Outline

• Bluetooth
• Quiz
• Wi-Fi
  • Physical Properties
  • Collision Avoidance
  • Distribution System
  • Frame Format
• Wi-Max
• Cell Technologies
• Packet Switching

Wireless

• Last day, we began to talk about wireless networks.
• We mentioned it is common to arrange such networks in a base station client set-up. Here the base station might be connected to a wired network.
• We distinguished different levels of mobility for the client. Almost zero: which might happen with a very directional base station such as in the early versions of WiMax. Mobility within the range of a base station: Bluetooth. Mobility between base stations: Cell phones and to a lesser degree Wi-Fi.
• It is also possible for wireless networks to be set-up in a mesh or ad-hoc fashion where there is no centralized base station. From a power consumption point of view this might make sense if one had a lot of low-powered devices. Research in this area is still in the early stages.
• We now describe each of the technologies: Bluetooth, Wi-Fi, Wi-Max, and cell phones in more detail.

Bluetooth

• Bluetooth (802.15) fills the niche of very short-range communication between mobiles phones, notebooks, and other peripherals.
• Some example uses might be to pair a cell phone to a headset or to send photos or business cards from a cell phone to a laptop. This size range of communication is often referred to as a Personal Area Network (PAN).
• Bluetooth uses a licensed excempt 2.45GHz radio band and has a typical range of 10m.
• Because its intended use only requires a limited range, power consumption is low.
• The standardizing body for Bluetooth, the Bluetooth Special Interest Group, has specified an entire suite of protocols that Bluetooth can support in addition to just the link layer protocols. These are called profiles. For example, there is a protocol describing how to sync PDAs with a computer over Bluetooth. There is also an object exchange protocol (OBEX).
• The basic Bluetooth network (a piconet) consists of a master device which is paired with up to seven slaves. Slaves do not directly communicate. The spec also describes how the pairing based on a shared passkey is established.
• To maximize the limited radio bandwidth that Bluetooth has, Bluetooth uses frequency spread spectrum techniques. It uses FDM amongst 79 channels, using each for 625 micro seconds at a time.
• Synchronous time division multiplexing is also used; that is, time is divided into slots. Only the master can send on even numbered slots. A frame can take either 1, 3, or 5 consecutive slots.
• In addition to seven active slaves; piconets are allowed up to 255 parked slaves, which are parked in a low-power consumption state and which can be woken up by the master.
• Bluetooth also specifies how devices can be discovered. A device can make a request of any devices (set to discoverable) in range for information such as device name, device class, list of services, technical information.

Quiz

Suppose a host has a frame to send on an 10Mbps Ethernet. The line though is busy. What action does the Host take?

1. It waits till the line is free and sends immediately.
2. It waits 51.2µs and tries to send again.
3. It sends immediately to cause a collision, and then the two colliding hosts use the exponential backoff algorithm.

Wi-Fi

• Wi-Fi is a trademark of the Wi-Fi alliance. This alliance certifies devices that meet the 802.11 spec.
• Wi-Fi is designed for the limited geographical area, local area network setting (home, offices, etc).
• Like Ethernet its main challenge is to mediate access to a shared media -- in this case, radio signals.
• Over the next few slides, we will explore its functioning in more detail...

Physical Properties

• 802.11 can run over six (so far) physical layer protocols: five based on spread spectrum radio, and one over diffused infrared.
• The fastest non-draft standard (802.11g) operates at 54Mbps (although the 802.10n draft can in theory do 300Mbps).
• The original 802.11 had two radio-based physical layer standards: one using frequency hopping amoung 79 1MHz channels, and the other using direct-sequence with an 11 bit chipping sequence. Both ran at 2Mbps.
• The original 802.11b used direct sequence spread spectrum and could run at 11Mbps.
• All of these used radio at the 2.4 GHz frequency range.
• 802.11a came next. It offers speeds of up 54Mhz. However, it ran on the more line of sight frequency of 5Ghz.
• 802.11g runs in the original frequency range and is backward compatible with 11b, but offers 54Mbps speeds.
• The 802.11n draft also operates at the 2.4Ghz frequency and can be backward compatible (probably slows down higher speed clients). To transmit at higher speeds requires more antennas and a technique called multiple input multiple output (MIMO) is used.

Collision Avoidance

• One complication that a wireless system must solve that didn't need to be solved in the Ethernet setting is that different people might be able to receive signals from different people:

  A

  B

  C

• Orange here is suppose to represent the mix of the red and yellow radio signals. i.e., in the above both A and C can hear and send to B, but A and C are not in range of each other. A and C are called hidden nodes with respect to each other.
• This can cause problems as a frame from A might collide with a frame from C at B, but neither A nor C is aware of the collision.
• There is also the exposed node problem. It might occur in a situation like:

  A B

  C D

• Suppose B is sending to A. C might think it cannot transmit because it can hear B. However, if would be okay to transmit to D, because D cannot hear B and A cannot hear C.

