Abstract
After reading this chapter from "Deploying License-Free Wide-Area Networks" (1587050692), you'll understand the International Organization for Standardization (ISO) Open Systems Interconnection (OSI) seven-layer reference model, and you'll know the best features for wireless backbone equipment, access points (APs), PBX, wireless network cards, mesh network nodes, and amplifiers. You'll also be able to recognize compatibility problems that can cause problems when mixing wireless equipment from different vendors.
The equipment that you select for your broadband wireless wide-area network (WAN)
plays a major role in the reliability, scalability, and profitability of your network. This
chapter helps you evaluate and select your wireless network equipment.
This chapter does not list feature information vendor by vendor. The quantity of information
would be overwhelming and the listing would quickly become outdated. Instead,
this chapter aims to help you understand the features and characteristics that are available
on wireless equipment. When you understand the features and their significance, you will
be in a position to select the equipment that best meets your network needs.
Any New Features This Week?
Wireless equipment is evolving rapidly. Wireless hardware and software features change
each week. I have attempted to describe all the significant wireless hardware and software
features that were offered (by at least one equipment manufacturer) at the time I wrote this
chapter in 2002. Because of rapid equipment evolution, I suggest that you supplement the
information presented here with your own feature research.
This chapter contains the following major sections:
A description of the equipment selection process.
A brief explanation of the International Organization for Standardization (ISO) Open
Systems Interconnection (OSI) seven-layer reference model. An understanding of this
model helps you understand how various wireless features fit into your network.
A list of equipment features, arranged by OSI layer. Following each feature is an
explanation of the feature.
A summary of the features that are the most desirable for wireless backbone
equipment, access points (APs), customer premises equipment (CPE), wireless
network cards, mesh network nodes, and amplifiers.
A discussion of compatibility issues that can cause problems when mixing wireless
equipment from different vendors.
Suggestions about evaluating and receiving vendor support.
OVERVIEW OF THE EQUIPMENT SELECTION PROCESS
Your equipment purchase can involve spending only a few hundred dollars, or it can involve
spending hundreds of thousands of dollars. The more money that you plan to spend, the
more important it is that you include all of the following steps in your selection process.
Reviewing Your Wireless Network Needs
Before you select your wireless equipment, take the time to review your wireless network
needs:
How many wireless end users do you want to serve?
What network architectural elements do you want your wireless network to include?
Do you need only point-to-point links or will you deploy point-to-multipoint APs?
Do you need wireless backbone bandwidth?
Do you need mesh network nodes or repeaters?
What features will your wireless network need so that it can connect to your wired
network?
Will you need routing or only bridging?
After you have reviewed both the wireless network features and the wired features that you
need, you are ready to begin researching specific wireless equipment features.
Researching Equipment Features
Now that you know your network needs, you can begin listing wireless equipment that
matches your needs. The most difficult part of the research process is not learning what
features a particular brand of equipment offers. The most difficult part is learning what
features are not offered or which features do not work the way you expect them to work.
If you have not worked with wireless equipment before, it can be difficult to get an accurate
picture by looking only at press releases and advertising flyers. Press releases are typically
loaded with attractive buzzwords that promise wireless performance and wireless benefits
that are sometimes exaggerated or theoretical. Advertising flyers and spec sheets do not lie
about equipment performance, but they sometimes omit information that would reveal
performance shortcomings.
Evaluate equipment that offers the specific features that you need, such as distance and
bandwidth capabilities, but before you decide to buy, visit a network where that particular
vendor’s equipment is deployed.
Visiting Deployment Sites
After you have researched equipment features, you will have one or more equipment
vendors who can provide equipment that appears (at least on paper) to meet your wireless
needs. It is appropriate and proper for you to ask the vendors to recommend one or two
existing wireless networks that have deployed their equipment. Visit these sites and talk
with the network operators who have deployed the equipment.
Your visit will allow you to learn what features work especially well and what features do
not work as expected. You will learn which expectations were exceeded (the good news) and
which expectations were not met (the not-so-good news). You will learn if the equipment is
easy or difficult to manage. You will also learn if vendor support is poor, good, or outstanding.
This is information that you cannot obtain from a spec sheet or an advertising flyer. With
the benefit of this information, can make an accurate and informed decision about which
equipment to purchase.
