Status and Challenges of CDN
Although the internet is ubiquitous now in people's life and work after decades of high-speed development, content access has always been one of its primary functions. Increases in content types, video bitrates and terminal types all lead to rises in content traffic. Consequently, the bottleneck of core networks becomes more severe and degrades user experience. This gives rise to a content delivery network (CDN) that can improve quality of experience (QoE), reduce backbone network traffic and properly distribute content traffic. By directing user requests to suitable service nodes through content delivery and service scheduling, CDN provides distributed content service.Depending on the content they deliver, CDNs fall into two categories: specialized CDNs and converged CDNs. A specialized CDN delivers a specific type of content. For example, an IPTV CDN specifically carries IPTV video content. A converged CDN can deliver multiple types of content and support many different terminal types. Operators mostly choose the converged CDN in their construction.
Currently operators run a converged CDN at the provincial level. The CDN employs a two-level architecture that comprises a provincial-level central node and city-level edge nodes (Fig. 1). In few large cities that include high-traffic districts, the edge nodes have moved closer to the broadband network gateway/service router (BNG/SR) level, forming a three-level CDN architecture.
The above architecture has both advantages and disadvantages. It can significantly cut backbone network traffic and long-distance (from prefecture-level city to provincial capital) transmission cost. Nodes at both levels can be massively deployed with a mature construction solution. The nodes can be co-located with edge and provincial data centers (DCs) to share a computing and storage infrastructure. However, the architecture also has obvious disadvantages. First, edge nodes are placed far from end users. As a result, many hops are needed in a service path, QoE is difficult to assure, and fault location and removal is complex. Second, the network segments from OLT to BNG and from BNG to boundary router (BR) tend to become bottlenecks as service traffic surges. Finally, this architecture does not leverage the low-cost and abundant-bandwidth advantages of an optical access network. The disadvantages will become ever more noticeable as 4K/8K/AR/VR video content become ubiquitous.
With the emergence of edge computing and the introduction of SDN and NFV, it has become a trend to introduce NFV infrastructure (NFVI) in the access office. If the NFVI in the access office can be fully utilized so that converged CDN services can be deployed near end users and a three-level CDN architecture can be built, the disadvantages mentioned above in the two-level CDN architecture can be effectively avoided. This article is intended to discuss this approach. By introducing a new converged CDN solution based on ZTE’s TITAN platform, the article gives good thought and suggestions for operators to deploy converged CDNs in their access offices.
Introducing A-CDN to Build a Three-Level Converged CDN Architecture
In hardware design, the blade servers employ a powerful SoC-based CPU, NVMe SSD drives, and 10G network ports. Each blade server occupies two service slots and can be inserted in any slot in the TITAN OLT. In software design, the embedded blade servers run on ZTE’s proprietary CDN software. Evaluation and verification show that the blade servers can deliver a converged CDN service capacity of 20 Gbps and provide 70% of the CDN service for 20,000 users in the same access office. This vastly reduces OLT’s uplink bandwidth of the converged and metro networks and delays the need for chain network upgrade. To suit the A-CDN scenario, ZTE's CDN software system has been optimized in storage and content.● A-CDN storage optimization: Store three types of most broadcast content including three-hour live-channel time-shifted TV programs, TVOD programs in a week, and 5,000 movies that are currently broadcast most. A-CDN has a total storage capacity of less than 16 Tbps but can cover 70% of the VOD service.
[Keywords] converged CDN, access office, A-CDN, TITAN