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SOHO Networking Basics: Concepts & Components
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SOHO Networking Basics
Author’s remarks
 A substantial amount of materials in this set of handout is adapted from Wikipedia
and Guide to Networking Essentials (2nd edition) published by Course Technology.
 This set of materials is co-developed by Chung, C.F. Jeffrey and Alvin C. M. Kwan.
What is Computer Networking?
Computer networking involves connecting computer systems for the purpose of sharing
information and resources. It requires a great deal of technology and there is a number of
decisions to be made regarding the choices for physical connection as well as related
communication software.
What Does Computer Networking Offer?
Some advantages of computer networking are as follows:
 It permits users to share information, e.g., through file sharing, as well as computer
hardware, e.g., network printers.
 Tasks of distributed nature can be processed by networked computer systems by
exchanging data and intermediate results among themselves. For example, Fedex
tracks the courier items during their delivery.
 It helps improve human communication by reducing physical document flow and
transposition error, e.g., through e-mail.
Communication Overheads
In addition to the extra software and hardware, data communications involve a number of
overhead costs too. Such overheads exhibit in form of extra control information and
processing time that are required to make the data communication feasible and reliable.
Some overheads are listed below:
 Each computer/terminal/node in a network must be assigned with a unique address so
that messages can be directed to the right destinations. This implies that every
message has to be tagged with a destination node’s address which is stored in the
header of a packet.
 Instead of transmitting entire message through the network in one shot, a message is
divided into small pieces, often referred to as packets, before it is directed to the
network so that a transmission error will only require the retransmission of the
problematic packet instead of the entire message. Typically a sequence number is
included in each packet header for the reconstruction of the original message.
 To ensure an error-free communication, messages are usually tagged with control
information (e.g., checksum) for error detection and probably error recovery too. To
avoid negotiating with the source host for a retransmission of a packet that encounters
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a transmission error, control information that support error recovery instead of error
detection only is often included in the trailer of a packet.
To reduce communication cost, messages are often compressed before transmission.
Compressed messages are decompressed at the receiving end.
For applications like e-commerce, data security is needed and thus messages are
encrypted before transmission and decrypted at the other end.
For large computer networks, communication nodes may be linked together in more
than one way (e.g., via different paths on the Internet). Thus, a decision must be made
to choose which communication path (or route) to use. In practice, the decision is
typically made by a kind of data communication equipment called router.
The above points indicate the necessity of including additional control information to the
message before transmission. Those control information may either be stored in the
header or trailer of the data packet. For example, node address is typically stored in the
header whereas checksum is typically stored in the trailer.
Teaching remark
 One way to introduce the topic communication overheads is to use the analogy of
posting a letter. The purpose of sending a letter is of course to bring a message across
to the recipient. However we need to write the message down on a piece of paper
(encoding), enclosing the letter with an envelope (like control information in data
communication) with an address written on (recipient address to locate the address).
The letter is to be carried forward to the recipient by a postman from the post office
(which is an external party). A similar analogy is on moving to a new home. In this
case, the idea of packing and unpacking belongings into boxes before and after the
moving would be useful to illustrate the idea of fragmenting and reconstruction of the
message before and after the transmission respectively. Protection film or foam
rubber that wraps up stuff in the moving example is analogous to the inclusion of
control information (in form of header or trailer) to help achieve a secured data
transmission in a networked environment.
Data Transmission Across Packet Switched Network
(Discussion on “circuit switching” is out of syllabus)
In a large network or a network of networks, there is often more than one path or data
link that a packet can traverse from a source host to a destination host. The OSI model
does not define how packets are transmitted across a network. Instead it specifies
decisions that a protocol needs to make when considering the issue. For example, must
all packets of a message be following the same data link? The dominant communications
paradigm, packet switching, allows packets to be individually routed over different data
links (see Figure 5). This contrasts with another paradigm, circuit switching, which sets
up a dedicated data link between the source and destination nodes for their exclusive use
for the duration of the communication.
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Figure 5. Data transmission across a packet switched network
There are several deficiencies in a circuit switched network.