Testing Wireless Equipment in the Lab
When you have completed your site visits, there will probably be one or two vendors that
you think would be good equipment providers. At this point, consider making a small equipment
purchase consisting of either a pair of wireless units or one AP and one CPE unit.
Set up these units indoors and become familiar with them. Configure the units and measure
their throughput in both directions. Learn to use the diagnostics.
TIP: Practice safety when you are working near wireless equipment. High amounts of microwave
energy can cause damage to the human body, so minimize your exposure to this type of
energy. Do not point a directional antenna at yourself or at any other nearby person. Turn
the wireless equipment off any time you are not testing it. Remember: When you double
the distance between yourself and a wireless antenna, you reduce the amount of radiation
reaching you to one-fourth the previous level. Whenever possible, maintain as much
distance as possible between yourself and a wireless antenna.
When your indoor testing is complete and you are comfortable with the units, proceed to
outdoor testing.
Testing Wireless Equipment Outdoors
Testing wireless equipment outdoors allows you to test the range, throughput, and
reliability of the equipment in the presence of real-world noise, interference, and weather.
For your outdoor testing, perform the following steps:
Step 1 Pick two locations that are as far apart as the maximum link distance that
you expect the equipment to cover. For example, if you plan to build a
wireless cell with a 4-mile (6.4 km) radius, pick an AP location that is
high enough to have at least two line-of-sight (LOS) paths that are at least
4 miles long.
Step 2 Test using an AP antenna system similar to the one that you expect to use
in your actual network deployment.
Step 3 Temporarily set up the CPE at first one and then the other of your two test
locations.
Step 4 Test during the busiest part of the day and repeat the throughput tests that
you performed indoors. It is important that you test the throughput from
the CPE to the AP. This is an important test of the AP’s capability to
receive in the presence of noise and interference. For more details about
throughput testing, see the description in Chapter 7, "Installing Outdoor
Wireless Systems."
Step 5 If possible, repeat your performance testing several times over a period
of several days or weeks. The equipment performance should remain
constant throughout the entire test period.
Your outdoor testing will not tell you how many customers the AP will handle at full load,
but it will give you a good preliminary performance indication. If all your test results are
good, proceed to the following purchase decision step.
Making Purchase Decisions
Your testing should bring you to the point where you are most comfortable with the performance
of one or two brands of wireless equipment. You can now make your purchase
decision and be fairly confident that the equipment you buy will meet your performance
expectations.
OSI AND TCP/IP REFERENCE MODELS
Several different but similar layered data architectures have been developed to allow
reliable data transfer between different computer systems and between different networks.
When you understand a little about the service layers and the protocols that these architectures
use, you will be in a good position to understand the similarities and the differences
between different brands of wireless equipment.
The seven-layer ISO OSI reference model was first proposed around 1983 to allow connectivity
(or interworking) between different computer systems. Prior to the OSI reference
model, computer systems made by one manufacturer could not easily communicate with
computer systems made by other manufacturers. The intent of the OSI reference model was
to allow computer systems to successfully communicate with each other even though
different vendors manufactured them. Figure 6-1 shows the OSI reference model alongside the TCP/IP architecture.
Beginning in the 1970s, the United States Department of Defense began promoting
computer networking between university research departments and government installations.
One of the primary goals of this internetworking effort was to develop a survivable
network—one that would be able to continue communicating even if some of the network
nodes or some of the communications links were destroyed. This new networking effort
was based on two primary protocols: the Transmission Control Protocol (TCP) and the
Internet Protocol (IP). TCP performed transport layer functions equivalent to the transport
layer in the OSI model. IP performed network layer functions that were equivalent to the
network layer (Layer 3) in the OSI model. When application layer (Layer 7) protocols
(Telnet, FTP, SMTP, and so on) and physical (Layer 1) and data link layer (Layer 2)
protocols were added, the result was an architecture that effectively contained five layers.
Figure 6-1 shows the TCP/IP model alongside the OSI reference model for comparison.
The TCP/IP architecture is functionally equivalent to the OSI reference model. The major
similarities and differences are as follows:
Both models have an application, a transport, and a network/Internet layer.
