LOCAL NETWORKS

       

1 INTRODUCTION

LOCAL AREA NETWORK (LAN)

Local area networks, more commonly known as LANs, typically connect a geographically restricted group of clients, such as a group of employees in an office building, to a server. Clients simply are stand-alone personal computers (PCs) or other types of workstations, while servers are faster computers that house the programs and data distributed to the workstations. Servers either can be mainframe computers or sophisticated PCs. Several network operating systems exist, including Microsoft Windows NT, Macintosh's AppleTalk, and Novell's NetWare. These systems are housed directly within the machine acting as the server. Related software within each client allows it to access programs and data on the server, just as if the applications and files were actually on the hard drive of the client's machine. Some LANs also allow clients to communicate with each another via e-mail messages or real-time chat programs. Particularly large LANs may require several dedicated servers, while smaller LANs may actually be nothing more than a peer-to-peer network, in which a few workstations act as servers by allowing the users at each station to access files and applications on one another's machines.
In some cases, clients must access a server for all the software applications and data files they need. However, LANs also can be set up with servers that only provide select applications to clients. For example, some workstations equipped with their own printers may access their LAN each time they perform a word processing or data processing function, yet not need the LAN when actually printing documents. Other LANs may link printers directly to servers to allow a single printer to be shared by several workstations, a cost saving technique used by many corporations, libraries, schools, and other institutions.
To actually transfer data, LANs use protocols like IBM's Token Ring, which typically arranges computers in a ring or star shape to facilitate connection. Ethernet, which was developed by Bob Metcalfe and Xerox Corp. in the early 1970s, is the most common LAN protocol. It uses coaxial cables or twisted pair wires to connect machines arranged most commonly in a bus layout, where all computers are connected to a central line. Another protocol, Fiber Distributed-Data Interface (FDDI), uses fiber optic lines to connect up to thousands of workstations as far as 124 miles apart. Networks much bigger than this typically begin to connect LANs together to form a wide-area network, or WAN. Data transmission speeds for these technologies range from roughly 1 million bytes per second for Ethernet and Token Ring to 10 million bytes per second for FDDI. Efforts to improve network speed have resulted in the creation of new technologies like Fast Ethernet, the even faster Gigabit Ethernet, and Fast Token Ring.

The importance of local network design, whether standing on its own merit or part of an overall national network, cannot be overstressed. In comparison to the long-distance sector, the local sector is not the big income producer per capital invested, but there would be no national network without it. Telephone companies or administrations invest, on the average, more than 50% in their local areas. In the larger, more developed countries the investment in local plant may reach 70% of total plant investment.

The local area, as distinguished from the long-distance or national network, was discussed in Section 4 of Chapter 1. In this chapter we are more precise in defining the local area itself. Let us concede that the local area includes the subscriber plant, local exchanges, and the trunk plant interconnecting these exchanges as well as those trunks connecting a local area to the next level of network hierarchy, or the point of presence (POP)∗ (USA) or primary center (CCITT).

To further emphasize the importance of the local area, consider Table 2.1, which was taken from Ref. 1 (CCITT). Figure 2.1 is a simplified diagram of a local network with five serving exchanges and illustrates the makeup of a typical small local area.

The design of such a network (Figure 2.1) involves a number of limiting factors, the most important of which is economic. Investment and its return are not treated in this text. However, our goal is to build the most economical network assuming an established quality of service. Considering both quality of service and economy, certain restraints will have to be placed on the design. For example, we will want to know

2 SUBSCRIBER LOOP DESIGN

2.1 General

The pair of wires connecting the subscriber to the local serving switch has been defined as the subscriber loop. It is a dc loop in that it is a wire pair supplying a metallic path for the following:

• Talk battery for the telephone transmitter (Chapter 1, Section 2).

• An ac ringing voltage for the bell on the telephone instrument supplied from a special ringing source voltage.

• Current to flow through the loop when the telephone instrument is taken out of its cradle (“off hook”), telling the serving switch that it requires “access,” thus causing a line seizure at that switch.