MACA

• 802.11 solves these problems by using multiple access with collision avoidance (MACA).
• When the sender wants to send, it sends a request to send (RTS) control frame to the receiver. This frame has a field indicating how long it wants to hold the medium.
• The receiver then sends back a clear to send (CTS) frame which contains the length field echo'd back to the sender.
• Any node that sees the CTS frame knows it is close to the receiver, and therefore cannot transmit for that time period.
• Any node that sees the RTS frame, but not the CTS frame, knows that it is out of range of the receiver so it is okay to transmit.
• The receiver sends an ACK after successfully receiving the sender's frame.
• If two senders' RTS's collide, then there won't be any acknowledgment CTS's. In this circumstance, the senders are supposed to wait a random amount of time and send again.

Distribution System

• In 802.11 terminology, base stations are called access points.
• Different base stations are connected to each other via a so-called distribution system. This might be Ethernet, Token ring, etc. The distribution network runs at layer 2.
• Each node associates itself with one access point. For example, node A might be associated with AP-1 and node E might be associated with access point AP-3.
• If node A needs to communicate with node E, it is up to the distribution network to get the frame to the correct place. This might involve using a bridging algorithm like what we'll learn when we cover Ch 3.
• To select an access point to associate itself, a node uses the following scanning algorithm:
  1. The node sends a Probe frame.
  2. All APs within reach reply with a Probe Response frame.
  3. The node selects one of these and sends an Association Request frame.
  4. The given AP responds with a Association Response frame.
• Another way, a node might associate itself with an access points relies on the fact that APs can also periodically send out Beacon frames which advertise the capabilities of the access point. A node can send an Association Request frame in response to join the access point, if it wants.

Frame Format

• The control contains three important subfield a 6-bit Type field (RTS or CTS, etc); and two single bit fields ToDS and FromDS. These are use to indicate whether the frame had to be forwarded across the distribution system.
• If both ToDS and FromDS are 0, then Addr2 identifies the source node and Addr1 identifies the receiving node. In the both non-zero case, Addr3 and Addr4 are used to denote the intermediate receiver and the original source, Addr2 is the intermediate source.

WiMax

• WiMax (Worldwide Interoperability for Microwave Access) is standardized as 802.16.
• It has a 1-6 mile range, with a maximum range of 30 miles and provides speeds up to 70Mbps. So it is used in metropolitan area networks often as a last-mile technology.
• Client systems are called subscriber stations. These are assumed to be not end-user computing devices but instead systems that multiplex all the communication devices of a building.
• WiMax defines several physical layer protocols. The original used frequencies in the 10GHz to 66 GHz range, and was limited to line of sight.
• Multiple antennas are typically used and pointed in different directions called sectors.
• For near line of sight and nonline sight circumstances several protocols were added that communicates at frequencies below 10GHz.
• Bandwidth is divided between upstream and downstream traffic using both TDM (take turns using the same frequency) and FDM (take turns using the same frequency). The proportion of upstream versus downstream time can be adaptively changed by the base station.

Cell Technologies

• Cell Phone technologies can also be used to get mobile computers to communicate.
• The frequency bands for cell phones vary slightly for different places around the world. In Europe, it is in 900MHz and 1800MHz bands; in North America, it is at 850 and 1900MHz bands. This can cause problems for people who travel a lot.
• Another issue is that there are a fair number of incompatible cell phone standards.
• Cell phones also rely on the use of base stations. The geographic area served by one station is called a cell.
• Cells overlap. When a phone (because it is being moved) detects one base station is getting weak , the base station can give control to the base station the phone is receiving most strongly. This is called a handoff.

More Cell Technologies

There have been several different generations of cell phone technologies:

1. (1G) which was analog and didn't support data communications to any degree (it did have something later called D-AMPS).
2. (2G) and (2.5G) which were digital technologies such as GSM (global system for mobile communication). North America was a late adopter of this technology so several other competing standard also existed. The different 2G technologies either combined FDM and TDM (this is what GSM does) or used a technique called CDMA (code division multiple access). The latter uses different chipping sequences for each sender. GSM used 8Kbps compressed voice and had a data part of its standard called GPRS (general packet radio service). Speeds can be 30-70Kbps or a little higher using EDGE.
3. (3G) makes use of a variant of CDMA called wideband-CDMA (w-CDMA). It can communicate at speeds up to 1.92 Mbps.

Packet Switching

• We are now going to look at how packet switching is done in networks.
• One problem a switch must solve is which line to forward a request out on.
• We will also look at how to handle when switches become congested.
• We will look at LAN switching which evolved out of Ethernet Bridging.
• We will also discuss switching in ATMs.