shorten latency, thus helping operators reduce network construction and maintenance costs.
5G has higher requirements for bandwidth, latency, reliability and security, which presents an enormous challenge for the bearer network. With a focus on user experience, 5G must offer data speeds of hundreds of Mbps (even of Gbps order) per user and up to tens of Gbps per base station. The expected super-high access rates requires that the bearer network to provide one or two orders of magnitude more bandwidth. 5G covers application scenarios with varying latency requirements, of which ultra-reliable low latency communication (uRLLC) requires the lowest latency. While NGMN suggests the one-way end-to-end latency for uRLLC should be less than 1 ms, 3GPP defines uRLLC air interface latency as 0.5 ms. Besides, there is latency in core network, and then the remaining latency budget for bearer network is extremely low. These requirements present huge challenges to both 5G fronthaul and middlehaul networks.
The ZTE TITAN optical access platform supports WDM-PON access at the user side, low-latency forwarding in the forwarding plane, and built-in OTN uplink at the network side, enabling the transport of high-bandwidth, low latency services, unified 5G fronthaul, middlehaul and backhaul bearing, and FMC. Connected to the transport network via the OTN interfaces on the uplink network side, the TITAN implements end-to-end mapping of user traffic into OTN timeslots, simplifies service deployment and O&M, and enables end-to-end deployment of large-granularity, low-latency services while ensuring transparency of transmission (fixed latency and no jitters). With a dedicated logical pipe, the TITAN ensures the quality and security of 5G services and provides a low-latency solution.
Big video services not only require high bandwidth but also low packet loss ratios and low latency during transmission. For 4K TV, the latency and the packet loss ratio should not be higher than 20 ms and 0.0168% respectively, which can be realized by optimizing the existing networks. For 8K TV, the latency and the packet loss ratio should not be higher than 16 ms and 0.001% respectively, while the thresholds for VR are 12 ms and 0.001%, which requires adjustments to the existing network architectures and equipment. The CDN is usually deployed centrally for better management and resource utilization. The OLT directly connects to the CDN servers via built-in OTN uplink interfaces to avoid latency and packet loss caused by L2 and L3 forwarding equipment. This scheme can significantly improve user experience, especially for latency-sensitive fast channel switching services and AR/VR services, which are susceptible to latency, packet loss and bandwidth.
Virtualization and Multi-Service Bearing Drive Convergence of Access Network with BNG
Convergence of OLT with BNG Spurred by SDN & NFV
Nowadays network transformation with SDN and NFV is an industry trend. A focus for the future modernization of metro and access networks is to build an access edge that integrates multiple services. BNG virtualization is a hot topic, for which, various approaches have been proposed. Since the forwarding performance of the existing x86 servers is inadequate, separating the control and forwarding planes is a widely accepted solution for the present. This solution uses universal servers to implement virtualization in the control plane and specialized devices to implement high-performance forwarding in the forwarding plane, as shown in Fig. 1. In the figure, the control plane is represented by the vBNG, and it uses an NFV architecture for virtualization. With the decoupling of services from the hardware platform, the vBNG can utilize a universal platform to enable flexibility, convenience, and reduced TCO, and allow dynamic creation of services and opening up of capabilities. In addition, a modular design and a flexible loading mechanism make the software system of the vBNG highly adaptive.
With the cloudification of the control plane, the OLT can converge with the traditional BNG forwarding plane and avoid the deployment of dedicated BRAS hardware for a forwarding plane. This scheme gives full play to TITAN’s fully distributed switching architecture to flatten the network and slash the network upgrade costs. The forwarding plane is centrally controlled through an SDN architecture. An SDN controller, which integrates management and control, and a service orchestrator are deployed to enable end-to-end automatic configuration and an intelligent and open network. The convergence of the OLT with the BNG also means that the L3 functions are moved to the OLT, allowing the bearing of 5G services and the future movement of the CDN into the AO.
L3 to the Edge to Enable 5G Bearing
Compared with 4G, the 5G network architecture will be flatter. Some old as well as new functions (such as mobile edge computing) will be deployed closer to the end users. The existing traffic pattern based on south-north convergence will undergo a major change. East-west traffic between different 5G network elements, for instances, between gNBs and between a CU/DU, will increase drastically. East-west traffic is more sensitive to latency, and should be forwarded at a location as close to the access layer as possible. That puts new demands on the east-west traffic scheduling of the backhaul or even the fronthaul and middlehaul networks, which can be met when the OLT provides flexible L3 connectivity. With L3 capabilities centrally managed and controlled via SDN, coordinated traffic across base stations can be forwarded close to the base stations to maximize synergy gain, and on-demand, real-time connections between wireless and core networks can be set up. By introducing new technologies like Segment Routing/EVPN/VXLAN, the network protocols and O&M can be simplified to better cope with future data-center-focused network architecture evolutions.
L3 to the Edge to Enable Movement of CDN into the AO
Deploying the CDN in a centralized way allows resources to be efficiently managed, maintained and utilized. However, it also has many drawbacks, including more traffic pressure on the convergence and metro networks, higher latency, and increased packet loss in the event of network congestion. These drawbacks hinder the high-quality transmission of big video services, especially latency-sensitive fast channel switching services as well as AR/VR services that can be affected by latency, packet loss, and bandwidth. If the CDN is moved to the AO, IPTV and OTT video services can be delivered from locations close to the end users to greatly improve service quality.
ZTE's TITAN optical access platform, based on a high-end distributed router architecture, supports large-capacity user access, convergence switch functions, built-in OTN boards, and convergence of the BNG forwarding plane with the OLT to realize network equipment integration and network flattening. With the built-in OTN boards, the TITAN can deliver the high bandwidth and low latency needed to transport 5G and big video services. With the convergence of the BNG forwarding plane with the OLT, the L3 functions can be moved to the OLT, which is also needed by 5G and big video services.
[Keywords] OTN, BNG, flat network , convergence, multi-service bearing, virtualization,