eCPRI protocol stack | eCPRI Protocol layers

This page describes eCPRI protocol stack along with eCPRI Protocol layers. The stack mentions all the protocol layers used in eCPRI interface to be carried over Ethernet/IP.

Introduction:
• eCPRI stands for Enhanced Common Public Radio Interface.
• The basic idea is to divide functionalities of BS (Base Station) into two blocks viz. REC (Radio Equipment Control) and RE (Radio Equipment) and connect them via packet based fronthaul transport network such as Ethernet or IP. This is shown in the figure-1. REC and RE terms are used with CPRI where as same terms are renamed as eREC and eRE in eCPRI interface.
• eCPRI handles following service points viz. User Plane Traffic, Synchronization and Control & Management.
• eCPRI can be used between single eRECs and eREs as well as multiple eRECs/eREs.
• It is used for 3GPP 4G (LTE) or 5G (NR) radio base station.

• Both eREC and eRE are physical separated where in eRE is kept near to antenna and eREC at far end.

eCPRI protocol stack | eCPRI Protocol layers

The figure-2 depicts eCPRI protocol stack. The stack has to take care of following information flows.
• 1. User Plane: User information transmitted to/from BS and to/from UE with format as per eREC/eRE functional decomposition. time critical C & M information, other services etc.
• 2. C&M Plane (Control and Management) : C & M information exchanged between C & M entities within eREC and eRE. Information flow is conveyed to higher layers.
• 3. Synchronization Plane: The data used for frame and time alignment.

• eCPRI message structure is as follows.
eCPRI message =
{
eCPRI header (4 bytes)
eCPRI payload (variable size)
}

eCPRI message common header consists of 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 2¹⁶-1.
}

The interface supports ethernet switched or IP routed fronthaul networks. Let us understand how these 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 eCPRI message format and message types >>.
• For User plane over ethernet, eCPRI messages are transmitted in standard ethernet frames. Minimum size of data field of ethernet frame is 46 octets. Frames with payload sizes larger than 1500 octets can also be supported.
• For User plane over IP, eCPRI messages are transmitted in UDP/IP packets.
➤C&M Plane: eCPRI specification is out of scope for this information flow. This information can use protocols (e.g. TCP/UDP/SCTP etc.) over IP/IPsec protocol.
➤Synchronization Plane: eCPRI specification is out of scope for this information flow. This information can use protocols (e.g. SyncE/PTP) as shown in the figure-2.
➤Connection OAM: eCPRI specification is out of scope for this information flow. For IP parts of eCPRI, ICMP protocol can be used. ICMP for IPv4 is defined in RFC 792 where as ICMP for IPv6 is defined in RFC 4443. An eCPRI node needs to have either a unique MAC address or a unique IP address. For ethernet parts of eCPRI, ethernet OAM is used as per IEEE 802.1Q and ITU-T recommendation G.8013/Y.1731.