Circuit to Packet Network Overview

CTP products are designed to accommodate the delay, delay jitter, and packet reordering characteristics of an IP network. Figure 1 shows examples of applications that use CTP products.

Figure 1: Sample Application Using CTP Products

Numerous processes must occur to adapt serial data to and from IP packets. These processes are summarized in Figure 2. You configure the characteristics of the processes by using the CTP menu interface or the CTPView graphical user interface.

Figure 2: Circuit-to-Packet Conversion Processes

Using the menu interface, you can configure the CTP products to accept a serial data stream and create an IP flow that will be transferred across an IP network. The connection provided by the CTP platform is a physical layer circuit between the end user equipment.

Serial Stream Processing

Rate selection and clock configuration allow the serial interface rate to be configured through the software. Rates supported range from less than 300 bps to 12.288 Mbps (in subhertz increments).

You can configure the CTP systems by using the menu interface to provide multiple prioritized node clock references. An external reference input and any of the serial interfaces may be used for the node reference clock. Reference frequencies must be 32 KHz, n x 64 KHz, or 1,544 KHz up to a maximum of 4096 KHz (2048 KHz maximum on the CTP1002).

The electrical characteristics and encoding of the CTP ports are software configurable. The available options are EIA530, EIA530A, RS-232, V.35, analog 4WTO, conditioned diphase, isochronous, T1, and E1.

An analog voice signal terminated on the 4WTO interface is converted into a 64-Kbps pulse-code modulation (PCM) digital bit stream before adaptation to and from an IP flow. The analog interface allows transmit and receive levels to be adjusted.

Transmit Packet Processing

The CTP platform is configured with the remote IP address of the device where the packets created from the local serial port are to be routed.

The CTP remote port is specified by the IP address and physical port number of the remote unit and port.

The packet size created by the CTP platform may be set from 32 to 1456 bytes. Larger packet sizes are more bandwidth-efficient but introduce more serialization delay when the packet is created. The menu interface verifies that the combination of packet size and data rate does not result in a packet rate exceeding 1200 packets per second.

Time to live (TTL) may be set from 0 to 255. The TTL is the maximum number of hops in the IP network that the packet may travel before it is discarded by the network. You can configure the service type byte, which some IP networks use to determine the quality of service provided to the IP flow.

Receive Packet Processing

A receive buffer is required to smooth the timing jitter of received packets because of the delay variance that is inevitably encountered in the IP network. The configuration allows you to configure both the size of the buffer (in 1-ms increments) and the maximum amount of buffering delay allowed before the buffer will recenter. The size of the buffer configured should depend on the performance and characteristics of the IP network.

The CTP platform automatically resequences packets when they arrive out of order. If a packet is not received, the CTP platform inserts all data in lieu of the packet information so that bit count integrity is maintained.

You can prompt the menu interface to display detailed information about the port status, such as packet counts, late packets, missing packets, and buffer fill.

Serial Stream Creation

The packet receive process allows the serial data rate to be configured through the software. Rates supported range from less than 300 bps to 12.288 Mbps in subhertz increments. Conditioned diphase and isochronous interfaces operate at rates up to 1.024 Mbps.

Clock Options

• Interface clocking options—The CTP platform allows complete configuration flexibility of interface clocking. This flexibility includes your ability to specify how clocks are generated (that is, from the node clock, which can be phase locked to an external clock input) and what clocks are used to process the data from the attached device. The CTP platform can synthesize over 1.5 billion rates between 1 bps and 12.288 Mbps.

• Asymmetric clocking—You can configure CTP circuits to synthesize asymmetric rates.

• Reference clock input—The CTP platform can phase lock its node clock to an interface clock or external reference input. Up to five prioritized references can be configured. The node provides a reference holdover if all references are lost.

• Plesiochronous operation—Calibrated Clock is a patented CTP feature that allows the one-time calibration of the CTP oscillator to a known reference. Depending on environmental factors, two units calibrated to the same clock will have a clock difference as small as 100 parts per billion. This calibration enables CTP circuits to operate for long periods of time before a buffer recenter occurs.

• Adaptive clocking—Although IP router networks do not transfer physical layer clocking, the CTP adaptive clocking feature, using patented Advanced Time Domain Processing (ATDP), allows the CTP platform to recover clocking information from the remote CTP port and adjust the local clock accordingly. ATDP provides rapid convergence to the correct clock, and does not vary due to changes in the average jitter buffer fill. As a result, a CTP circuit will continuously operate without a buffer recenter, even when clock references are not used.