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Chapter 6 - Supporting Popular WAN Technologies

Cisco TCP/IP Routing Professional Reference
Chris Lewis
  Copyright © 1999 The McGraw-Hill Companies, Inc.

Additional WAN Technologies
To finish this chapter, we'll discuss some general terms, technologies, and devices used in WAN implementations. First, let's take a closer look at the Cisco serial ports that interface to WAN connections.
Cisco Serial Interfaces
Cisco serial interfaces are either built in, as with the fixed-configuration routers such as the 2500-series, or installed, as with the Fast Serial Interface Processor (FSIP) card, for modular routers such as the 7500-series. Either way, the interface itself is basically the same device that will support speeds up to T-1 (1.544 Mbps) in the United States, and E-1 (2.048 Mbps) elsewhere. The only restriction here is that the FSIP card has two 4-port modules, with each module capable of supporting four T-1 connections; only three E-1 connections can be supported simultaneously because the aggregate throughput on this card cannot exceed 8 Mbps.
The Cisco serial port uses a proprietary 60-pin connector and configures itself according to the cable and equipment that are connected to it. As discussed in Chap. 3, a Cisco serial port will configure itself as DTE or DCE depending on the cable end connected to it. Cisco serial port cables will terminate in EIA-232, EIA-449, X.21, or V.35 connectors. These standards have differing capabilities regarding the distance over which they can transport communications at varying speeds. Table 6.1 shows the distances over which it is safe to deliver data at varying speeds for these interface standards. These are not necessarily hard-and-fast rules; it is possible to use longer cables—but if you do so, you start running the risk of frame errors being introduced by interference on the cable.
Table 6-1: Serial Communications Distance Limitations
Rate
EIA-232 Distance (feet)
EIA-449, X.21, V.35 Distance (feet)
9600
50
1,025
56,000
8.5
102
1,536,000
N/A
50
EIA-232 cannot transmit a given speed of data at the same rate as the other interface standards because it uses unbalanced signals. The terms balanced and unbalanced are often used in data communications, so we will explain the difference between these two types of communication here.
First we need to understand a little of what voltage is. Volts measure what is known as the electrical potential difference between two objects. Measuring a difference requires a common reference point for measurement against in order to make the number meaningful. It's like measuring the height of a mountain with reference to sea level rather than against the next closest mountain. It's the same with volts. They normally are quoted as so many volts above what is known as ground, which is taken as 0 volts, i.e., the electrical potential of the planet Earth.
Just as there is nothing inherently dangerous about standing on top of a tall building, there is nothing inherently dangerous about being at a certain voltage level. The danger comes if you fall off the building and come in to contact with the ground at high velocity. The same is true if you are at a given voltage level and come into contact with something at a different voltage level. When two objects at different voltage levels come into contact with each other, they will try to equalize their voltage by having current (amperes) flow between them.
In data communications, signals that represent data are measured in terms of a particular voltage level. In unbalanced communications, there is only one wire carrying the data signal, and its voltage level is measured against ground (i.e., the Earth's voltage level). With balanced communications, two wires are used to transmit the signal, and the signal voltage level is measured between these wires. Using two wires for data transmission significantly improves a cable's ability to deal with electrical interference.
The idea behind why two wires are better at dealing with electrical interference is simple. If a two-wire system is transferring data and is subject to electrical interference, both wires are similarly affected. Therefore, the interference should make no difference to the circuitry receiving the two-wire transmission, which determines the voltage level, and hence the signal, as the difference in electrical potential between the two wires. With unbalanced single-wire transmission, any electrical interference on the line directly affects the voltage potential of that wire with respect to ground. Therefore, unbalanced data transmission is less able than balanced transmission to deliver uncorrupted signals in the presence of electrical interference.
Serial Interface Configuration.     If a serial interface is acting as a DCE and is generating a clock signal with the clockrate command, the normal operation is for the attached DTE device to return this clock signal to the serial port. If the attached DTE device does not return the clock signal to the serial port, you should configure the port with the transmit-clock-internal command.
Encoding of binary 1 and 0 is normally done according to the Non-Return to Zero (NRZ) standard. With EIA-232 connections in IBM environments, you may need to change the encoding to Non-Return to Zero Inverted (NRZI) standard as follows.
