Chapter 9: Managing Link Performance with CSU-DSUs

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9.1 Introduction

Just as data transmission over analog lines requires modems, digital services require CSUs and DSUs to interconnect data terminal equipment (DTE) at the customer premises with the public network. CSU-DSUs are used for connecting bridges and routers, cluster controllers, and other DTE over point-to-point leased-line networks.

The CSU is positioned at the front end of a circuit to equalize the received signal, filter both the transmitted and received waveforms, and interact with the carrier’s test facilities. FCC Part 68 registration rules require that every digital circuit be terminated by a CSU. ^[1] The DSU element transforms the encoded waveform between alternate mark inversion (AMI) and a standard business equipment interface, such as RS-232C or V.35. It also performs data regeneration, control signaling, synchronous sampling, and timing. Both CSUs and DSUs must comply with AT&T Publication 62310 for interconnection with digital facilities and services.

Because digital transmission systems are capable of transporting signals between DTE in a form that is close to their native form, the serial data does not need to be compressed with complex modulation and demodulation techniques, as is the case when sending data via modems over analog facilities. Instead, customer-provided CSUs or DSUs encode serial data from terminals or computers and perform wave-shaping of the transmit signal before it is sent over the digital facility to ensure an acceptable level of network performance (see Figure 9.1). Lower-speed (2.4 to 56 Kbps) DTE interconnected via DDS by means of standard RS-232C or V.35 interfaces requires both the CSU and DSU functions. Some CSU-DSUs have multiple interfaces and can multiplex traffic from several sources onto the link. For example, two serial data ports may be available to mix data traffic types. There also may be a DSX-1 interface for connection to a local PBX. ^[2]

Figure 9.1: The original digital signal from a terminal’s serial data output (a) is translated into a bipolar format through AMI (b). The unshaped bipolar waveform is first reduced to a 50% bipolar waveform and then filtered (c), producing an output spectrum with zero energy at the data rate and reducing high-frequency components that may interfere with other services sharing the same cable binder. The output spectrum of a 9.6-Kbps transmission, for example, will exhibit no energy at either dc or 9,600 Hz and will peak at 4,800 Hz.

In a T1 environment, where up to 24 individual DS0s of 56/64 Kbps are available, these devices can operate as simple TDMs, inserting traffic on the correct time slot so it can be demultiplexed in the carrier’s network or at the customer’s remote site.

^[1]Part 68 addresses connection of terminal equipment to the public telephone network, permitting consumers to connect equipment from any source to the public network if such equipment fits within the technical and safety parameters outlined in the guideline. Part 68 was first adopted in 1975 as part of the FCC’s Wide Area Telecommunications Service (WATS) rulemaking process in response to telephone company slowness in modifying tariffs to permit consumers to attach their own equipment to the public network. Since the introduction of Part 68, the FCC has consistently applied the principle that any device, which is privately beneficial without being publicly detrimental, may be attached to the network. In fact, without Part 68, users of the PSTN would not have been able to connect their computers to the network, which would have stalled the development and expansion of the Internet.

^[2]DSX-1 stands for digital signal cross-connect, and the “1” defines the T1 signal as measured at a point of connection inside the customer premises. The only difference between DSX-1 and regular DS1 is the signal strength. The DSX-1, which is always inside the customer premises, can have line lengths of only up to 655 feet (196.5m). But DS1, which is outside the customer premise, can handle lengths up to about 1 mile (1.6 km).

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