• The telephone dials that, when operated, makes and breaks the direct current on the closed loop, which indicates to the switching equipment the number of the distant telephone with which communication is desired; alternatively, a touch-tone pad with digit buttons. Unique pairs of audio tones, representing digits 1–0, are transmitted to the serving exchange switching equipment. The typical subscriber loop is powered by means of a battery feed circuit at the switch. Such a circuit is shown in Figure 2.2. Telephone battery source voltage has been fairly well standardized at −48 V dc.

2.2 Quality of a Telephone Speech Connection

2.2.1 Loudness Rating. Loudness rating is the principal parameter for measuring the quality of a speech connection. CCITT/ITU-T Recs. P.11, P.76, P.78, and P.79 provide information on subjective and objective methods for the determination of loudness ratings (LRs). The currently recommended values of loudness loss in terms of loudness ratings are given in ITU-T Recs. G.111 and G.121 [25, 26]. In simple terms, loudness rating is a measure of speech level (volume). The term replaces reference equivalent, used in previous editions and older texts.

2.2.1.1 Customer Opinion. Customer opinion, as a function of loudness loss, can vary with the test group† and particular test design.

These are based on representative laboratory conversation test results for telephone connections in which other characteristics such as circuit noise have little contribution to impairment.

2.2.1.3 Determination of Loudness Rating. The designation of loudness ratings (LRs) in an international connection are given in Figure 2.3. Telephone sensitivity must be measured. The measurement can be made using guidelines in ITU-T Recs. P.66, P.76, P.78, and P.79. Telephone sensitivity includes both microphone and earpiece sensitivity. Overall loudness rating (OLR) is then calculated using the following formula:

OLR = SLR + CLR + RLR (2.1)

OLR is defined as the loudness loss between the speaking subscriber’s mouth and the listening subscriber’s ear via a connection.

The send loudness rating (SLR) is defined as the loudness loss between the speaking subscriber’s mouth and an electric interface in the network. (The loudness loss here is defined as the weighted decibel average of driving sound pressure to measured sound pressure.)  interface in the network and the listening subscriber’s ear. (The loudness loss in this case is defined as the weighted decibel average electromotive force to measured sound pressure.)

The circuit loudness rating (CLR) is the loudness loss between two electrical interfaces in a connection or circuit, each interface terminated by its nominal impedance, which may complex. (The loudness loss here is approximately equivalent to the weighted decibel average of the composite electric loss.)

     2.3 Subscriber Loop Design Techniques

                            

      2.3.1 Introduction. Consider the following drawing of a simplified subscriber loop:

The distance D, the loop length, is a critical parameter. The greater the value of D, the greater the attenuation that the loop suffers, and signal level drops as a result. Likewise, there is a limit to D due to dc resistance, so signaling the local switch can be affected. When we lift the telephone off hook, there

† Note: The terms on hook and off hook derive from old-fashioned telephones. This type of telephone usually had a wooden instrument box hanging on a wall. Extending from the box through a hole on the side was a lever held normally in the “up-position.” There was a metal ring on the end of the lever. The telephone instrument had a wire pair connected to it, several feet (∼1 meter) long, SUBSCRIBER LOOP DESIGN 47 must be enough current flow in the loop to actuate the local switch where the loop terminates. Of course it will follow that the greater the wire diameter of the loop pair, the less resistance there is per unit length; also, the less attenuation there is per unit length. On a particular subscriber loop we must set an attenuation limit and a minimum current flow. The current flow is usually stated as a resistance in ohms.

We expect a common battery voltage of −48 V. This is what a high-impedance voltmeter will read anywhere in the loop when no current is drawn, such as a telephone instrument off hook.

When designing a subscriber loop, we would be vitally interested on what its maximum length would be. There are two variables that must be established:

(1) The maximum loop resistance. This value is a function of the circuit in the switch where the loop terminates. One current value that comes to mind is 2400 _. (2) The maximum loss or attenuation on the loop. This will be taken from the national transmission plan. In Europe, 6 dB is commonly used for this value. This is 6 dB at the reference frequency of 800 Hz. In North America the reference frequency is 1000 Hz. The loss value may be as high as 9 dB.

