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Reference Manual for the Wireless Cable Modem Gateway CG814WG v2
Networks, Routing, and Firewall Basics
B-11
Stateful Packet Inspection
Unlike simple Internet sharing routers, a firewall uses a process called stateful packet inspection to
ensure secure firewall filtering to protect your network from attacks and intrusions. Since
user-level applications such as FTP and Web browsers can create complex patterns of network
traffic, it is necessary for the firewall to analyze groups of network connection “states.” Using
Stateful Packet Inspection, an incoming packet is intercepted at the network layer and then
analyzed for state-related information associated with all network connections. A central cache
within the firewall keeps track of the state information associated with all network connections.
All traffic passing through the firewall is analyzed against the state of these connections in order to
determine whether or not it will be allowed to pass through or rejected.
Denial of Service Attack
A hacker may be able to prevent your network from operating or communicating by launching a
Denial of Service (DoS) attack. The method used for such an attack can be as simple as merely
flooding your site with more requests than it can handle. A more sophisticated attack may attempt
to exploit some weakness in the operating system used by your router or gateway. Some operating
systems can be disrupted by simply sending a packet with incorrect length information.
Wireless Networking Overview
The CG814WG v2 Gateway conforms to the Institute of Electrical and Electronics Engineers
(IEEE) 802.11b standard for wireless LAN
s (WLANs). On an 802.11b wireless link, data is
encoded using direct-sequence spread-spectrum (DSSS) technology and is transmitted in the
unlicensed radio spectrum at 2.5GHz. The maximum data rate for the wireless link is 11 Mbps, but
it will automatically back down from 11 Mbps to 5.5, 2, and 1 Mbps when the radio signal is weak
or when interference is detected.
The 802.11b standard is also called Wireless Ethernet or Wi-Fi by the Wireless Ethernet
Compatibility Alliance
(WECA, see
), an industry standard group promoting
interoperability among 802.11b devices. The 802.11b standard offers two methods for configuring
a wireless network - ad hoc and infrastructure.
Infrastructure Mode
With a wireless Access Point, you can operate the wireless LAN in the infrastructure mode. This
mode provides wireless connectivity to multiple wireless network devices within a fixed range or
area of coverage, interacting with wireless nodes via an antenna.
Page 72 / 122
Reference Manual for the Wireless Cable Modem Gateway CG814WG v2
B-12
Networks, Routing, and Firewall Basics
In the infrastructure mode, the wireless access point converts airwave data into wired Ethernet
data, acting as a bridge between the wired LAN and wireless clients. Connecting multiple Access
Points via a wired Ethernet backbone can further extend the wireless network coverage. As a
mobile computing device moves out of the range of one access point, it moves into the range of
another. As a result, wireless clients can freely roam from one Access Point domain to another and
still maintain seamless network connection.
Ad Hoc Mode (Peer-to-Peer Workgroup)
In an ad hoc network, computers are brought together as needed; thus, there is no structure or fixed
points to the network - each node can generally communicate with any other node. There is no
Access Point involved in this configuration. This mode enables you to quickly set up a small
wireless workgroup and allows workgroup members to exchange data or share printers as
supported by Microsoft networking in the various Windows operating systems. Some vendors also
refer to ad hoc networking as peer-to-peer group networking.
In this configuration, network packets are directly sent and received by the intended transmitting
and receiving stations. As long as the stations are within range of one another, this is the easiest
and least expensive way to set up a wireless network.
Network Name: Extended Service Set Identification (ESSID)
The Extended Service Set Identification
(ESSID) is one of two types of Service Set Identification
(SSID). In an ad hoc wireless network with no access points, the Basic Service Set Identification
(BSSID) is used. In an infrastructure wireless network that includes an access point, the ESSID is
used, but may still be referred to as SSID.
An SSID is a thirty-two character (maximum) alphanumeric key identifying the name of the
wireless local area network. Some vendors refer to the SSID as network name. For the wireless
devices in a network to communicate with each other, all devices must be configured with the
same SSID.
Page 73 / 122
Reference Manual for the Wireless Cable Modem Gateway CG814WG v2
Networks, Routing, and Firewall Basics
B-13
Authentication and WEP
The absence of a physical connection between nodes makes the wireless links vulnerable to
eavesdropping and information theft. To provide a certain level of security, the IEEE 802.11
standard has defined two types of authentication methods, Open System and Shared Key. With
Open System authentication, a wireless PC can join any network and receive any messages that are
not encrypted. With Shared Key authentication, only those PCs that possess the correct
authentication key can join the network. By default, IEEE 802.11 wireless devices operate in an
Open System network.