Router1(config)#interface serial 0
Router1(config-int)#nrzi-encoding
Cisco serial ports use a 16-bit Cyclic Redundancy Check (CRC) Frame Check Sequence (FCS). If a Cisco serial interface is communicating directly with another Cisco serial interface, better performance might be obtained by using a 32-bit CRC, as fewer retransmissions occur with a 32-bit CRC. To enable 32-bit CRCs, perform the following in interface configuration mode:
Router1(config-int)#crc32
High-Speed Serial Interface.     I have mentioned technologies in this book that make use of speeds higher than T-1 speeds, for example, SMDS or frame relay delivered on a T-3 circuit. To accommodate these higher transmission rates, a modular Cisco router uses a High-Speed Serial Interface (HSSI). The HSSI, just one, is delivered on an HSSI Interface Processor (HIP) card. The HIP card is inserted in any slot on a 7x00-series router and interfaces directly to the router bus architecture, the Cisco eXtended bus, or cxbus.
The HSSI is now a recognized standard, known as the EIA612/613 standard and operating at speeds of up to 52 Mbps. This enables the interface to be used for SONET (51.82 Mbps), T-3 (45 Mbps), and E-3 (34 Mbps) services. The HSSI on the HIP is different from other Cisco serial ports in that it uses a 50-pin Centronics connector and requires a special cable to connect to the incoming line's DSU device. In other words, a regular SCSI cable will not do. Once installed and connected, the HSSI can be configured and monitored just as any other serial port. The following example applies an IP address of 193.1.1.1 to an HSSI on a HIP inserted in slot 2 in a 7x00 series router.
Router1(config)#interface hssi 2/0
Router1(config-int)#ip address 193.1.1.1 255.255.255.0
All Cisco serial ports are numbered starting at 0, and as there is only one HSSI on a HIP, it will always be port 0. Any other special encoding necessary, such as framing type or linecode, will be specified by the supplier of the high-speed line.
Line Types
For more than 25 years, telephone companies have been converting from analog to digital transmission their networks that carry voice transmission. Nearly all of the bulk communications between telephone company switching points is digital. It is only over the last mile or so from a CO to a home or small business that the communication is analog.
You may have wondered why so much of data communications is based upon 64 kbps circuits or multiples thereof. The reason is that, in digital terms, it used to take 64 kbps to cleanly transmit a voice signal. This single voice channel is referred to as a DS0. Data communications came along after the digitization of voice traffic, and therefore "piggybacked" on the voice technology in place. Let's take a brief look at the basic unit of digital transmission lines, the 64 kbps circuit.
Dataphone Digital Service (DDS).     Dataphone Digital Service, or DDS as it is commonly referred to, actually gives you only 56 kbps throughput. The additional 8 kbps is not available for data transfer and is used to ensure synchronization between the two ends of the DDS circuit. This is a function of the Alternate Mark Inversion (AMI) data encoding technique. DDS is essentially supplied over the same pairs of copper wires used for regular analog telephone connections, and two pairs, for a total of four wires, are needed for the service.
In the United States, the telephone company will install the line up to what is known as the "demarc," a point of demarcation with respect to troubleshooting responsibility. The demarc is a RJ-48 connector, to which you must connect a CSU/DSU device. In Europe and other parts of the world, the CSU/DSU is typically supplied with the circuit. The CSU/DSU is really two devices in one; the Channel Service Unit (CSU) interfaces to the telephone company's network for DDS and T-1 services, whereas the Data Service Unit (DSU), interfaces to your equipment, in this case a router.
In many locations, 64 kbps lines are now available through the use of B8ZS (Bipolar with 8 Zero Substitution) encoding that replaces AMI. This gives you back the full 64 kbps by use of a more intelligent line coding mechanism. This 64 kbps service is known as Clear Channel Capability or Clear 64. All you need to make sure of in your CSU configuration is that it has AMI encoding for 56 kbps services and B8ZS for 64 kbps service.
T-1, Fractional T-1, and T-3 Services.     A T-1 is a collection of 24 DS0 circuits, and often is referred to as a DS1. T-1 service may be delivered as either channelized or unchannelized. The unchannelized option is the easiest to understand. In effect, the unchannelized T-1 acts as a 1.536 Mbps pipe that connects to one serial interface.
In its channelized form, the T-1 can be used to deliver 24 time slots, each having 64 kbps throughput, or the PRI service previously discussed in the section on ISDN. The channelized T-1 has two typical applications. The first is to supply 24 DS0 connections over one physical link. A T-1 configured this way can be directly connected to one of the two ports on a MIP card inserted in a 7x00-series router, or the one available port on the CT1 card inserted in a 4700-series router. Once connected, the T-1 can be addressed as 24 separate serial interfaces. The telephone company can "groom" each of the separate DS0 channels to any location on its network that is serviced by an individual DS0 circuit. This is done via a device termed the Digital Access Cross Connect (DACC).
