eCPRI Protocol Stack and Layers Explained

This document describes the eCPRI protocol stack and its various layers. The stack details all protocol layers used in the eCPRI interface when carried over Ethernet/IP networks.

Introduction to eCPRI

• eCPRI stands for Enhanced Common Public Radio Interface.
• The core concept involves dividing Base Station (BS) functionalities into two blocks:
  • REC (Radio Equipment Control)
  • RE (Radio Equipment) These blocks are connected via a packet-based fronthaul transport network, such as Ethernet or IP. This architecture is illustrated in Figure 1.

Figure 1: eCPRI Fronthaul Connection

While the terms REC and RE are used with CPRI, in eCPRI, they are referred to as eREC and eRE, respectively.

• eCPRI manages the following service points:
  • User Plane Traffic
  • Synchronization
  • Control & Management
• eCPRI can be deployed between single eRECs and eREs, as well as multiple eRECs/eREs.
• It’s designed for 3GPP 4G (LTE) and 5G (NR) radio base stations.
• The eREC and eRE are physically separated, with the eRE positioned closer to the antenna and the eREC located further away.

eCPRI Protocol Stack

Figure 2: eCPRI Protocol Stack

Figure 2 illustrates the eCPRI protocol stack. This stack handles the following information flows:

1. User Plane: User information is transmitted to/from the BS and UE, formatted according to the eREC/eRE functional decomposition. This includes time-critical C&M information and other services.
2. C&M Plane (Control and Management): C&M information is exchanged between C&M entities within the eREC and eRE. This information flow is conveyed to higher layers.
3. Synchronization Plane: Data used for frame and time alignment.

eCPRI Message Structure

The eCPRI message structure is defined as:

eCPRI message = { eCPRI header (4 bytes) eCPRI payload (variable size) }

The eCPRI message common header consists of the following fields:

eCPRI header = { eCPRI protocol revision (4 bits) Reserved (3 bits) C bit (1 bit), C = 0 indicates that the current eCPRI message is the last one inside the eCPRI PDU C = 1 indicates that another eCPRI message follows this current one within the eCPRI PDU. eCPRI message type (8 bits), type of service carried by the message. eCPRI payload size (16 bits), maximum supported payload size is 2^16 -1. }

The interface supports Ethernet-switched or IP-routed fronthaul networks. Let’s explore how different information types flow through the eCPRI protocol stack.

User Plane

• For eCPRI over Ethernet, UDP/IP is not used.
• For eCPRI over IP, Ethernet might be used. Refer to eCPRI message format and message types for more details.
• Ethernet: eCPRI messages are transmitted within standard Ethernet frames. The minimum size of the data field in an Ethernet frame is 46 octets. Frames with payload sizes larger than 1500 octets are also supported.
• IP: eCPRI messages are transmitted in UDP/IP packets.

C&M Plane

The eCPRI specification does not cover this information flow. Protocols like TCP/UDP/SCTP over IP/IPsec can be used.

Synchronization Plane

The eCPRI specification is also out of scope for this information flow. Protocols such as SyncE/PTP can be used, as shown in Figure 2.

Connection OAM

The eCPRI specification doesn’t define Connection OAM. For the IP parts of eCPRI, ICMP can be used. ICMP for IPv4 is defined in RFC 792, while ICMP for IPv6 is defined in RFC 4443. An eCPRI node requires either a unique MAC address or a unique IP address. For Ethernet parts of eCPRI, Ethernet OAM is used, according to IEEE 802.1Q and ITU-T recommendation G.8013/Y.1731.

References

• eCPRI Specification V1.2 (2018-06-25)