The TCP/IP model does not have a session layer (Layer 5 of the OSI reference model)
or a presentation layer (Layer 6 of the OSI reference model).
Both models have a lower layer that connects the upper layers to the actual physical
network. In the OSI reference model, the lower layer (Layer 1) is called the physical
layer. In the original TCP/IP model, the lower layer was called the host-to-network
layer. In present-day use, TCP/IP networks use the combination of a Layer 2 sublayer
called the medium access control (MAC) sublayer along with Layer 1 to provide
connectivity over the wireless link.
Virtually all the wireless equipment features that you evaluate operate at the physical, data
link, and network layers of the OSI and TCP/IP reference models. The wireless features and
functionality (modulation type, data rate, and so on) take place at the physical layer. Access
to (and sharing of) the wireless medium takes place at the data link layer. Routing takes
place at the network layer.
Peer Protocols
Peer protocols run across the Internet but provide communication only between same-layer
processes. One example of this same-layer communication process is a Hypertext Transfer
Protocol (HTTP) web browser running on the application layer of one network. The HTTP
browser retrieves information from its peer web server running on the application layer of
another network. Although the HTTP communication is application-layer-to-application
layer (peer-to-peer), both networks communicate downward through their lower network
layers.
Services
Information is passed from the top (application) layer of one network down through the
lower layers. Each layer provides a set of services for the layer just above it and utilizes the
services provided by the layer just below it. The set of services between two layers is
referred to as the interface between the two layers. For example, Layer 6 provides services
for Layer 7; Layer 5 provides services for Layer 6; and so on. In this way, Layer 7 (the
application layer) can communicate all the way down to Layer 1 (the physical layer).
The following list illustrates how services and protocols operate. When your web browser
uses HTTP over a wireless network, the information flow is as follows:
The HTTP information request originates at the application layer on the originating
network.
The HTTP request travels downward from the application layer (using the services
provided by all the intermediate layers) to the physical layer on the originating
network. The physical layer uses the appropriate wireless protocol (for example, the
appropriate direct sequence spread spectrum modulation or DSSS) to communicate
the request over the air wirelessly to the physical layer on the other network.
The physical layer on the other network uses the DSSS protocol to receive the request
from the originating network. The physical layer then passes the information up
through its interface to the data link layer. Using higher and higher layer services,
the request passes upward until it eventually reaches the application layer. There,
the HTTP protocol processes the request and replies with a response.
Using services of lower and lower layers, the response travels downward to the
physical layer. Using the Layer 1 DSSS protocol, the response is transmitted over
the air back to the physical layer of the originating network.
Using services, the originating network passes the response upward to the application
layer where the HTTP protocol receives the response to its original request.
Basic Packet Structure and Frame Types
Packet switching store-and-forward techniques underlie the operation of layered architectures.
Packet switching uses an underlying data structure, called a packet. A packet is like
a hamburger on a bun. Although the exact packet structure varies from layer to layer and
protocol to protocol, most packets contain a data payload section. The data payload is the
hamburger in the middle of the bun. Other fields encapsulate (surround) the data payload
and make up the bun. The bun typically provides the following:
Packets prepared by Layer 2 (the data link layer) are called frames. Not all frames contain
payload data. Wireless APs and wireless stations exchange three types of frames, each with
the following functions:
Data frames carry user payload data (the hamburger) between different wireless
network nodes.
Control frames carry information such as request-to-send (RTS) and clear-to-send
(CTS) messages as well as frame acknowledgments (ACK).
Management frames carry association and authentication requests and responses in
addition to beacon information.
Application Layer Functions and Protocols
The application layer is where the end user programs run. Telnet, Simple Mail Transfer
Protocol (SMTP), File Transfer Protocol (FTP), and HTTP are examples of application
layer protocols. Wireless equipment that you evaluate will likely have network
management software that operates at the application layer level.
Transport Layer Functions and Protocols
The transport layer’s job is to provide reliable communications from application to application
regardless of the lower-layer protocols and communications links. The transport
layer encapsulates data from the application layer (and the session layer, if used) and passes
it down to the network layer.
Typical transport layer protocols are TCP and User Datagram Protocol (UDP). Wireless
equipment that you evaluate does not usually have features that operate at the transport
layer level.
