Chapter 9

Read pages 276 to 289 and this lesson. Then, complete the homework according to the schedule .

Lesson 9 is the first of several lessons on Wide Area Networks or WANs. In particular Lesson 9 covers circuit switching.

Circuit Switching and Switching Networks
Switching is a process of connecting incoming signals to the proper outgoing channel. Oh so long ago when telephone operators handled all long distance calls, the operators manually interconnected the appropriate outgoing line with the incoming call.

Consider a mechanical switch, which directs a signal from an incoming line to the appropriate outgoing line. You might be most familiar with a telephone switch, but there are also data switches.

Telephone Switch: uses the phone number to determine which outgoing line to connect to.

Data Switch: uses hardware address to decide which outgoing line to choose.

We can organize switching into two types:

Circuit Switching: An incoming line is connected to an outgoing line and the lines are dedicated for use by this single connection. In series, the connections build an end-to-end connection. The Public Switched Telephone Network (PSTN) is a circuit switched network.

Packet Switching: Individual packets, frames, or cells are accepted on one incoming line and sent out one outgoing line. The lines are only used when the packet needs them, thus many connections can share the use of these lines. The Internet is packet switched.

We will investigate packet switching in Lesson 9. From this point, we'll concentrate on the public telephone network and how it uses circuit switching.

The Circuit Switched Telephone Network
All circuit switched connections, including the telephone system, set up a pathway between two end points that is dedicated for the duration of the "call". Here, the word "call" can refer to both a voice call or a data call. A dedicated circuit has some nice advantages.

User gets exclusive use of this pathway. It is not shared.
Very little delay, once the call has been set up.
User information rate can be guaranteed.
Information arrives in the same order that it was sent in.

Of course there are some disadvantages as well.

Very inefficient use of the media because of idle time.
Other users trying to use the same circuit are blocked. A good example is a busy telephone signal.
Both ends have to establish identical connections to the network. An example here is modem speed. Your modem has to accept data at the same rate that the data was sent at.
Complete, end-to-end call set up is required before transmission of information can begin.

The above advantages and disadvantages are typical features of the PSTN, but also occur in circuit switched data networks.

One feature of circuit switched connections is the need for complete call set up before any messages can be passed. Sometimes, as you know, call setup fails [43k sound clip]. After the call is set up users exchange full or half duplex messages (most likely full duplex) across the end-to-end circuit that is dedicated for their use. Upon completion of the exchange, one user disconnects and the resources along the circuit are released becoming available for use by other users.

The following two graphics show two callers separated by three links. Two switches perform circuit selection to route the call request through the network to the "callee." If the callee accepts the call, a circuit is established.

The network layout:

The call set up would proceed in this manner:

For the telephone system, circuit establishment would follow these steps, which agree with the small network and set up graphics above.

User picks up handset of telephone to make a call, electrical current begins to flow (an "off-hook" signal). Current sensor in the telephone company's central office triggers the sending of dial tone to caller (nearly instantaneously).

Caller dials number and the switch, knowing the dialed number, selects the appropriate outgoing line by checking a database. Then the switch requests a connection with the next switch along the chosen path, which we'll call Switch 2.

Switch 2 confirms the connection to Switch 1 and requests that the dialed number be sent. Switch 1 sends the dialed number.

Switch 2, in this case, is the last switch before reaching the callee. Switch 2 checks the callee's line to determine if it is in use. If it is, a busy signal is returned to the caller. If the callee's line is not in use, switch 2 will send a ring signal back through the network (which becomes a ringing tone on the callers line) and also rings the callee's phone.

The end-to-end circuit is established if the callee picks up the phone. From this point, there is a dedicated pathway through the network, a circuit from end to end.

Users communicate as necessary until one disconnects by hanging up. The "on-hook" signal releases the connection to the switch. The switch releases its connection to the other switch. The release of each link cascades through the network all the way to the final link between the other user and the nearby switch. This link is not released until the user hangs up.

Usually the user to network link is a permanent, dedicated connection that sits idle when not in use. When the phone is "on-hook" there is no current flowing and the link idle. It is, however, still dedicated for the use of the service subscriber.

The Telephone System
The PSTN, or public switched telephone network, has three main parts, the local loop, the central office and interoffice trunks. In this section we will take a look at each of these.

The Central Office
The CO houses equipment to operate the telephone network. The primary piece of equipment is the switch. At the CO, an incoming call request must be directed to an appropriate outgoing line. the address, a phone number in this case, provides the necessary information that a switch needs to consult a table and look up which outgoing line the call request should be routed to. Outgoing lines between central offices are called trunks.

Trunks
Between two central offices are groups of links. There are not enough links to support calls by all users simultaneously, but that many links are not necessary. It is extremely unlikely that every subscriber would pick up their phone and make calls simultaneously that need to be routed across the same set of trunks. A single voice line between two central offices counts as one trunk. Usually, there are many trunks between central offices and this bundle is called a trunk group.

By designing carefully, traffic engineers can provision the network with just enough trunks to meet demand. This demand is not current demand, but the expected demand at the end of the "engineering interval".

For example, an engineer might specify that 32 trunks should be installed between two COs and that these should be sufficient to support the expected demand for service at the end of 5-years of use.

The engineering interval would include the design time, the installation time and the time in service. At the end of this interval, the trunk group should meet the expected demand.

Unfortunately, it isn't always easy to specify the number of trunks to install. Usually a T1 is used to provide service. This means that trunks are installed in groups of 24. In the example above, we could install one T1 with 24 trunk lines. Twenty-four trunks is not very close to 32. The engineer would expect the service on the trunk group to be overloaded, with customers getting (fast) busy signals if they couldn't be rerouted.

Our other choice is to install two T1 connections, or 48 trunks. The traffic engineer has to make some decisions about which way to go here. Perhaps an E1 could nearly meet the engineering interval.

The "Local Loop"
The telephone system between the central office, or CO, and your home, including all the wires, poles, terminal boxes, manholes and other equipment is collectively called the "local loop" and sometimes "the last mile".

The pair of copper cables that run from your home to the CO are solely dedicated to you. Each subscriber has a unique pair of wires running all the way to the CO's switch. This link is not shared, unless you have a party line, which is pretty rare these days. The local loop is the most expensive part of the telephone system because of the amount of copper used and the installation time required to it.

Once your call reaches a telephone switch it has to compete for the limited number of outgoing lines, unless it immediately goes on to another local loop line.

The design of our current PSTN has been driven in the past by the need to support voice service. Very little consideration was given to transmitting data across the network.

Now, we are trying to retrofit data systems so they can work over this network. Digital Subscriber Line, or DSL, is one example of how the phone system can be reworked to support high data rates.

A migration of design goals is taking place now as data transmission becomes more and more important. Your humble instructor predictions that data will become the primary engineering goal for the telephone network, that is if it hasn't already.

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