Using a T-1 in this way simplifies central site equipment management considerably. Consider a head office that needs to be connected to (conveniently) 23 remote branches. All that is necessary in the head office is one T-1 connected to a MIP or CT1 card; no additional CSU/DSU or cabling requirements are necessary.
The following is an example of how to get into configuration mode for the 22nd DS0 channel on a channelized T-1 connected to port 0 in a MIP card that is inserted in slot 1 of a 7x00-series router.
Router1(config)#interface serial 1/0:22
Router1(config-int)#
From here, an IP address can be assigned, encapsulation defined, and any other configuration entered, just as for any other physical serial port.
A T-1 also can be used as an efficient way to deliver analog phone services, but to do this, an additional piece of equipment, a channel bank, is necessary to convert the digital T-1 signals to 24 analog telephone lines. This can be useful if you need to configure many centrally located dial-up ports, into which roving users will dial into with analog modems. The Cisco AS-5200 has a built-in channel bank and modems so that simply by connecting a single T-1 connector to it, you can have up to 24 modem calls answered simultaneously. In fact, the AS-5200 is even a little more clever than that. The AS-5200 also has a built-in T-1 multiplexer, giving it hybrid functionality for call answering. This means that if you connect a T-1 configured as a PRI to an AS-5200, the AS-5200 will autodetect if the incoming call is from a digital ISDN or analog caller and will answer the call with the appropriate equipment.
As we discussed previously for DS0 channels, a T-1 can use either AMI or B8ZS encoding and either the Extended Super Frame (ESF) or D4 framing format. As long as your T-1 equipment (I'm assuming it's a router) is configured to be the same as that used by the telephone company, you should be okay.
In Europe and elsewhere, multiple DS0 services are delivered on an E-1, which comprises 32 DS0 channels, giving 2.048 Mbps throughput. The E-1 uses High Density Bipolar 3 (HDB3) encoding. If you order a 256 kbps or 384 kbps circuit from your carrier, you will get a T-1 installed and be allocated only the appropriate number of DS0 channels needed to give you the desired throughput. This service is known as fractional T-1 and is a good idea. The one constant in data networking is the increased need for bandwidth, so it makes sense to install capacity that is easily upgraded, as it probably will be needed at a later stage anyway.
A T-3 or DS3 connection is a collection of 672 DS0 circuits of 64 kbps each, which gives a total throughput of 43,008 kbps. (DS3 is the term used for this speed communication over any medium, whereas T-3 is specific to transmission over copper wires.) The actual circuit speed is somewhat faster than that, but some effective bandwidth is lost to synchronization traffic. An HSSI is the only interface that a Cisco router can use to connect to a T-3.
The hierarchy of these circuits just discussed is as follows: a single DS3 comprises seven DS2 channels, which break out to 28 DS1 channels, generating a total of 672 DS0 channels.
All this potential for faster and faster throughput does not always materialize. Imagine that a company needs to perform a mission-critical file transfer across the Atlantic five times a day. The file transfer uses TCP as the layer 4 protocol to guarantee correct sequencing of packets and to guarantee delivery. The file that is being transferred is getting bigger and taking longer to transfer, so the company is prepared to spend money to upgrade the link to speed up the file transfer. The transatlantic link is currently running at 384 kbps, and the plan is to upgrade the link to 512  kbps. Will this speed up the transfer? It depends.
TCP works on the basis of requiring from the receiving device an acknowledgment confirming that the packets sent have arrived safely before it will send more packets. The number of packets TCP will send before it stops and waits for an acknowledgment is defined by the Window size.
Let's say the Window size is set initially at 7500 bytes. Now, if you measure the round-trip delay across the Atlantic, it normally will come to around 160 milliseconds (0.16 seconds). So we have to ask ourselves, "How long does it take to clock 7500 bytes onto the link?" If it takes less than 160 milliseconds, the sending device stops transmitting and waits for an acknowledgment from the receiver before it will send any more packets. If this occurs, clearly the full bandwidth is not being utilized. So let's work out what will happen.
Multiplying 7500 by 8 bits per byte yields 60,000 bits. A 384,000 bps link will take 0.156 seconds (60,000/384,000) to clock this number of bits onto the line. You can see that if the round-trip time is 0.16 seconds, the transmitter already will have been waiting for 0.04 second before it transfers any more data. Increasing the speed of the link to anything above 384 kbps means that the sending device will just spend more time idle, waiting for acknowledgments, and the speed of the file transfer will not improve.
The only way to improve the effective throughput if a higher-speed line is installed is to increase the Window size. If this value is changed on one machine, it must be changed on all other machines with which the machine communicates—which might be something of an implementation challenge.

 


 
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