Network Layer Functions and Protocols
The essential network layer protocol is IP. In addition to IP, the network layer often utilizes
other routing protocols such as Routing Information Protocol (RIP) and Border Gateway
Protocol (BGP).
The network layer encapsulates data (the hamburger) from the transport layer between IP
source, IP destination, and IP routing information. Packet routing typically goes from intermediate
network to intermediate network before the packets finally arrive at their destination
network.
Data Link Layer Functions and Protocols
The data link layer includes the logical link control (LLC) sublayer and the MAC sublayer. The
data link layer normally performs a wide variety of functions, including segmenting the bit
stream into frames, error handling, flow control, and access control.
Examples of data link layer protocols include Point-to-Point Protocol (PPP) and Spanning
Tree Protocol.
LLC Sublayer Functions and Protocols
The LLC sublayer makes up the top half of the data link layer and interfaces to the network
layer (above) and the MAC sublayer (below). The LLC Sublayer encapsulates the Layer 3
data by adding sequence and acknowledgment numbers. The LLC Sublayer might provide
different service options, depending on the network software.
MAC Sublayer Functions and Protocols
The MAC sublayer makes up the bottom half of the data link layer. The MAC sublayer
interfaces to the physical (wireless) layer and provides the following functions:
Reliable delivery — The MAC sublayer provides a reliable delivery mechanism that
looks for an acknowledgment for every frame that is sent. If an acknowledgment is
not received, the MAC sublayer retransmits the frame.
Access control — The MAC sublayer controls access to the wireless channel. The two
basic types of access control are carrier sense multiple access with collision avoidance
(CSMA/CA) and polling. CSMA/CA is a distributed coordination function (DCF)
because the decision about when to transmit is distributed to all wireless stations.
Each wireless station listens before transmitting. If a station hears that the frequency
is busy, it backs off (waits) a random amount of time and tries again. When the
frequency is clear, the station proceeds to transmit. In addition, a request-to-send/clear-to-send (RTS/CTS) mechanism can be enabled. Large packets are more likely
to collide; therefore, stations that have packets larger than the RTS/CTS threshold
must request and receive clearance from the AP MAC before they can transmit their
packets. Finally, in networks that have heavy traffic and many end users, a point
coordination function (PCF) can be used. One single point (the MAC in the AP)
coordinates transmissions from all stations. The PCF polls each wireless station and
then tells each end user when it can transmit.
NOTE: You might have heard of wireless networks with a hidden node problem. This problem can
occur in a network that uses DCF. In most wireless networks, certain wireless stations
cannot hear all the other wireless stations. Under heavy traffic loads, several stations might
try to transmit at the same time. This can happen even when the stations are using RTS/CTS.
When stations transmit at the same time, collisions occur and network throughput drops
drastically. The solution to hidden-node problems is to use wireless equipment that can
support PCF.
Encryption —— MAC also provides encryption. The most frequently used
encryption method is wired equivalent privacy (WEP).
MAC control frame subtypes include RTS, CTS, and ACK. Examples of management
frame subtypes include Beacon, Probe Request, Authentication, and Association Request.
Physical Layer Functions and Protocols
The physical layer transports encapsulated data from the data link layer and transmits it
wirelessly to the distant network. There are several physical layer wireless standards. There
are also many proprietary physical layer wireless protocols. In addition to your wireless
feature evaluation, you will evaluate physical layer wired-interface features such as
Ethernet and serial port features.
PHYSICAL LAYER WIRED-INTERFACE FEATURES
This section describes the wired-interface feature options that you will encounter when you
begin to research and select your wireless equipment.
NOTE: No wireless equipment vendor offers all the listed features in any model of their wireless
equipment—nor should they. Each wireless network is built to serve a specific set of end
user needs. These end user needs dictate the best set of wired and wireless features for that
particular network. Each feature listed in the following sections is offered on at least one
brand and model of wireless equipment. It is important that, as you consider the available
features, you keep your wireless network needs in mind. Your equipment research involves
finding the best match between your network needs and the wireless equipment feature set
offered on a particular model of wireless equipment.
