Wireless Networking

Technology has now reached the point where vendors and users are both wondering whether the continual task of running cables and connecting computers through ethernet ports is even worth the effort. A number of standards are designed to integrate wireless networking with TCP/IP. The following sections describe some of those technologies. You’ll learn about

• 802.11 Networks

• WAP

• Mobile IP

• Bluetooth

Many of the details for how these technologies are incorporated into products and services depend upon the vendor. The following sections introduce you to some of the concepts.

802.11 Networks

As you learned in Hour 3, the details of the physical network reside at the Network Access layer of the TCP/IP protocol stack. The easiest way to imagine a wireless TCP/IP network is simply as an ordinary network with a wireless architecture at the Network Access layer. The popular IEEE 802.11 specifications provide a model for wireless networking at the Network Access layer.

The 802.11 protocol stack is shown in Figure 9.10. The wireless components at the Network Access layers are equivalent to the other network architectures you learned about in previous hours. In fact, the 802.11 standard is often called wireless Ethernet because of its similarity and compatibility with the IEEE 802.3 Ethernet standard.

In Figure 9.10, note that the 802.11 specification occupies the MAC sublayer of the OSI reference model. The MAC sublayer is part of the OSI Data Link layer. Recall from Hour 2, “How TCP/IP Works,” that the OSI Data Link and Physical layers correspond to the TCP/IP Network Access layer. The various options for the Physical layer represent different wireless broadcast formats, including Frequency Hopping Spread Spectrum (FHSS), Direct Sequence Spread Spectrum (DSSS), Orthogonal Frequency Division Multiplexing (OFDM), and High Rate Direct Sequence Multiplexing (HR/DSSS).

One quality that distinguishes wireless networks from their wired counterparts is that the nodes are mobile. In other words, the network must be capable of responding to changes in the locations of the participating devices. As you learned in earlier hours, the original delivery system for TCP/IP networks is built around the assumption that each device is in some fixed location. Indeed, if a computer is moved to a different network segment, it must be configured with a different address or it won’t even work. By contrast, devices on a wireless network move about constantly. And, although many of the conventions of Ethernet are preserved in this environment, the situation is certainly more complicated and calls for some new and different strategies.

Independent and Infrastructure Networks

The simplest form of wireless network consists of two or more devices with wireless network cards communicating with each other directly (see Figure 9.11). This type of network, which is known as an Independent Basic Service Set (Independent BSS, or IBSS), is often adequate for small collections of computers in a compact space. A classic example of an Independent BSS is a laptop computer that networks temporarily with a home PC when the owner returns from a road trip and transfers files through a wireless connection. Independent BSS networks sometimes occur spontaneously at workshops or sales meetings when participants around a table link through a wireless network to share information. The Independent BSS network is somewhat limited, because it depends on the proximity of the participating computers, provides no infrastructure for managing connections, and offers no means of linking with bigger networks such as the local LAN or the Internet.

Another form of wireless network, called an Infrastructure Basic Service Set (Infrastructure BSS), is more common on corporate networks and other institutional settings—and it is now quite popular as an option for the home and coffee shop due to a new generation of inexpensive wireless routing devices. An Infrastructure BSS depends on a fixed device called an access point to facilitate communication among the wireless devices (see Figure 9.12). An access point communicates with the wireless network through wireless broadcasts and is wired to an ordinary Ethernet network through a conventional connection. Wireless devices communicate through the access point. If a wireless device wants to communicate with other wireless devices in the same zone, it sends a frame to the access point and lets the access point deliver the message to its destination. For communication to or from the conventional network, the access point acts as a bridge. The access point forwards any frames addressed to the devices on the conventional network and keeps all frames addressed to the wireless network on the wireless side.

The network shown in Figure 9.12 lets the computers function much as they would with an ordinary wired ethernet network. The Infrastructure BSS configuration also offers benefits if you consider a larger area served by a collection of access points connected by conventional ethernet (see Figure 9.13).

802.11 was devised to address situations like the network depicted in Figure 9.13. The idea is for the roving device to remain connected as it travels anywhere within the area served by the network. The first thing to notice is that, if the device is to receive any network transmissions, the network must know which access point to use to reach the device. This concern is, of course, compounded by the fact that the device is possibly moving, and the appropriate access point might change without warning. Another thing to notice is that the classic concepts of a source address and destination address are not always sufficient for delivering data on a wireless network. In fact, the 802.11 frame makes provision for four addresses:

• Destination address— The devices to which the frame is addressed.

• Source address— The device that sent the frame.

• Receiver address— The wireless device that should process the 802.11 frame. If the frame is addressed to a wireless device, the receiver address is the same as the destination address. If the frame is addressed to a device beyond the wireless network, the receiver address is the address of the access point that will receive the frame and forward it to the Ethernet distribution network.

• Transmitter address— The address of the device that forwarded the frame onto the wireless network.

