Transport is the first concern for 5G commercial deployment. As further research is done in 5G wireless standards, it is pressing to carry out research into 5G transport standards. At the June 2017 plenary meeting of ITU-T SG15, Chinese enterprises and organizations including China Mobile, ZTE, and CAICT submitted several 5G contributions related to 5G transport, actively promoting the research on 5G transport standards. Experts at the meeting agreed that a transport network meeting 5G requirements was a very important new topic, and at the summary meeting, they officially approved the initiation of a technical report Transport Network Support of IMT-2020/5G (GSTR-TN5G), and the research plan for 5G transport standards. This marked a key step towards 5G transport research in ITU-T and also a major contribution made by Chinese enterprises to the research.
The research on 5G transport technologies and standards involve several standards organizations such as ITU-T, IEEE, OIF, and IETF. ITU-T SG15 focuses on 5G transport requirements and solutions, IEEE 802.1 TSN on 5G time-sensitive network requirements and solutions, IEEE 1914 Working Group on 5G fronthaul requirements, network architecture, and data encapsulation and mapping, OIF on FlexE interfaces and links, and IETF Segment Routing on simplifying MPLS TE signaling protocols for network traffic engineering and rapid protection switching.
At the meeting held in January 2015, OIF officially launched the FlexE project, aiming to expand capabilities of standard Ethernet interfaces in three aspects:
● Bonding: Multiple standard Ethernet interfaces are bonded to transport higher-rate traffic.
● Channelization: Multiple Ethernet data streams at any bit rates are multiplexed for transport through a standard Ethernet interface.
● Sub-rate: Sub-rate emphasizes transporting services at rates lower than Ethernet PHY rates.
In November 2016, OIF started research into FlexE 2.0 that involves support of FlexE groups composed of 200 Gb/s and 400 Gb/s Ethernet PHY and transport of frequency or time by the FlexE group. ZTE actively participated in the research of FlexE 2.0 and submitted several contributions at the OIF meetings. During the OIF meetings, ZTE submitted multiple contributions concerning the requirements of using FlexE technologies for 5G transport. As discussed at the meeting, if these requirements are reasonable, different projects may be started to standardize these technologies.
At the ITU-T SG15 plenary meeting held in June 2017, ZTE was the first in the insustry to submit a contribution FlexE Layer Network Model. The contribution creatively extends the FlexE technology from link to network, which includes 66B path layer and 66B section layer. The former implements 66B cross connection, client service OAM insertion and extraction, and protection, while the latter is identical to FlexE 1.0 defined in OIF and provides rate adaption, section-layer OAM insertion and extraction, and multiplexing and demultiplexing.
Ultra-High Accuracy Time Synchronization
For ultra-high accuracy time synchronization, the timing performance of enhanced primary reference time clock (ePRTC, G.8272.1), enhanced reference clock (eEEC, G.811.1), and enhanced synchronous Ethernet equipment slave clock (eEEC, G.8262.1) is carried out, which involves frequency precision, noise generation, noise tolerance (wander and jitter tolerance), noise transfer, transient response and holdon performance, and interface requirements. The development of LTE-A, CoMP, accurate location service for base stations, and future 5G has raised higher requirements for precision synchronization (100 ns for an end-to-end synchronization requirement). In PRTC lock mode, requirements for time error and wander tolerance are even stricter and the time error should be within 30 ns (max|TE|) or smaller. The PRC frequency precision (longer than a week) increases from 10-11 to 10-13. More effort will be put in the research on partial timing support, SyncE, new time synchronization architecture definition (including 5G transport and fronthaul networks), and synchronous OAM and management.
IEEE1914.1 was started in February 2016 and will be completed in December 2018. The research focuses on transport architecture for mobile fronthaul services including user services, and management and control plane services. The research also involves fronthaul network definition and requirements including data rates, timing, synchronization, and QoS. The 0.3 draft version has been completed, and the follow-up research will focus on OAM, latency, network management, and convergence requirements.
Time-sensitive networking (TSN) was originated from the former IEEE802.1 audio/video bridging (AVB) project. The target of TSN TG is to provide determinate services through the Ethernet, for example, to ensure low latency and jitter and very few packet loss.
● Frame preemption: 802.3br and 802.1Qbu are used to divide low-priority data that can be preempted into smaller segments, so that high-priority data can be handled and transmitted before low-priority data. This ensures a faster transmission path.
● Frame replication and elimination: 802.1CB is used to guarantee frame loss rate and ensure that a copy of key traffic can be transported in disjoint paths of the network and two pieces of incoming data can be merged and deleted for seamless redundancy.
● Stream reservation protocol (SRP) enhancement: 802.1Qcc is used to configure TSN traffic, providing more enhanced functions than the original SRP. The TSN traffic can be configured in fully distributed mode, fully centralized mode, network centralized mode, and user distributed mode.
The IETF Segment Routing working group was established in September 2013, responsible for the research on simplifying MPLS traffic engineering and signaling protocols. Segment IDs are advertised through IGP to construct specified forwarding paths for traffic engineering and fast switching. The Segment Routing data plane uses two formats: MPLS label stack and IPv6, which are compatible with existing MPLS and IPv6 networks. Segment Routing, being studied currently, is regarded as the basic network technology for 5G network transport and slicing.
Standards organizations are carrying out in-depth research on 5G transport standards in terms of low latency, large bandwidth, huge connectivity, physical isolation, network slicing, and high-accuracy time synchronization. The research will definitely provide strong technical guidance and reliable technical guarantee for commercial deployment of future 5G transport networks.
Standardization, 5G transport, FlexE, Time-sensitive networking, Segment routing, Ultra-high accuracy time synchronization