Your physical layer wired-interface feature evaluation includes some or all of the following:
Low-speed data ports
Ethernet ports
High-speed data ports
Voice interfaces
Low-Speed Data Ports
Most wireless equipment contains at least one low-speed port, such as the following:
Low-speed serial data ports — On some equipment, a low-speed serial data port is
used for initial system con.guration. The serial port speed generally ranges between
4800 bits per second (bps) and 19.2 kilobits per second (kbps).
Low-speed user data ports — Early wireless modems were frequently designed to
transport only low-speed serial data over the wireless link. Port speeds range from
4800 bps to 128 kbps.
Dialup telco interfaces — Dialup telco interfaces provide low-speed dial backup
connectivity for times when a higher-speed primary link, such as a T1 or digital
subscriber line (DSL) link, is unavailable.
Ethernet Ports
Ethernet interfaces allow network data to access the wireless network. Wireless equipment
can include one or more of the following Ethernet interfaces:
10Base-T — This is the most common Ethernet interface.
100Base-TX — This interface is found on higher-speed wireless equipment.
Ethernet hubs or switches — This interface is found on some wireless APs.
High-Speed Data Ports
In addition to Ethernet interfaces, it is often desirable to use wireless bridges or routers to
transport other high-speed non-Ethernet data streams. To transport these streams, wireless
equipment can include the following interfaces:
Digital subscriber line (DSL) interfaces — DSL interfaces enable a wireless bridge
or router to extend or share a DSL connection.
Cable interfaces — Cable interfaces enable wireless equipment to extend or share a
cable Internet connection.
Asynchronous Transfer Mode (ATM) interfaces — ATM interfaces enable wireless
equipment to connect to and from an ATM network.
T1/E1 interfaces — T1/E1 interfaces enable wireless equipment to extend a 1.544
megabit per second (Mbps) T1 line or a 2.048 Mbps E1 line from point A to point B,
for example, between two private automatic branch exchanges (PABXs). Some
models of wireless equipment provide T1/E1 connectivity only. Other wireless
equipment models provide simultaneous wireless Ethernet and T1/E1 connectivity.
TIP: Telecommunications managers who want to provide both Ethernet and voice-PABX
connectivity between buildings find wireless equipment that simultaneously provides both
Ethernet and T1 connectivity to be especially useful.
T3/E3 interfaces — 45-Mbps T3 interfaces enable full-duplex wireless equipment to
provide T3 connectivity between two points.
Optical Carrier 3 (OC-3) interfaces — 155-Mbps OC-3 interfaces enable full-duplex
wireless equipment to provide OC-3 connectivity between two points.
Wireless OC-3 equipment usually has the capability to carry several T1 or E1 circuits
in addition to the OC-3 circuit.
Optical Carrier 12 (OC-12) interfaces — 622-Mbps OC-12 interfaces allow full-duplex
wireless equipment to provide OC-12 connectivity between two points.
TIP: Remember that, in general, there is an inverse relationship between wireless bandwidth and
wireless distance; as bandwidth goes up, distance goes down. OC-12 wireless equipment
typically operates only over distances up to approximately 1312 ft. (400 m).
Voice Interfaces
Voice interfaces enable wireless equipment to carry voice in addition to data. The following
types of voice interfaces are possible:
Voice over Internet Protocol (VoIP) interfaces — VoIP interfaces allow IP
telephones to connect directly to the wireless equipment and to make on-network
voice calls. Making calls to the public switched telephone network (PSTN) requires
the use of an external telephone gateway.
Talkback/orderwire interfaces — A talkback interface (sometimes called an order
wire) provides a two-way voice circuit. Maintenance personnel normally use this
circuit for end-to-end voice communication over the wireless link while servicing the
wireless equipment.
Wired-Interface Security Features
Physical layer wired-interface security features limit user access to the system administration
console via a serial port or an Ethernet port. Successful entry of a password is
required before gaining access to system administration functions. Additionally, some
equipment allows a management station IP address to be configured. Attempts to access
the system administration console from other IP addresses are refused.
Disclaimer:
Reproduced from the book Deploying License-Free Wide-Area Networks. Copyright 2003, Cisco Systems, Inc. Reproduced by permission of Pearson Education, Inc., 800 East 96th Street, Indianapolis, IN 46240. Written permission from Pearson Education, Inc. is required for all other uses. Visit www.ciscopress.com for more information about this title.
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