Circuit Emulation

Key Terms

What Is CEM?

CEM treats data as an arbitrary bit stream — the actual Layer 1/Layer 2 format is irrelevant to the transport. This makes it ideal for carrying legacy or opaque traffic over modern packet-switched networks.

Common CEM Use Cases

• 2G / 3G mobile backhaul traffic
• T1 / E1 circuit emulation over packet networks
• PBX-to-PBX connectivity
• Inter-MSC (Mobile Switching Center) connectivity
• Already-encrypted traffic with no defined structure (government, high-security)
• Proprietary synchronous or asynchronous data streams
• Leased line emulation

CEoP SPAs (for Cisco 7600)

Platforms Supporting CEM

Key Configuration Note

Creating a channel-group under a T1 controller automatically creates the associated serial interface.

Clock Distribution

• The hub router owns the clock
• The spoke router recovers the clocks from 0/0/0
• The spoke router uses that clock, to sync lines 0/0/1, 0/1/0, and 0/1/1`

        Hub Router                                                      Spoke Site Router      
                                                                                               
 <Reference Clock>                                                                             
     │                                                                                         
┌────┼─────────────────────┐                                     ┌──────────────────────────┐  
│    ▼                     │                                     │                          │  
│┌────────┬──────► Internal│ 0/0/0─────────────────────────0/0/0 │Line->Network-Clock ──┐   │  
││PLL (1) │                │                                     │                      │   │  
││ Clock  ├──────► Internal│ 0/0/1─────────────────────────0/0/1 │Internal ◄─────────┐  │   │  
│└────┬──┬┘                │                                     │                   │  ▼   │  
│     │  └───────► Internal│ 0/0/2─────────────────────────0/1/0 │Internal ◄─────┬───┴─────┐│  
│     │                    │                                     │               │PLL  (2) ││  
│     └──────────► Internal│ 0/0/3─────────────────────────0/1/1 │Internal ◄─────┤ Clock   ││  
│                          │                                     │               └─────────┘│  
└──────────────────────────┘                                     └──────────────────────────┘  
                    4 Port card in Slot 0     2x 2 port cards Slot 0 and Slot 1        │       
                                                                                       ▼       
 PLL = Phase Locked Loop                                             <clock recovered from hub>

Wireshark Decoding

• SAToP traffic can be decoded using the pwsatopcw protocol keyword
• If Wireshark does not auto-detect the encapsulation, right-click the frame → Decode As and manually select the correct protocol
  • You need to already know what format the traffic is in — there is no auto-detection for pseudowire types

CEM Command Reference

Key Configuration Notes

• TDM byte = one timeslot
• xconnect requires matching VCIDs on both ends to bring up the pseudowire connection