Wired Equivalent Privacy
(WEP) data encryption is used when the wireless devices are configured
to operate in Shared Key authentication mode. There are two shared key methods implemented in
most commercially available products, 64-bit and 128-bit WEP data encryption.
802.11b Authentication
The 802.11b standard defines several services that govern how two 802.11b devices communicate.
The following events must occur before an 802.11b Station can communicate with an Ethernet
network through an access point such as the one built in to the CG814WG v2:
1.
Turn on the wireless station.
2.
The station listens for messages from any access points that are in range.
3.
The station finds a message from an access point that has a matching SSID.
4.
The station sends an authentication request to the access point.
5.
The access point authenticates the station.
6.
The station sends an association request to the access point.
7.
The access point associates with the station.
8.
The station can now communicate with the Ethernet network through the access point.
An access point must authenticate a station before the station can associate with the access point or
communicate with the network. The IEEE 802.11b standard defines two types of authentication:
Open System and Shared Key.
•
Open System Authentication
allows any device to join the network, assuming that the device
SSID matches the access point SSID. Alternatively, the device can use the “ANY” SSID
option to associate with any available Access Point within range, regardless of its SSID.
•
Shared Key Authentication
requires that the station and the access point have the same WEP
Key to authenticate. These two authentication procedures are described below.
Page 74 / 122
Reference Manual for the Wireless Cable Modem Gateway CG814WG v2
B-14
Networks, Routing, and Firewall Basics
Open System Authentication
The following steps occur when two devices use Open System Authentication:
1.
The station sends an authentication request to the access point.
2.
The access point authenticates the station.
3.
The station associates with the access point and joins the network.
This process is illustrated in below.
Figure B-4:
802.11b open system authentication
Shared Key Authentication
The following steps occur when two devices use Shared Key Authentication:
1.
The station sends an authentication request to the access point.
2.
The access point sends challenge text to the station.
3.
The station uses its configured 64-bit or 128-bit default key to encrypt the challenge text, and
sends the encrypted text to the access point.
4.
The access point decrypts the encrypted text using its configured WEP Key that corresponds
to the station’s default key. The access point compares the decrypted text with the original
challenge text. If the decrypted text matches the original challenge text, then the access point
and the station share the same WEP Key and the access point authenticates the station.
5.
The station connects to the network.
FVM318
Access Point
1) Authentication request sent to AP
2) AP authenticates
3) Client connects to network
802.11b Authentication
Open System Steps
Cable or
DLS modem
Client
attempting
to connect
Page 75 / 122
Reference Manual for the Wireless Cable Modem Gateway CG814WG v2
Networks, Routing, and Firewall Basics
B-15
If the decrypted text does not match the original challenge text (i.e., the access point and station do
not share the same WEP Key), then the access point will refuse to authenticate the station and the
station will be unable to communicate with either the 802.11b network or Ethernet network.
This process is illustrated in below.
Figure B-5:
802.11b shared key authentication
Overview of WEP Parameters
Before enabling WEP on an 802.11b network, you must first consider what type of encryption you
require and the key size you want to use. Typically, there are three WEP Encryption options
available for 802.11b products:
1.
Do Not Use WEP:
The 802.11b network does not encrypt data. For authentication purposes,
the network uses Open System Authentication.
2.
Use WEP for Encryption:
A transmitting 802.11b device encrypts the data portion of every
packet it sends using a configured WEP Key. The receiving 802.11b device decrypts the data
using the same WEP Key. For authentication purposes, the 802.11b network uses Open
System Authentication.
3.
Use WEP for Authentication and Encryption:
A transmitting 802.11b device encrypts the
data portion of every packet it sends using a configured WEP Key. The receiving 802.11b
device decrypts the data using the same WEP Key. For authentication purposes, the 802.11b
network uses Shared Key Authentication.
FVM318
Access Point
1) Authentication
request sent to AP
2) AP sends challenge text
3) Client encrypts
challenge text and
sends it back to AP
4) AP decrypts,and if correct,
authenticates client
5) Client connects to network
802.11b Authentication
Shared Key Steps
Cable or
DLS modem
Client
attempting
to connect

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