The 802.11 frame format is shown in Figure 9.14. Some important fields are as follows:

• Frame control— A collection of smaller fields describing the protocol version, the frame type, and other values necessary for interpreting the contents of the frame.

• Duration/ID— A field that provides an estimate of approximately how long the transmission will last. This field may also request buffered frames from the access point.

• Address fields— 48-bit physical address fields. As was noted earlier, 802.11 sometimes requires up to four different addresses. The addresses fields are used differently depending on the type of frame. The first field is typically the receiver and the second field is typically the transmitter.

• Sequence control— The fragment number (used for defragmentation) and a sequence number for the frame.

• Frame body— The data transmitted with the frame. As you learned in Hour 2, the data transmitted with a frame also contains upper-layer protocol headers.

• Frame Check Sequence (FCS)— A cyclic redundancy check, used to check for transmission errors and verify that the frame has not been altered in transit

Note that, because 802.11 is a Network Access layer protocol set, the addresses used in 802.11 frames are the 48-bit physical addresses you learned about in Hour 3, not IP addresses. As the device moves across the wireless network, it registers itself with the nearest available access point. (Technically, it registers itself with the access point that has the strongest signal and least interference.) This registration process is known as association. When the device roams closer to another access point, it reassociates with the new access point. This association process lets the network determine which access point to use to reach each device.

802.11 Security

As you can probably guess, an unprotected wireless network is extremely insecure. To eavesdrop on a conventional network, you must at least be somehow connected to the transmission medium. A wireless network, on the other hand, is vulnerable from anywhere within broadcast distance. Not only can an intruder listen in, but an enterprising attacker can also simply show up with a wireless device and start participating in the network if the network has no protections to prevent such activities.

To address these concerns, IEEE developed an optional security protocol standard to accompany 802.11. The Wired Equivalent Privacy (WEP) standard is designed to provide a level of privacy approximately equivalent to the privacy provided by a conventional wired network. The goal of WEP is to address the following concerns:

• Confidentiality— Protection from eavesdropping

• Integrity— Assurance that the data is unaltered

• Authentication— Assurance that the communicating parties are who they say they are, and that they have the necessary authorization to operate on the network

WEP handles the confidentiality and integrity goals through encryption using the RC4 algorithm. The sending device generates an Integrity Check Value (ICV). The ICV is a value that results from a standard calculation based on the contents of the frame. The ICV is then encrypted using the RC4 algorithm and transmitted to the receiver. The receiving device decrypts the frame and calculates the ICV. If the calculated ICV value matches the value transmitted with the frame, the frame has not been altered.

WEP, unfortunately, has met with objections from security experts. Most experts now regard WEP as ineffective. Some of the objections to WEP are actually objections to the implementation of the RC4 encryption algorithm. WEP theoretically uses a 64-bit key, but 24 bits of the key are used for initialization. Only 40 bits of the key are used as a shared secret. This 40-bit secret is too short, according to the experts, and WEP is therefore insufficient for effective protection. Experts also point to problems with the key management system and with the 24-bit initialization vector used to begin the encryption.

An update to WEP known as WEP2 increased the initialization vector to 128 bits and added Kerberos authentication to organize the use and distribution of secret keys. However, WEP2 didn’t solve all the problems of WEP. Several other protocols, such as Extensible Authentication Protocol (EAP) have appeared to address the concerns about WEP.

The 802.11i draft standard for a better wireless security protocol appeared in 2004 and was incorporated into the 802.11 standard in 2007. This new approach, which is known as WPA2, uses an AES block cypher for encryption rather than RC4 and also comes with more secure procedures for authentication and key distribution. WPA2 appears to be a big advance in wireless security; however, as of this writing, WEP is still running on a large number of devices throughout the world.

Many wireless devices also support other security measures. For instance, many wireless routers let you enter the MAC addresses of computers that are authorized to operate on the network. These kinds of measures are often effective for stopping your next door neighbor from embezzling onto your bandwidth, but be aware that experienced intruders have ways to get around these kinds of controls.

Wireless Application Protocol (WAP)

Standards such as the 802.11 series focus on integrating wireless devices with conventional networks at a local level. The true mobile networking paradigm, in which users go wherever they want to go with a handheld device and access the Internet through the mobile telephone network, requires some additional consideration. The Wireless Application Protocol (WAP) is a protocol stack designed specifically for wireless devices. Whereas 802.11 networks are similar in character to other forms of ethernet, WAP is significantly different from ordinary TCP/IP. WAP is actually a specification for the upper application-related protocol layers. WAP was designed for delivering information to and from mobile handheld devices.

The protocols of the WAP stack are tailored to the needs and influences of the wireless world. The WAP specification includes a markup language called Wireless Markup Language (WML) that is based on XML but specifically designed to provide the features necessary for constructing Web content for the small screens of handheld devices. WAP provides its own protocol layers that are roughly modeled on the upper layers of the OSI stack. The designers, however, did not mind diverging from strict OSI conformance to address the particular needs of the wireless environment.