1. The overhead of setting up a dedicated link before any application data is transferred
can be costly especially when the amount of data to be transferred is small.
2. When any network node in the dedicated data link malfunctions, a new end-to-end
connection is needed to be established before any remaining data can be transmitted.
3. Any spare data transmission capability (which is more commonly known as
bandwidth) that is not taken up by a data transfer in a circuit switched network will be
wasted, e.g., when the source host is unable to transmit data to the network at a speed
that reaches the network bandwidth.
Although it may appear that packet switching is far better than circuit switching, such an
understanding is not always correct because of the following reasons.
1. A routing decision is to be made for the transmission of each packet but a routing
decision is made once only in a circuit switched network.
2. In packet switched networks, such as the Internet, each data packet is labeled with the
complete destination address and routed individually. However circuit switched
networks, such as the voice telephone network, allow large amounts of data be sent
without continually repeating the complete destination address as a dedicated data
link is used exclusively.
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In general, packet switching can optimize the use of the network bandwidth (as it can be
shared by multiple data transfers between multiple source and destination hosts) and
increase robustness of communication (as data transfer can be conducted on different data
links and any failure on a network node will have minimal impact to a packet switched
network). However circuit switching is not of no value. It aims to achieve minimal data
delay and thus a better quality of services (which is often defined by a maximal tolerable
data delay). Such a property is critical to computer applications that require a smooth
data transfer between the source and destination hosts, e.g., audio and video data.
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Applications of Small Office/Home Office (SOHO) Networking
A SOHO network is a small office/home office local area network. A local area network
(LAN) is a collection of computers and other networked devices that fit within the scope
of a single physical network. LAN covers a small local area, like a home, office, or small
group of buildings such as a university. Communication media are owned by the LAN
owner. This contrasts to wide area network or WAN which is a computer network
covering a wide geographical area, involving a vast array of computers, e.g. the Internet.
SOHO networks generally are confined to a single room. Such networks generally
connect communicating devices to a router, small switch, or hub through physical cables
(in a wired network) or wirelessly (in a wireless network). Conceptually the networking
technology and basic network components involved in SOHO networking are not much
different from large networks. The major differences are in the scale and complexity.
Generally SOHO networks are used to share information and hardware like files and
printers as well as to share an Internet access connection. A SOHO network may also
have a server, e.g., a web server, which needs to be accessed.
SOHO networking facilitates a new way of work arrangement called telecommuting,
telework or working from home (WFH). Employees enjoy flexibility in working location
and hours (within limits). The motto is that “work is something you do, not something
you travel to”. A successful telecommuting programme requires a management style
which is based on results, i.e., “managing by objective”, and not on close scrutiny of
individual employees, i.e., “managing by observation”.
Wikipedia has the following description about the potential benefits of telecommuting.
Telecommuting is seen as a solution to traffic congestion (due to single-car
commuting) and the resulting urban air pollution and petroleum use. Initial
investments in the network infrastructure and hardware are balanced by an
increased productivity and overall greater well-being of telecommuting staff
(more quality family time, less travel-related stress), which makes the
arrangement attractive to companies, especially those who face large office
overhead and other costs related to the need for a big central office (such as the
need for extensive parking facilities).
The above excerpt indicates that the impact of networking technology is far beyond the
technology arena. In fact, many large companies in the United States (of America) have
successfully taken advantage of the networking technology to save their operational costs.
One example is that many USA companies establish their telephone support services in
India. When their clients make a phone enquiry to them, the calls are actually connected
to their staff in India with the use of Internet phone technology. The labour cost in India
is perhaps less than one-tenth of the America counterpart.
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A Computer Network Scenario
To help explain concepts about SOHO networking, the following scenario is created (see
Figure 6). Note that the computer network being described is a LAN instead of a SOHO
network. The LAN is composed of three smaller LANs and a web server which are
separated by a firewall (which will be introduced later). The network adopts the TCP/IP
protocol and thus each of the network devices is allocated with an IP address. Note that
some IP addresses are reserved for special purposes. For instance, some IP addresses are
used for message broadcasting and some others support message multicasting to
predefined groups of network devices.
