Outline

• Wi-Fi
  • Collision Avoidance
  • Distribution System
  • Frame Format
• Quiz
• 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.
• Also, last we discussed Bluetooth and talked about the physical properties of Wi-Fi.
• Today, we discuss Wi-Fi, Wi-Max, and cell phones in more detail.

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 with, 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 intermediate source.

Quiz

Which of the following is true?

1. Media Access Control Algorithm used by Ethernet is a deterministic algorithm.
2. FDDI is a dual ring network.
3. A Bluetooth slave device, once parked, can only be woken up by hard-resetting the device.

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.
4. (3.something G) consists of several technologies some of which promote themselves as 4G. For example, the 3GPP (3G partnership project) family of standards such as HSPA and LTE, 3GPP2 family of standards such as EV-DO, and the IEEE family of standards such as Wi-Max.
5. (4G) again several competing technologies. Should support 100Mbps for highly mobile clients, and 1Gbps for more stationary ones. Some candidate systems are: IEEE 802.16m (WirelessMAN-Advanced) and LTE Advanced.

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.

Switches

• We said we were interested in how switches know which line to forward on and how switches handle congestion.
• A switch is multi-input, multioutput device, which transfers packets from an input to one or more outputs.
• In terms of the topologies for networks we've seen so far, we have point-to-point links, Bus (Ethernet), and rings. Switches add to this a star topology.
• Some advantages of the star topology related to switches include:
  • Even keeping the degree of the switch vertex small (even as small degree 3), by connecting several switches, we can build a large network with a complicated structure.
  • We can connect switches to each other and to hosts using long-hau,l point-to-point links, which means that we can build networks which span large geographic distances.
  • Adding a new host to the network by connecting it to a switch does not necessarily reduce the performance of hosts already connected to the network. So switched networks are considered more scalable than things like a strict Ethernet approach.

Forwarding

• A switch is connected to a set of links. Each link uses a data link layer protocol to communicate with the node or nodes on the other end of the link.
• As an example, a switch might have a T1, a T3, and a STS-1 line coming into it. So the data link protocol is not necessarily the same for each line.
• Where a link comes into a switch is called a port. Each port usually has a number associated with it. i.e., port 1, port 2, etc. of the switch.
• A switch needs to be able to receive data on one link and be able to send it out of the correct outgoing link. This job is called forwarding or switching.
• There are two common approaches to solving the switching decision problem: a datagram (also called connectionless) approach and a virtual circuit (also-called connection-oriented) approach.
• There is also a less common approach called source routing.
• MAC addresses are used to uniquely identify all switches/hosts in a switched network. That is why Ethernet cards have to have a unique address.

Datagrams

• In this approach, to decide how a switch forwards a packet, the switch consults a forwarding table that it has inside its memory.
• This table consists of two columns one for the destination host and the other for the port to use as the outgoing line. For example, for switch 2 above, the table would look like:

  Destination	Port
  A	3
  B	3
  C	1
  D	0
  E	0

More on the Datagram Approach

• In the datagram approach, we can imagine that the network operator configures all the cables and sets up the the routing table statically.
• For small networks this is possible, but for large, dynamically evolving networks, this is close to impossible. This harder problem is called routing, and we will talk about routing tables later in the semester.
• Connectionless approaches have the following characteristics:
  • A host can send a packet anywhere any time. This contrasts with connect-oriented approaches where a "connection-state" might need to be established first.
  • When a host sends a packet, the host does not know if the network is capable of delivering it or if the receiver is even alive.
  • Each packet is forwarded independently of the previous packets. Thus, if a forwarding table gets changed between two packets being sent, then the two packets might take different routes through the network.
  • A switch or link failure might not have any serious effect if it is possible to find an alternate route around the failure.