The WAP protocol layers, and the protocols associated with those layers, are shown in Figure 9.15. Like the other protocols described in this book, the WAP protocols describe a specification that is then implemented by vendors, who create the actual software. The WAP protocols include the following:

• WAP Session Protocol (WSP)— The WAP equivalent of HTTP. WSP provides a system for exchanging data between applications.

• WAP Transaction Protocol (WTP)— A protocol that provides handshake and acknowledgment services to initiate and confirm WAP transactions.

• WAP Transaction Layer Security (WTLS)— A security protocol modeled on SSL (see Hour 23, “TCP/IP Security”).

• WAP Datagram Transport Protocol (WDP)— A connectionless Transport layer protocol modeled on UDP (see Hour 6, “The Transport Layer”).

The WAP protocols represent the upper layers of the protocol stack. Note that WDP is similar to the UDP protocol, which resides on the Transport layer of TCP/IP’s stack. The WAP stack therefore resides mostly at the TCP/IP Application layer. However, the real relationship of WAP with TCP/IP is a bit more complex.

Because wireless networks are inherently slower and less reliable than cable-based networks, the WAP protocols are designed to deliver maximum performance. Some WAP protocols are in a binary format that must be translated to the text-based format of the TCP/IP protocols for the WAP device to receive Internet-related data transmissions. A device called a WAP gateway translates the WAP protocol information to an Internet-compatible format (see Figure 9.16).

The WAP suite includes other related protocols and languages not depicted in Figure 9.15, such as WMLScript (a scripting language) and WBMP (a bitmap format).

More recent WAP standards have proposed greater compatibility with TCP/IP and also greater compatibility with XML and HTML through XHTML, which will replace WML as the WAP markup language.

Mobile IP

You might have noticed that devices moving around the world pose a significant problem for delivering responses to Internet requests: The Internet addressing system is organized hierarchically with the assumption that the target device is located on the network segment defined through the IP address. Because a mobile device can be anywhere, the rules for communicating with the device become much more complicated. To maintain a TCP connection, the device must have a constant IP address, which means that a roaming device cannot simply use an address assigned by the nearest transmitter. Significantly, because this problem relates to Internet addressing, it can’t be solved strictly at the Network Access layer and requires an extension to the Internet layer’s IP protocol. The Mobile IP extension is described in RFC 3220.

Mobile IP solves the addressing problem by associating a second (care-of) address with the permanent IP address. The Mobile IP environment is depicted in Figure 9.17. The device retains a permanent address for the home network. A specialized router known as the Home Agent, located on the home network, maintains a table that binds the device’s current location to its permanent address. When the device enters a new network, the device registers with a Foreign Agent process operating on the network. The Foreign agent adds the mobile device to the Visitor list and sends information on the devices current location to the Home Agent. The Home Agent then updates the mobility binding table with the current location of the device. When a datagram address to the device arrives on the home network, the datagram is encapsulated in a packet addressed to the foreign network, where it is delivered to the device.

Bluetooth

The Bluetooth protocol architecture is another specification for wireless devices that is gaining popularity throughout the networking industry. Bluetooth was developed by IBM and a group of other companies. Like 802.11, the Bluetooth standard defines the OSI Data Link and Physical layers (equivalent to the TCP/IP Network Access layer).

Although the Bluetooth standard is often used for peripheral devices such as headsets and wireless keyboards, Bluetooth is also used in place of 802.11 in some cases, and Bluetooth backers are always eager to state that some of the security problems related to 802.11 do not apply to Bluetooth. However, IBM’s official line is that Bluetooth and 802.11 are “complementary technologies.” Whereas 802.11 is designed to provide an equivalent to Ethernet for wireless networks, Bluetooth focuses on providing a reliable and high-performing environment for wireless devices operating in a short range (10 meters). Bluetooth is designed to facilitate communication among a group of interacting wireless devices in a small work area defined within the Bluetooth specification as a Personal Area Network (PAN).

Like other wireless forms, Bluetooth uses an access point to connect the wireless network to a conventional network. (The access point is known as a Network Access Point, or NAP in Bluetooth terminology.) The Bluetooth Encapsulation Protocol encapsulates TCP/IP packets for distribution for delivery over the Bluetooth network.

Of course, if a Bluetooth device is to be accessible through the Internet, it must be accessible through TCP/IP. Vendors envision a class of Internet-ready Bluetooth devices accessible through a Bluetooth-enabled Internet bridge (see Figure 9.18). A Bluetooth NAP device acts as a network bridge, receiving incoming TCP/IP transmissions and replacing the incoming Network Access layer with the Bluetooth network access protocols for delivery to a waiting device.