Figure 6. A computer network scenario.
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The given network scenario describes the computer network of a small trading company.
It has a sales department and an inventory section. The company is managed by a
manager who has a personal assistant. All the parties mentioned above need to use
computers to support their duties in the company. Considering the confidentiality issue,
computers of the manager and his assistant are connected to a peer-to-peer subnetwork
(which will be detailed later) which is separated from the other two subnetworks of the
company network – one for the inventory section and another for the sales department.
The subnetwork for the inventory section is a wireless network composed of wireless
access points (which will be introduced later) and a combination of desktop and handheld
computers. Some access points are installed in the warehouse to enable the inventory
clerks to update the inventory database online during inventory checks. The last
subnetwork is owned by the sales department. It is a client-server subnetwork (which
will be detailed later). In order to save cost, a printer server is set up to allow users to
share the network printer. Besides, instead of allocating one computer to each staff
member in the department, a pool of computers is kept. To access a computer, a user
needs to log in. All user files are kept in the file server instead of the local machines so
that the sales staff does not need to remember on which computer systems that they have
stored their file in the past. For promotion purpose, the company has set up its company
website. All computers in the company are Internet enabled.
Peer-to-Peer Network vs. Client-Server Network
Server Computer
A server computer or simply a server is a computer that provides a (remote) service to
other computer(s) by some kind of network. As shown in Subnet A in Figure 6, the
services can lead to sharing of information (e.g. file sharing), hardware (e.g., printer
sharing) or other types of resource sharing (e.g., IP address sharing through the use of a
DHCP server which will be elaborated later). Web services provided by a web server is
another example on resource sharing (see top of Figure 6).
Client Computer
A client computer or simply a client is a computer that accesses a (remote) service on
another computer by some kind of network. In Subnet A (in Figure 6), four computers
can access the services of the DHCP, file and printer servers within the subnet and the
services of its own web server (outside the subnet).
Peer-to-Peer Network
In a peer-to-peer network, any computer can function as either a client or a server, e.g.
one computer shares its DVD-recorder while another shares its printer for one another.
No one computer has any higher priority to access, or heightened responsibility to
provide, shared resources on the network. The user access privilege for each computer
resource in a peer-to-peer network is maintained separately.
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The advantages of peer-to-peer networking are:
 Easy to install and configure
 Needs no dedicated administrator
 Not dependent on a dedicated server (and thus no single point of failure)
 Individual users control their own shared resources
 Needs no additional equipment or software beyond a suitable operating system, e.g.,
MS Windows XP
 Inexpensive to purchase and operate
 Works best for simple networks with a few users
The disadvantages of peer-to-peer networking are:
 Network security applies only to a single resource at a time
 Users may be forced to use as many passwords as there are shared resources (unless
some “centralized” coordination effort
 Each machine must be backed up individually to protect all shared data
 Access of a shared resource causes a reduced performance of the machine where the
resource resides suffers
 No centralized organizational scheme to locate or control access to data
 Does not work well as the number of users grows or for complex networks
Client-Server Network
In a client-server network, user computers act as clients of dedicated server machines that
handle network requests from their clients. As a server needs to respond to the requests
of a number of clients, it usually requires a more powerful machine.
The advantages of client-server networking are:
 Simplified network administration due to the use of centralized user accounts,
security, and access controls
 More powerful equipment enables clients to have more efficient access to network
resources
 Appropriate for networks with five or more users or any networks where resources
are used heavily
The disadvantages of client-server networking are:
 Server failure can result in a network unusable, or at least in loss of network resources
 Complex, special-purpose server software requires allocation of expert staff, which
increases expenses
 Dedicated hardware and specialized software add to the cost
Basic Network Components
A number of network components are used in the computer network scenario given in
Figure 6. They are client computers, server computers (e.g., web server, file server and
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printer server), dial-up and cable modems, hubs, switches, routers including the Internet
Service Provider (ISP) (broadband) routers, gateways, and wireless access points. Other
network components that are not explicitly shown in the diagram include network
interface cards (NIC) and networking media, etc. There are also some network
components which are omitted in the diagram such as repeaters and bridges. All those
components will be introduced below.
