
CEM Overview
Circuit Emulation (CEM) is a way to carry TDM circuits over packet switched network. CEM embeds the
TDM circuits into packets, encapsulates them into an appropriate header, and then sends that through Packet
Switched Network. The receiver side of CEM restores the TDM circuits from packets.
Modes of CEM
•Structure Agnostic TDM over Packet (SAToP) (RFC 4553) –SAToP mode is used to encapsulate
T1/E1 or T3/E3 unstructured (unchannelized) services over packet switched networks. In SAToP mode,
the bytes are sent out as they arrive on the TDM line. Bytes do not have to be aligned with any framing.
In this mode, the interface is considered as a continuous framed bit stream. The packetization of the
stream is done according to IETF RFC 4553. All signaling is carried transparently as a part of a bit
stream.
•Circuit Emulation Service over Packet (CEP) (RFC 4842) - CEP mode is used to encapsulate SDH
payload envelopes (SPEs) like VC11, VC12, VC4, or VC4-Nc over PSN. In this mode, the bytes from
the corresponding SPE are sent out as they arrive on the TDM line. The interface is considered as a
continuous framed bit stream. The packetization of the stream is done according to IETF RFC 4842.
Table 1: SDH CEM Channelization Modes
PortsCEMSDH Modes
STM16CEPVC4-16c
STM4, STM16CEPVC4-4c
STM1, STM4, STM16CEPVC4
STM1, STM4, STM16SAToPTUG-3-E3
STM1, STM4, STM16SAToPTUG-3-T3
STM1, STM4, STM16CEPTUG-2-VC11
STM1, STM4, STM16CEPTUG-2-VC12
STM1, STM4, STM16SAToPTUG-2-T1
STM1, STM4, STM16SAToPTUG-2-E1
Services Provided by SDH Configuration
The following services are provided by SDH Configuration:
1 port OC-48/ 4 port OC-12/OC-3 + 12 port T1/E1 + 4 port T3/E3 CEM Interface Module Configuration Guide, Cisco
IOS XE Fuji 16.7.x (Cisco NCS 4200 Series)
4
Configuring SDH
CEM Overview