Communication Equipment

For servers and clients to communicate with each other, certain communications equipment is required. The communications equipment used depends on the method of data transmission.

The following sections discuss three types of communication equipment: multiplexers (muxes), Channel Service Unit/Data Service Units (CSU/DSUs), and modems.

Multiplexers

Multiplexing combines multiple (analog or digital) low-speed signals into a single high-speed signal for transmission over a communications line. Following are examples of different multiplexing methods:

• Frequency Division Multiplexing (FDM) Each signal is assigned a different frequency. FDM allocates a portion of the bandwidth for use all the time.

• Time Division Multiplexing (TDM) Each signal is assigned a fixed time slot in a fixed rotation. TDM allocations the entire bandwidth for a short amount of time.

• Statistical Time Division Multiplexing (STDM) Time slots are assigned to signals dynamically to make better use of bandwidth.

• Wavelength Division Multiplexing (WDM) Each signal is assigned a particular wavelength for use all the time. WDM is used only on optical fiber.

Multiplexers, also known as muxes, do the actual signal combination for transmission. The demultiplexer reverses the process by separating the high-speed signal into the original low-speed signals. The multiplexer and demultiplexer are often a single device capable of processing both outgoing and incoming signals, as illustrated in the Figure 4-1.

Figure 4-1. Multiplexer Combining Four Low-Speed Lines into a Single High-Speed Line

CSU/DSU

Channel Service Unit/Data Service Units are devices that terminate a digital signal on a customer's premise. CSU/DSUs perform line coding, line conditioning and equalization, and some (remote and local) test functions, such as loopback testing. CSU/DSUs terminate the local loop from the network service provider and translate the bipolar (WAN) digital signal into a unipolar (LAN) digital signal, as illustrated in Figure 4-2 and 4-3.

Figure 4-2. CSU/DSU with Four-Wire Interface to the Telco and V.35/RS-232/449 Interface to the End-user Equipment

Figure 4-3. Unipolar/Bipolar Digital Signal Conversion (by CSU/DSU)

NOTE

Some CSU/DSUs have built-in data compression; however, most IXCs/LECs do not support this compression. If a private (leased) line service is implemented in which a customer location is terminating both ends of the service, data compression can be implemented as the serving IXCs/LECs are passing the data through their networks (instead of terminating the service).

Figures 4-4 and 4-5 illustrate these compression and no-compression scenarios.

Figure 4-4. Compression (Often) Not Supported by Service Provider

Figure 4-5. Compression Supported

The CSU/DSU device performs two distinct functions, as detailed in Table 4-3.

Table 4-3. CSU/DSU Functions

Device	Function
CSU	Receives and transmits signals to and from the WAN line. Provides a barrier for electrical interference from either side of the unit. Echoes loopback signals from the service provider for remote testing purposes.
DSU	Manages line control and converts input and output signals between RS-232C, RS-449, or V.35 frames from the LAN (router or bridge interface) DS-x frames on the DS0/DS1/DS3 line. The DSU manages timing errors and signal regeneration. The DSU provides a modem-like interface between the computer as data terminal equipment (DTE) and the CSU. The DSU is often integrated with the CSU, is called a CSU/DSU, and is sometimes a component of a T1 WAN card (in a voice PBX or router).

DTE/DCE Interfaces

A DTE/DCE interface is the point of interconnection between the user DTE and the communications facilities. The data circuit-termination equipment (DCE) is the termination point and interface for these communications facilities.

The DTE/DCE components are described as follows:

• DTE is the component that converts user information into signals for transmission and reconverts the received signals back into user information. A user interacts with the DTE, or the DTE may be the user. The DTE interfaces with the DCE.

• DCE performs functions such as signal conversion and coding at the network end of the line between the DTE and the line. The DCE can be a separate or an integrated part of the DTE or the intermediate equipment.

DTE/DCE interfaces are often RS-232C/, RS-422/449, V.35, V.90, High-Speed Serial Interface (HSSI), or High Performance Parallel Interface (HIPPI/HPPI). DTE/DCE interfaces also have a direct impact in terms of supportable bandwidth and maximum distance between DTE and DCE.

RS-232C

RS-232C is a standard ("C" is the current version) that describes the physical interface and protocol for low-speed serial data communication between computers and related devices. RS-232 is used for both asynchronous (PC or terminal server) and synchronous data transfer (such as Frame Relay or SDLC/HDLC) DTE/DCE interfaces.

The RS-232 standards restrict transmission to 20 kbps or less and line lengths of 15 m (50 feet) or less. However, RS-232 is more robust than the traditional specified limits of 20 kbps over a 15 m line. Most 56 kbps DSUs are supplied with both V.35 and RS-232 ports because RS-232 can support speeds up to 200 kbps (over the same 50-feet distance).

NOTE

The 15 m limitation for RS-232C cable length can be stretched to about 30 m for ordinary cable, if it is well screened and grounded, and about 100 m if the cable is of low capacitance (ratio of electrical charge to potential in a conductor).

RS-422/449

The RS-422/449 standards are designed to replace the older RS-232 standard because they support higher data rates and distances (2 Mbps at 200 feet). RS-422/449 also provides for greater immunity to electrical interference, or crosstalk, than does RS-232.

NOTE

Crosstalk is a disturbance caused by electromagnetic interference along a circuit or a cable pair. Telecommunications signals can disrupt a signal in an adjacent circuit, causing the signals to become confused and cross over each other, resulting in crosstalk. Crosstalk is similar to two people talking over each other during the course of a conversation; the signals (in this case, the words from each speaker) talk over each other, resulting in a conversation that neither party can understand.

V.35

V.35 is a standard for high-speed data transmission (up to 2.048 Mbps). In the U.S., V.35 is the interface often used by routers and CSU/DSUs to connect to T1/E1 services.

V.90

V.90 is an asymmetrical transmission standard in which upstream transmission (keystroke and mouse-click commands) are sent at speeds up to 33.6 Kbps; and downstream transmission is sent at speeds up to 53.3 Kbps.

NOTE

V.90 modems receive data at speeds of up to 56 Kbps; however, due to Federal Communications Commission (FCC) rulings on maximum permissible transmit power levels during down load transmissions, speeds of 53.3 Kbps are the maximum allowed. The actual data speeds achieved vary depending on line conditions.

HSSI

The HSSI is a DTE/DCE interface that enables transmission rates of up to 52 Mbps. HSSI was developed by Cisco Systems and T3plus Networking to address the need for high-speed communication over WAN links.

HSSI can handle the T3 speeds (45 Mbps) of many WAN technologies, such as High-Speed Frame Relay and Asynchronous Transfer Mode (ATM), as well as the Office Channel-1 (OC-1) speeds (52 Mbps) of the synchronous digital hierarchy (SDH).

HIPPI/HPPI

HIPPI, previously called HPPI is a DTE/DCE standard enabling speeds of 800 Mbps or 1.6 Gbps (simplex or full-duplex). HIPPI is used for short distances (up to 10 km depending on cable type) to interconnect DTE/DCE devices, such as a high-speed server with an ATM switch in a data-center.