Networking Media
A networking medium, which may be tangible (e.g., cables in a wired network) and
intangible (e.g., radio signal in a wireless network), is a medium across which network
data can travel in the form of a physical signal, whether it is a type of electrical
transmission or some sequence of light pulses. Examples of tangible media are coaxial
cable, twisted pair cable, and fiber-optic cables. Examples of intangible media are
infrared, microwave and radio wave. Details about networking media will be given later.
Figure 7. A network cable.
Network Interface Card
A network interface card (NIC)or network adaptor establishes and manages the network
connection of a network device. It translates parallel digital computer data into serial
signals appropriate for transmission along the network medium and serial signals into
parallel digital computer data for incoming network data.
Figure 8. A network interface card.
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Teaching remark
 To test an NIC, issue a ping command to the loopback IP address 127.0.0.1 in a
command window. Virtually any data written to a network that starts with the
number 127 will be written to the output buffer of the NIC and then read in form the
input buffer of the same NIC. If the NIC works properly, a screen output similar to
the one below will be displayed.
Dial-up Modem
Telephone lines are not suitable for carrying digital signal as it was designed for carrying
voice which is analog in nature. A dial-up modem (a short form of modulatordemodulator) modulates digital signal from a source host to analog signal before it gets
into the telephone network and analog signal is demodulated back to digital signal for the
destination host at the other end. A dialup modem can be either internal (like a PCI card)
or external (see Figure 9). Due to the slow data rate (i.e., bandwidth) of the telephone
network, it is almost obsolete nowadays.
Figure 9. A dial-up modem.
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A specific type of modem is called the Asymmetric Digital Subscriber Line (ADSL or
DSL) modem. For most Internet users, the download data rate is far more important than
the upload rate as most of their data traffics are of the download type. ADSL modems
enable faster data transmission over copper telephone lines by supporting faster data flow
in one direction than the other, i.e., asymmetrically. The basic design rationale is that
there is likely to be more crosstalk (i.e., undesirable electrical interference) from other
circuits at the digital subscriber line access multiplexer end (where the wires from many
local loops are close together) than at the customer premises. Thus the upload signal is
weakest at the noisiest part of the local loop, while the download signal is strongest at the
noisiest part of the local loop. This explains why the download data rate is configured to
be higher than the upload data rate.
Cable Modem
A cable modem (see Figure 10) is a special type of modem that is designed to modulate a
data signal over cable television infrastructure by taking advantage of unused bandwidth
on a cable television network (e.g. i-CABLE of CableTV). It is primarily used to deliver
broadband Internet access. Cable modems usually deliver speeds comparable to that of
ADSL modems though the latter generally have better upload speeds. Users in a
neighborhood share the available bandwidth provided by a single coaxial cable line.
Therefore, connection speed can vary depending on how many people are using the
service at the same time. Since cable networks tend to be spread over larger areas than
ADSL services, more care should be taken to ensure good network performance.
Figure 10. A cable modem.
Hub
A hub is a device for connecting multiple network devices together (see Figure 11),
making them act as a single segment and providing bandwidth which is shared among all
the connected devices. A hub typically provides four or more ports (through which data
are sent and received) into which a plug or cable connects.
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Figure 11. A hub.
Nowadays most hubs are active in the sense that they serve as a repeater too. A repeater
is an electronic device that receives a weak or low-level signal, then amplifies, reshapes,
retimes, or performs a combination of any of these functions on the received signal and
finally retransmits it at a higher level or higher power, so that the signal can cover longer
distances without degradation. Data signals are weakened or degraded as they traveled
along the media due to energy loss. For example, data signals in form of electrical pulse
lose energy, usually in form of heat, as they pass along a conductive wire. Such a
phenomenon is known as signal attenuation. An attenuated signal may be too weak to be
discerned and that is why repeaters are sometimes introduced in a computer networks.
Teaching remark
 Hubs, repeaters and network cables work at the physical layer of OSI Model.
