When it comes to private line or TDM business services, network providers have both a challenge and an opportunity. They must evolve away from their outdated TDM networks – which have already reached end-of-life status – and migrate to modern systems based on packet-networking technologies. The cost to overlay and maintain parallel (and often redundant) TDM and packet networks is as high as it is complex. Not to mention most technical support has retired, moved on, or is extremely hard to find.

As part of Ciena’s Packet Networking Summer Webinar Series, I recently gave a session on Digital Modernization: Understanding Circuit and Pseudowire Emulation, where I discussed the above-mentioned challenges. I also covered the opportunity to simplify network modernization to minimize network transition costs, enable next-generation collaborations and communications, and provide an elegant migration path to supporting TDM and Packet requirements over an updated and supported network infrastructure that’s simpler and more cost-effective to own and operate.

TDM-to-Packet modernization is a converged approach that enables service providers to deliver legacy TDM services over a performance-grade packet-based network. This approach involves Circuit Emulation (CEM) that enables network providers to emulate private line circuits (T1/E1, T3/E3) and SONET/SDH Virtual Connections (VCs) over a modern, performance-grade packet-based network.

TDM Voice, Video, and Data

Most humans can hear frequencies between 20 and 20,000 Hertz (Hz). Over time, as we age, this range (unfortunately) narrows. Interestingly enough, dogs can hear between 67 and 45,000 Hz. As humans, the reason we don’t hear a dog whistle is because the pitch is at a much higher frequency then we’re able to hear. These higher frequencies are also the first to be lost when diminished by aging.

When it comes to human voice, most range between 80 to 3,000 Hz, but some high range voices can get into the 6,000 Hz range. For most, the dynamic or usable voice range is 300 to 3,400 Hz – commonly referred to as Ultra Low Frequency (ULF) band or 300 to 3,000 Hz voice frequency band. This ULF band is important in the telecom world as it sets the bandwidth for a single voice frequency transmission. For digital transmission, that voice frequency bandwidth is slightly higher at 4,000 Hz, which also includes guard bands.

Global TDM transmission and switching systems use the Nyquist sampling rate of 2 times the frequency or an 8 kHz sampling rate (8,000 samples/sec) Pulse Code Modulation (PCM) signal as the fundamental basis for voice. There are two types of digital TDM carriers, T-carrier (T1) and E-carrier (E1) supporting digital voice and data services. T1 transmission supports a data rate of 1.544 Mb/s for US, Canada, and Japan. E1 supports a data rate 2.048 Mb/s for Europe, Australia, and other parts of the world. TDM multiplexes 28 T1’s into a T3 (44736 kb/s) and 16 E1’s into a E3 (34368 kb/s) digital signals. In a previous article I wrote on how T-carriers were the fundamental basis for TDM networks architectures.

For the past 40 years, Primary Rate Interface (PRI) transmission protocols have successfully supported voice calling, and data. PRI voice is based on using ITU-T Recommendation G.711 Pulse Code Modulation (PCM) of Voice Frequencies. The standard was released in 1972 and primary used for telephony, and the basis for circuit switching of the Public Switched Telephone Network (PSTN). G.711 is a narrowband audio codec used in other technologies for fax and audio/video codecs, including popular one like H.320 and H.323, and the standard form of digital audio in computers, compact discs, and many other digital audio applications. PRI is also the interface standard used of Integrated Services Digital Network (ISDN) for carrying multiple Digital Signal Zero (DS0) data transmissions, which are subsequently carried by a T1 circuit, for example.

The Motion Picture Experts Group (MPEG), and other video standards bodies and groups, have been using PCM techniques, albeit at a higher sampling rating along with the audio/video codecs for many decades.  Most video is recorded, produced, and transported in its native digital format. 

TDM data traffic can be created and terminated for a variety of encapsulation protocols on the TDM network, including Generic Framing Procedure – Framed (GFP-F), Packet-over-SONET/SDH (PoS), Frame Relay (FR), Multi-Link Frame Relay (MLFR), and Point-to-Point Protocol (PPP).

Framing TDM Services

In TDM framing there are two basic framing structures channelized and unchannelized for carriers, SONET and SDH.

Channelized voice had its beginning in the 1950s as a Trunk Level 1 or T1, having 24 separate and distinct channels of 8 bits – each running at 8 kb/s – for a total of 64 kb/s. In the digital switching world, this channel is the basic digital switching rate and referred to as Digital Signal Level 0 (DS0) for T-carriers and E0 for E-carriers. From there (DS0), T-carrier and E-carrier took separate digital hierarchy paths - until Optical Carrier (OC) signal rates in SONET and SDH.

