Regardless of the portion of the band in which Wi-Fi operates, sharing with Bluetooth is inevitable.
Perhaps these two technologies are not competing after all, but in fact may work together to make your computing experience wire-free.
There is a battle brewing between two major wireless networking standards: Bluetooth and IEE802.11b or Wi-Fi. There are some other wireless networking technologies out there such as HomeRF, which is a force in its own right, but Bluetooth and Wi-Fi are the major contenders. At first glance, it would seem that Bluetooth would have it all over 802.11b. After all, you have to admit that a technology named after a 10th century Viking King (Harald Bluetooth) has got a certain amount of curb appeal. But upon closer inspection, you will see that there is not a clear winner, and perhaps these two technologies are not competing after all, but in fact may work together to make your computing experience wire-free.
First, let's start with the similarities. Both systems use frequencies in the 2.4GHz band licensed for industrial, science and medical uses (ISM). Both systems allow users to connect computer devices together to form networks without wires. Both systems allow peer-to-peer networking, and both systems allow you to connect a wireless-capable device to the Internet via a network.
|DATA RATE||MODULATION SCHEME|
|1Mb/s||Differential Binary Phase Shift Keying (DBPSK)|
|2Mb/s||Differential Quaternary Phase Shift Keying (DQPSK)|
|5.5Mb/s and 11Mb/s||Quaternary Phase Shift Keying/Complementary Code Keying (QPSK/CCK)|
|Table 1. Wi-Fi uses Direct Sequence Spread Spectrum (DSSS) at the above data rates and modulation schemes.|
Here are some of the differences. Bluetooth allows users to create Wireless Personal Area Networks (WPANs). Transfer speed is about 1Mb/s. The technology is designed for short distances (up to about 30 feet) and can be used as a wire replacement technology, interconnecting devices such as laptops, cell phones and PDAs. It permits a total of eight devices to be connected together through something called a piconet. Devices on a piconet all use the same “channel.” Devices on different channels can overlap, operating on separate piconets. It is possible to join piconets together into larger networks, but Bluetooth technology seems to be targeted more at small ad hoc networks. These are created whenever two devices using Bluetooth come within 30 feet of each other. (See Figure 1.) Bluetooth manufacturers frequently mention synchronization and exchange of files and information between personal devices such as cell phones and PDAs as typical applications for piconets.
Out of a total of eight possible connections on a piconet, Bluetooth supports three synchronous connection oriented (SCO) links, which can be used for headsets or cordless telephony. (Think about wireless headsets for your cell phone, and you will be on the right track.) Bluetooth has an eye on low power consumption and small form factor. A primary design goal was for Bluetooth technology to be implemented in CMOS on a single chip. Many of the user requirements for Bluetooth came from the cell phone and PDA community. Bluetooth uses a frequency hopping scheme (1600 hops/sec) and Gausian Frequency Shift Keying (GFSK) modulation. While bridging to a wired LAN is possible using Bluetooth, protocol conversion is required.
The other technology — 802.11b, or Wi-Fi — is used to create Wireless Local Area Networks (WLANs). Transfer speed is about 11Mb/s, but 53Mb/s is coming soon. Range for Wi-Fi systems is between 300 and 900 feet outdoors. For those of you familiar with wired networks, Wi-Fi will seem very familiar. It supports true multipoint networking, including functionality such as broadcast, multicast and unicast. Since Wi-Fi uses MAC addresses, a problem caused by common device IDs is virtually impossible. The system uses the same sort of collision and backoff technology as conventional Ethernet with some modifications. Collisions are caused when more than one device tries to talk at the same time. Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is a technology used by Wi-Fi to avoid collisions. In this system, fast acknowledgement of packets received is key. A device wishing to transmit checks to see if the channel is clear. If it is, the device begins to transmit. It then pauses to receive an acknowledgment from the receiving device. The technology is designed so that acknowledgements are given high priority, and they are generated very quickly. If another device wants to transmit while the channel is in use, it defers access and then backs off for a predetermined amount of time and tries again. (See Figure 2.) This is similar to existing technology used on wired Ethernet. There is one catch to this scheme: It assumes that all devices can hear all other devices. If there are “hidden devices,” then an alternate scheme is employed whereby a device can reserve space for a given amount of time. This information is relayed through the network, and devices communicate based upon this timeslot negotiation.
Wi-Fi uses Direct Sequence Spread Spectrum (DSSS) at four different data rates. The modulation used depends on the speed of the link. Table 1 on page 28 lists different data rates and modulation schemes.
While you can create ad hoc networks using Wi-Fi, its history is in providing wireless access to a network. Figure 3 shows how a large network of wired and wireless devices can be connected using Access Points or AP. Since the same protocols are used on the wired and wireless network, a protocol converter is not required.
Obviously, Wi-Fi is an IEEE standard. 802.11b is the current version. A new version of 802.11a is being drafted with enhanced features and increased speed.
The Bluetooth Special Interest Group is submitting their work to the IEEE 802.15 committee for consideration as the standard for Wireless Personal Area Networks (WPAN).
As mentioned earlier, Bluetooth and Wi-Fi may coexist in one facility. However, there may be problems using them in close proximity at the same time. Bluetooth selected FHSS, using 1MHz channels and a hop rate of 1600 hops/sec (625 microseconds in every frequency channel). Bluetooth uses 79 different channels in the United States and most of the rest of the world. IEEE 802.11b (Wi-Fi) opted for DSSS, using 22MHz of bandwidth (passband) to transmit data with speeds of up to 11Mb/s. A Wi-Fi system can use any of 11 22MHz subchannels across the allocated 83.5MHz of the 2.4GHz frequency band. A maximum of three Wi-Fi networks can coexist without interfering with one another. Geographies outside of the United States may support more or fewer than 11 selectable subchannels. However, regardless of the portion of the band in which Wi-Fi operates, sharing with Bluetooth is inevitable. There is a good chance that the two will interfere with each other under the right circumstances. As wireless systems become more common, you should be aware of this fact. Furthermore, as Bluetooth technology begins to be deployed in cell phones, PDAs and other devices, it could cause havoc if someone carrying one of these devices walks into an area with critical equipment operating on a Wi-Fi network.
Finally, a word about security. Both Bluetooth and Wi-Fi have security measures including encryption and authentication. But as you know, there is nothing one person can invent that another cannot figure out. As with all networking technologies, you should design your network with security in mind whether it is wired or wireless.
There is a wealth of information about Bluetooth (www.bluetooth.com), Wi-Fi (www.wi-fi.org) and HomeRF (www.homerf.org) on the Web. A search on any of these terms will bring up a vast amount of information.
Brad Gilmer is president of Gilmer & Associates, executive director of the AAF Association and technical facilitator of the Video Services Forum.
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