Switch
A switch (see Figure 12) offers the link management that a hub can provide, with greater
bandwidth and intelligence. Unlike hubs which are designed to connect network devices
in a particular way (i.e., network topology), a switch can be “programmed” to support a
variety of networking topologies.
Figure 12. A switch.
A switch can also be configured to organize groups of devices into virtual LANs to route
transmission among one or more groups of selected attached devices. Data received by a
hub is broadcast to all connected devices including any non-destination nodes through the
hub’s port and it is up to those devices to decide whether they need to act on the received
data. Switches are intelligent enough to identify and use only the port(s) to which the
destination devices are connected. Thus, unlike a hub, a switch allows multiple data
transmissions across a switch at the same time as long as the data transmissions do not
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involve the use of the same ports. This means that a switch can potentially support a
larger bandwidth than a hub. Nowadays, switches are often used to replace network hubs
and some people may refer a switch to as an intelligent hub.
Teaching remark
 Switches work at the physical layer of OSI Model.
Bridge (out of syllabus)
A network bridge or bridge connects multiple segments of a local area network together.
Unlike repeaters which work at the physical layer, bridges work along the data link layer
of the OSI Model. The key advantage of bridges over repeaters is that bridges can filter
traffic to ease congestion of network traffic. A bridge keeps a list of MAC addresses and
the network segment of each address. When the bridge receives a data packet, it
compares the packet’s source and destination addresses to its bridge table. If the two
addresses are found to be on the same network segment, the bridge discards the data
packet as there is no need to forward it to another network segment. Otherwise, the
bridge sends the packet to all segments except the one that received the packet. As a
bridge table will be examined for each data transfer, the speed of bridges is slower than
that of repeaters.
Router
A router (see Figure 13) forwards data packet across different networks, if necessary,
through a process known as routing until it reaches its destination.
Figure 13. A router.
Teaching remark
 Routing work at the network layer of OSI Model.
Wikipedia gives a brief description of routers (including Figure 14) as follows:
In non-technical terms, a router acts as a junction between two networks to
transfer data packets among them. A router is essentially different from a switch
that connects devices to form a Local Area Network (LAN). One easy illustration
for the different functions of routers and switches is to think of switches as
neighborhood streets, and the router as the intersections with the street signs.
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Each house on the street has an address within a range on the block. In the same
way, a switch connects various devices each with their own IP address(es) on a
LAN. However, the switch knows nothing about IP addresses except its own
management address. Routers connect networks together the way that onramps or
major intersections connect streets to both highways and freeways, etc. The street
signs at the intersection (routing table) show which way the packets need to flow.
Figure 14. Routers are like intersections whereas switches are like streets.
In the above diagram, the disc symbols represent routers whereas the rectangles represent
switches. Other network devices are shown by their IP addresses only. As a router
connects two networks together and thus it uses two IP addresses, one in each network.
A router that connects clients to the Internet, usually provided by an Internet Service
Provider (ISP), is called an edge router or ISP router.
Wireless Access Point
A wireless access point (WAP or AP) is a device that connects wireless communication
devices together to form a wireless network (see Figure 15). The WAP usually connects
to a wired network, and can relay data between wireless devices and wired devices.
Figure 15. A wireless access point.
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In SOHO networking, a wireless broadband router is often used instead of a WAP as
most wireless broadband router is really three devices in one box. First, there is a WAP.
Second, it serves as a hub to connect to several networking devices. Finally, the router
function ties it all together and lets the whole network share a high-speed cable or DSL
Internet connection.
Gateway
Gateways, also called protocol converters (see Figure 16), can operate at any layer of the
OSI model. Typically, a gateway converts one protocol stack into another. It is much
more complex than that of a router or switch. A gateway is commonly positioned at the
common intersection between a LAN and a WAN (which is typically the Internet in a
SOHO network). There the gateway commonly performs address translation (NAT),
presenting all of the LAN traffic to the WAN as coming from the gateway’s WAN IP
address and doing packet sorting and distribution of return WAN traffic to the local
network.
Figure 16. A gateway.
