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Page 176 / 193 Scroll up to view Page 171 - 175
176
Glossary
Multicast
A
Multicast
sends the same message to a select group of recipients. Sending an e-mail message
to a mailing list is an example of multicasting. In wireless networks, multicast usually refers to an
interaction in which the access point sends data traffic in the form of
IEEE 802.1x Frame
s to a
specified set of client stations (
MAC
addresses) on the network.
Some wireless security modes distinguish between how unicast, multicast, and broadcast frames
are encrypted or whether they are encrypted. See also
Unicast
and
Broadcast
.
NAT
Network Address Translation
is an Internet standard that masks the internal IP addresses being
used in a
LAN
. A NAT server running on a gateway maintains a translation table that maps all
internal IP addresses in outbound requests to its own address and converts all inbound requests
to the correct internal host.
NAT serves three main purposes: it provides security by obscurity by hiding internal IP addresses,
enables the use of a wide range of internal IP addresses without fear of conflict with the addresses
used by other organizations, and it allows the use of a single Internet connection.
Network Address
See
IP Address
.
NIC
A
Network Interface Card
is an adapter or expansion board inserted into a computer to provide
a physical connection to a network. Most NICs are designed for a particular type of network,
protocol, and media, for example,
Ethernet
or wireless.
NTP
The
Network Time Protocol
assures accurate synchronization of the system clocks in a network
of computers. NTP servers transmit
Coordinated Universal Time
(UTC, also known as
Greenwich
Mean Time
) to their client systems. An NTP client sends periodic time requests to servers, using
the returned time stamp to adjust its clock.
OSI
The
Open Systems Interconnection
(OSI) reference model is a framework for network design.
The OSI model consists of seven layers:
• Layer 1, the Physical layer, identifies the physical medium used for communication between
nodes. In the case of wireless networks, the physical medium is air, and radio frequency (RF)
waves are a components of the physical layer.
• Layer 2, the Data-Link layer, defines how data for transmission will be structured and formatted,
along with low-level protocols for communication and addressing. For example, protocols such
as
CSMA/CA
and components like
MAC
addresses, and
Frame
s are all defined and dealt with
as a part of the Data-Link layer.
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177
Glossary
• Layer 3, the Network layer, defines the how to determine the best path for information traversing
the network.
Packet
s and logical
IP Address
es operate on the network layer.
• Layer 4, the Transport layer, defines connection oriented protocols such as
TCP
and
UDP
.
• Layer 5, the Session layer, defines protocols for initiating, maintaining, and ending communication
and transactions across the network. Some common examples of protocols that operate on this
layer are network file system (NFS) and structured query language (SQL). Also part of this layer
are communication flows like single mode (device sends information bulk), half-duplex mode
(devices take turns transmitting information in bulk), and full-duplex mode (interactive, where
devices transmit and receive simultaneously).
• Layer 6, the Presentation layer, defines how information is presented to the application. It includes
meta-information about how to encrypt/decrypt and compress/decompress the data. JPEG and
TIFF file formats are examples of protocols at this layer.
• Layer 7, the Application layer, includes protocols like hypertext transfer protocol (
HTTP
), simple
mail transfer protocol (SMTP), and file transfer protocol (FTP).
Packet
Data and media are transmitted among nodes on a network in the form of
packets
. Data and
multimedia content is divided up and packaged into
packets
. A packet includes a small chunk
of the content to be sent along with its destination address and sender address. Packets are
pushed out onto the network and inspected by each node. The node to which it is addressed is
the ultimate recipient.
Packet Loss
Packet Loss
describes the percentage of packets transmitted over the network that did not reach
their intended destination. A 0 percent package loss indicates no packets were lost in transmission.
QoS
features are designed to minimize packet loss.
PHY
The Physical Layer (PHY) is the lowest layer in the network layer model (see
OSI
). The Physical
Layer conveys the bit stream - electrical impulse, light or radio signal — through the network at
the electrical and mechanical level. It provides the hardware means of sending and receiving
data on a medium, including defining cables,
NIC
s, and physical aspects.
Ethernet
and the
802.11
family are protocols with physical layer components.
PID
The
Process Identifier
(PID) is an integer used by Linux to uniquely identify a process. A PID is
returned by the
fork()
system call. It can be used by
wait()
or
kill()
to perform actions
on the given process.
Port Forwarding
Port Forwarding
creates a ‘tunnel’ through a firewall, allowing users on the Internet access to a
service running on one of the computers on your
LAN
, for example, a Web server, an FTP or
SSH server, or other services. From the outside user’s point of view, it looks like the service is
running on the firewall.
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178
Glossary
PPP
The Point-to-Point Protocol is a standard for transmitting network layer datagrams (
IP
packets)
over serial point-to-point links. PPP is designed to operate both over asynchronous connections
and bit-oriented synchronous systems.
PPPoE
Point-to-Point Protocol over Ethernet (PPPoE) is a specification for connecting the users on
a
LAN
to the Internet through a common broadband medium, such as a single DSL or cable
modem line.
PPtP
Point-to-Point Tunneling Protocol
(PPtP) is a technology for creating a
Virtual Private Network
(
VPN
) within the
Point-to-Point Protocol
(
PPP
). It is used to ensure that data transmitted from
one VPN node to another are secure.
Proxy
A
proxy
is server located between a client application and a real server. It intercepts requests,
attempting to fulfill them itself. If it cannot, it forwards them to the real server. Proxy servers have
two main purposes: improve performance by spreading requests over several machines and filter
requests to prevent access to specific servers or services.