Channelized voice for Private Branch eXchange (BPX) or key telephone system are used to keep phone lines separate. Channelized channels are separated individual voice channels for the PBX, which could be set up for local and/or long-distance service and as incoming only, outgoing only, or both. Features like Caller ID and Automatic Number Identification (ANI) were not supported.

When using ISDN PRI, channelized channels for T1 and E1 are set up differently. To carry voice or data, PRI uses 64 kb/s 8-bit sampled G.711 Bearer-channels (B-channels) and Delta-channels (D-channels) for control. T-carriers have 23 B-channels and 1 D-channel, and E-carriers have 30 B-channels and 2 D-channels. T-carriers can also use Robbed-bit Signaling (RBS) for Channel-Associated Signaling (CAS), which are limited to 56 kb/s per channel for data.

Basically, a channelized service is a circuit where the circuit can be subdivided into different channels using multiplexers and efficiently groomed. In some cases, TDM services can be time slot dependent, so a specific service channel must have the service arrive to the other end of the service at that specific channel or time slot.

An unchannelized circuit is a circuit where the full bandwidth of the line is dedicated to a single channel, which is applicable to data-centric services. Unchannelized TDM is commonly used for high bandwidth leased lines and each timeslot is unchanged. 

Circuit Emulation (CEM) and Pseudowire Emulation (PWE)

During the Digital Modernization: Understanding Circuit and Pseudowire Emulation session, I also discussed how CEM and PWE are key to enabling circuit switched services over a packet-based network, providing connectivity for Plesiochronous Digital Hierarchy (PDH), T1/T3, SONET/SDH, Add-Drop Multiplexer (ADM), Multi-Service Provisioning Platform (MSPP), Digital Cross-connect System (DCS), and Digital Access Cross-connect System (DACS) 3/3 and 3/1 while supporting Ethernet and IP protocols.

There are three types of CEM:

  1. Structured-Agnostic TDM over Packet (SAToP)
  2. Circuit Emulation Service over Packet Switched Network (CESoPSN)
  3. Circuit Emulation over Packet (CEP)

SAToP and CEMoPSN encapsulate payloads on low-speed services, while CEP encapsulate payload based on VC, and as shown below all three work flawlessly with PWE.

TDM PWE Service Protocol Stacks

TDM PWE Service Protocol Stacks


PWE is an emulation of point-to-point connection over a packet-switching network (PSN). CEM use PWEs mapped over traffic-engineered tunnels through an Ethernet, MPLS, or IP network.

Over the past few decades, many TDM-based technology platforms were used to deliver switched voice, video, and data services. Although these technologies served their customers very well, they are definitely showing their age.

Leaving TDM equipment to rust is not a viable option.

For today’s data transport needs, network providers use packet optical networks to offer IP and Ethernet services for residential enterprise customers to offer broadband, voice, cloud-computing, mobile backhaul, data center interconnects, and other modern network services, as well as voice services.

Network operators can modernize their TDM networks with CEM-capable Small Form-factor Pluggables (SFPs), FPGA modules, and ASIC-based platforms depending on scale and Circuit Emulation Service (CES).

Structure-Aware TDM over Packet (SAToP)

SAToP encapsulates TDM (T1, E1, T3, E3) as pseudowires over the PSN. SAToP only transports the TDM structure and is agnostic to any TDM data and signaling. SAToP is agnostic to all services. For example, a T1, T3, Virtual Tributary Group (VTG) of a SONET circuit, or CES are all treated the same way.

Circuit Emulation Service over Public Switched Network

CESoPSN encapsulates structured (DS0) TDM signals as pseudo wires over the PSN. This structure-aware TDM CES emulation enables packet bandwidth savings, supports DS0 level grooming and cross-connections.

Circuit Emulation over Packet (CEP)

CEP encapsulates SONET and SDH circuits and services as pseudowires over the PSN. CEP emulates Synchronous Payload Envelope (SPE)/Virtual Container (VC-n) and Virtual Tributary (VT)/Virtual Container (VC-n) digital signals, including OC-192/STM-64.

New Tricks for a Legacy TDM Network

The good news is, older dogs/networks can indeed learn new tricks. Faced with obsolescence and a network that’s decades old, CEM from Ciena’s pluggable transceiver family, 3926 and 6500 Packet Transport System (PTS) may be just the right trick to consolidate TDM services on a converged packet network. New tricks that can provide up to 5x lower power, 10x space saving, and 4x more TDM capacity than competing platforms.

So, don’t leave your network in the doghouse, teach it new tricks on how to connect all your network services across a single network, providing a migration to the future for legacy services, and enabling new opportunities.

Ask us how Ciena helps Evolutionize Your Packet Network.