Loss values will be the SLR or the RLR from equation 2.1. Then the maximum length of a subscriber loop will be governed by a resistance limit and a loss limit.

Which is the more important of the two parameters? The resistance limit wins every time. If we cannot signal over the loop (i.e., cause a line seizure at the serving switch), the loop will not work.

Remember when budgeting parameter values for the resistance limit on a subscriber loop, we must budget something for the telephone subset itself. Use 300 _ for this value. Again, the maximum loop resistance is set by the local serving switch design. Prior to the days of digital switches in the United States, the value was 1300 _. Allow 300 _ for the telephone subset leaving only 1000 _ for the loop itself. Some earlier digital switches advanced this value to 1800 _; some Northern Telecom switches provide 2400 ohms.

2.3.2 Calculating the Resistance Limit. To calculate the dc loop resistance for copper conductors, the following formula is applicable: Rdc = 0.1095 d2 (2.2) where Rdc is the loop resistance in ohms per mile (statute) and d is the diameter of the conductor (in inches).  earpiece on the top and a mouthpiece on the bottom and a hook on the end. When not in use, the telephone instrument hook engaged the ring on the lever, pulling the lever down with its own weight.

When someone wished to use the telephone, he/she unhooked the instrument from its ring, and the spring-loaded lever moved upwards. This caused a contact closure on the loop and current would flow in the loop. This became known as the “off-hook” condition. When the user was finished, he/she would replace the instrument, engaging the ring on the lever with the hook on the instrument, pulling the lever down. The contact now would open and current would stop flowing in the loop.

This was called the “on-hook” condition. [7, 8]

48 LOCAL NETWORKS

If we wish a 10-mile loop and allow 100 _ per mile of loop (for the stated

1000-_ limit), what diameter of copper wire would be needed?

100 = 0.1095

d2

d2 = 0.1095

100

d = 0.033 in. or 0.76 mm (round off to 0.80 mm)

Using Table 2.4, we can compute maximum loop lengths for 1000-_ signaling resistance. Use a 26-gauge loop. We then have

1000

83.5

= 11.97 kilo feet or 11,970 ft

Let’s use the 2400-_ switch as another example. Subtract 300 _ for the telephone subset, leaving us with a net of 2100 _. We will use a 26-gauge wire pair on the loop,  then from Table 2.4 we have

2100

83.5

= 25.149 kft or 25,149 ft

Thus we are dealing here with what some call the signaling limit on a subscriber loop, and not the loss (attenuation) limit, or what some call the transmission limit. This is described in Section 2.3.3. Another term we introduce is resistance design. This is a method of designing subscriber loops where resistance is the only limiting parameter. If we cover for sufficient resistance, the loop loss will take care of itself.

2.3.3 Calculating the Loss Limit. Attenuation or loop loss is the basis of transmission design of subscriber loops. The attenuation of a wire pair varies with frequency, resistance, inductance, capacitance, and leakage conductance.

Also, resistance of the line will depend on temperature. For open-wire lines, attenuation may vary by ±12% between winter and summer conditions. For buried cable, which we are more concerned with in this context, variations due to temperature are much less.

If we are limited to 6 dB (loss) on a subscriber loop, then by simple division we can derive the maximum loop length permissible for transmission design considerations for the wire gauges shown. 

28 = 6/0.666 = 9.0kft

26 = 6/0.51= 11.7kft

24 = 6/0.41= 14.6kft

22= 6/0.32= 19.0 kft

19 = 6/0.21= 28.5 kft

2.3.4 Loading. In many situations it is desirable to extend subscriber loop lengths beyond the limits described in Sections 2.3.2 and 2.3.3. Common methods to attain longer loops without exceeding loss limits are to increase conductor diameter, use amplifiers and/or range extenders,‡ and use inductive loading.

Inductive loading tends to reduce transmission loss on subscriber loops and on other types of voice pairs at the expense of good attenuation-frequency response beyond 3000 Hz. Loading a particular voice-pair loop consists of inserting inductance in series (loading coils) into the loop at fixed intervals. Adding load coils tends to decrease the velocity of propagation and increase the impedance. Loaded cables are coded according to the spacing of the load coils.