Firewall
A firewall aims at preventing any communications forbidden by the security policy. It
can be implemented in a piece of hardware (see Figure 17) and/or software. It has the
basic task of controlling traffic between different zones of trust. Typical zones of trust
include the Internet (a zone with no trust) and an internal network (a zone with high trust).
The goal is to provide controlled connectivity between zones of different trust levels
through the enforcement of a security policy and connectivity model based on the least
privilege principle (see Figure 18). Proper configuration of firewalls requires
considerable understanding of network protocols and of computer security. Small
mistakes can render a firewall worthless as a security tool.
Figure 17. A (hardware) firewall.
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Figure 18. Controlling traffic between different zones of trust with firewalls.
The DMZ indicated in Figure 18 stands for a demilitarized zone. It is a network area (a
subnet) that sits between an organization’s internal network and an external network such
as the Internet. Connections from the internal and the external network to the DMZ are
permitted, whereas connections from the DMZ are only permitted to the external network
– hosts in the DMZ may not connect to the internal network. This allows the DMZ’s
hosts to provide services to the external network while protecting the internal network in
case intruders compromise a host in the DMZ. For someone on the external network who
wants to illegally connect to the internal network, the DMZ is a dead end. The DMZ is
typically used for connecting servers that need to be accessible from the outside world,
such as e-mail, web and domain name servers.
Internet Access Methods
A network can access the Internet through a dedicated leased line or a usual phone line of
the public telephone network (using a dial-up modem), or the cable TV network (using
cable modem) or other ISP broadband networks (using ADSL modem, for instance).
Broadband connections to the Internet through cable or ADSL modems support both
wired and wireless networks.
Table 1 gives the characteristics of various Internet access methods in terms of the
equipment required, cost, data transfer rate, service reliability and number of users that
the Internet access can support. Although it may sound reasonably to use a broadband
Internet access instead of a leased line from a cost view point, the latter has the advantage
of being more reliable due to the use of a dedicated line. For some time critical
applications which require a guarantee quality of service in the response time, there may
be a point to stick to the seemingly more expensive leased line option.
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Modem dialup
Leased line
Broadband
Dialup modem,
telephone lines
Modem,
telephone lines
Cable or ADSL modem,
ISP router,
Category 5e/6 cable
and/or optical fiber cable
Very low. Less than
HKD$100
High. Typically costs
HK$1000+
Low to high. HKD$1001000+ per month
Equipment
required
Monthly
cost
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Data
transfer
rate
Slow, support up to
56Kbps only
Service
reliability
Low. Internet
connection can be
interrupted by
incoming phone calls.
No. of users
(rough
estimate)
Single user only. May
consider it as a backup
resource.
Medium to fast.
128Kbps (ISDN
connection) to 45Mbps
(T3 connection)
Excellent as the
connection is not
shared with any other
people
The bandwidth is
adequate for supporting
dozens to a few
hundreds of users.
Fast to very fast.
Typically 1.5Mps1000Mbps
Good. Data noise may
occur occasionally due to
bandwidth sharing within
the same building.
The bandwidth is
adequate for supporting a
few to a few dozens of
users.
Table 1. Characteristics of various Internet access methods in terms of equipment
required, cost, data transfer rate, service reliability and number of supported users.
Wired LAN vs. Wireless LAN
Table 2 compares the characteristics of wired and wireless networks.
Wired network
Wireless network
Cost
Data transfer rate
(for home use)
Network interface card (NIC)
Network cable
Lower
100-1,000Mbps (various Ethernet
implementations)
Data security
Not a serious concern
Reliability
Network reliability is good. Data
retransmission is rarely required.
Mobility
Little
Restricted by network structure
(which is set during physical
network configuration)
Wireless NIC (or WNIC)
Wireless Access Point
Higher
Supported up to 54Mbps (IEEE
802.11g)
An important issue (as data are
broadcast over the air)
Network reliability can be
seriously affected by the
surrounding environment. Data
retransmission is almost a norm.
Good
Equipment
required
LAN
Interconnection
No pre-defined network structure
restriction
Table 2. Characteristics of wired network and wireless network.
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