PSK
Pre-Shared Key
(PSK), see
Shared Key
.
Public Key
A
public key
is used in public key cryptography to encrypt a message which can only be decrypted
with the recipient’s private or secret key. Public key encryption is also called asymmetric encryption,
because it uses two keys, or Diffie-Hellman encryption. Also see
Shared Key
.
QoS
Quality of Service (QoS) defines the performance properties of a network service, including
guaranteed throughput, transit delay, and priority queues. QoS is designed to minimize
Latency
,
Jitter
,
Packet Loss
, and network congestion, and provide a way of allocating dedicated bandwidth
for high priority network traffic.
The
IEEE
standard for implementing QoS on wireless networks is currently in-work by the
802.11e
task group. A subset of
802.11e
features is described in the
WME
specification.
RADIUS
The
Remote Authentication Dial-In User Service
(RADIUS) provides an authentication and
accounting system. It is a popular authentication mechanism for many
ISP
s.
RC4
A symmetric stream cipher provided by
RSA Security
. It is a variable key-size stream cipher with
byte-oriented operations. It allows keys up to 2048 bits in length.
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179
Glossary
Router
A
router
is a network device which forwards packets between networks. It is connected to at
least two networks, commonly between two local area networks (
LAN
s) or between a
LAN
and
a wide-area network (
WAN
), for example, the Internet. Routers are located at gateways—places
where two or more networks connect.
A router uses the content of headers and its tables to determine the best path for forwarding
a packet. It uses protocols such as the Internet Control Message Protocol (ICMP), Routing
Information Protocol (RIP), and Internet Router Discovery Protocol (IRDP) to communicate with
other routers to configure the best route between any two hosts. The router performs little filtering
of data it passes.
RSSI
The
Received Signal Strength Indication
(RSSI) an
802.1x
value that calculates voltage relative
to the received signal strength. RSSI is one of several ways of measuring and indicating
radio
frequency
(RF) signal strength. Signal strength can also be measured in mW (milliwatts), dBms
(decibel milliwatts), and a percentage value.
RTP
Real-Time Transport Protocol
(RTP) is an Internet protocol for transmitting real-time data like audio
and video. It does not guarantee delivery but provides support mechanisms for the sending and
receiving applications to enable streaming data. RTP typically runs on top of the
UDP
protocol,
but can support other transport protocols as well.
RTS
A
request to send
(RTS) message is a signal sent by a client station to the access point, asking
permission to send a data packet and to prevent other wireless client stations from grabbing
the radio waves. This message is a part of the IEEE 802.11
CSMA/CA
protocol. (See also
RTS
Threshold
and
CTS
.)
RTS Threshold
The
RTS threshold
specifies the packet size of a request to send (
RTS
) transmission. This helps
control traffic flow through the access point, and is especially useful for performance tuning on
an access point with a many clients.
Shared Key
A
shared key
is used in conventional encryption where one key is used both for encryption and
decryption. It is also called
secret-key
or
symmetric-key
encryption.
Also see
Public Key
.
SNMP
The
Simple Network Management Protocol
(
SNMP
) was developed to manage and monitor
nodes on a network. It is part of the
TCP/IP
protocol suite.
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180
Glossary
SNMP consists of managed devices and their agents, and a management system. The agents
store data about their devices in
Management Information Bases
(MIBs) and return this data to
the SNMP management system when requested.
SSID
The
Service Set Identifier
(SSID) is a thirty-two character alphanumeric key that uniquely identifies
a wireless local area network. It is also referred to as the
Network Name
. There are no restrictions
on the characters that may be used in an SSID.
Static IP Address
See
IP Address
.
STP
The
Spanning Tree Protocol
(STP) an IEEE 802.1 standard protocol (related to network
management) for
MAC
bridges that manages path redundancy and prevents undesirable loops
in the network created by multiple active paths between client stations. Loops occur when there
multiple routes between access points. STP creates a tree that spans all of the switches in an
extended network, forcing redundant paths into a standby, or blocked, state. STP allows only one
active path at a time between any two network devices (this prevents the loops) but establishes
the redundant links as a backup if the initial link should fail. If STP costs change, or if one network
segment in the STP becomes unreachable, the spanning tree algorithm reconfigures the spanning
tree topology and reestablishes the link by activating the standby path. Without spanning tree in
place, it is possible that both connections may be simultaneously live, which could result in an
endless loop of traffic on the LAN
Subnet Mask
A
Subnet Mask
is a number that defines which part of an IP address is the network address and
which part is a host address on the network. It is shown in dotted-decimal notation (for example,
a 24-bit mask is shown as
255.255.255.0
) or as a number appended to the IP address (for
example,
192.168.2.0/24
).
The subnet mask allows a router to quickly determine if an IP address is local or needs to be
forwarded by performing a bitwise AND operation on the mask and the IP address. For example,
if an IP address is
192.168.2.128
and the netmask is
255.255.255.0
, the resulting Network address is
192.168.2.0
.
The bitwise AND operator compares two bits and assigns 1 to the result only if both bits are 1.
The following table shows the details of the netmask:
Supported Rate Set
The
supported rate set
defines the transmission rates that are available on this wireless network.
A station may be able to receive data at any of the rates listed in this set. All stations must be
able to receive data at
the rates listed in the
Basic Rate Set
.
IP address 192.168.2.128 11000000 10101000 00000010 10000000
Netmask 255.255.255.0 11111111 11111111 11111111 00000000
Resulting network address 192.168.2.0 11000000 10101000 00000010 00000000

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