Introduction to Wireless Networking ECE 401WN Spring 2008

Transcription

1 Introduction to Wireless Networking ECE 401WN Spring 2008 Lecture 2: Communication Networks The first major topic we will study will be WLANs. But before that, we need to consider a few basics of networking. I. LANs, MANs, and WANs Figure 3.1 Chapter 3 Communication Networks Wide Area Networks Large geographic area May rely on circuits provided by a common carrier. Traditionally have only provided modest capacity Businesses for many years purchased T1 circuits (1.544 Mbps) Lecture 2, Page 1 of 11

2 With fiber optics, higher speed services are more widely available. 10s to 100s of Mbps UMKC has connection to a WAN of 55 Mbps UMKC connects to the Missouri Research and Education Network (MOREnet) as their WAN carrier WANs rarely use wireless. Although in the past, WANs (really long distance telephone) did use wireless. Many point-to-point microwave links. Local Area Networks Connect together end devices, servers, printers, etc. The scope of a LAN is smaller, typically a single building or cluster of buildings. A LAN is usually owned by the same organization that owns the end devices. What are the implications of this as compared to a WAN? Substantial investment. Managed by the owner. Data rates of LANs are much greater than WANs 100 Mbps up to 10 Gbps The primary technology is Ethernet. Figure 3.2, a picture of a LAN A major topic of this course is making LANs wireless. Lecture 2, Page 2 of 11

3 Metropolitan Area Networks II. Switching Middle ground between LANs and WANs. Two approaches Use WAN technology, only on a smaller scale. Or use LAN technology on a larger scale. - This one has become the most popular. - For example, using Ethernet extended throughout a city using fiber optics. Some call this Metro Ethernet when a commercial carrier provides Ethernet service. Primary customers: Those with campuses or that are spread throughout a city, and that need high data rates. In wireless, we have a special technology for MANs Switching nodes. Intermediate points to transfer data between networks or LANs. They are not concerned with content. They just move data from node to node to the destination. Connected together in some topology. - Rings, stars, fully connected, or a general topology. Lecture 2, Page 3 of 11

4 For example in the figure above, data is to be sent from A to F First sent to node 4. Then routed via nodes 5 and 6. Or alternatively 7 and 6. Or even longer routes if some links fail. Some nodes only connect to other nodes, but not to end users. Like node 5. We commonly call this a. End stations generally connect to nodes with point-to-point links. Usually a network is not fully connected. For example, node 5 is not directly connected to node 7. Why? Costly, unnecc. N*(N-1) connects Lecture 2, Page 4 of 11

5 Circuit Switching Was in the past been the dominant technology for voice and data communications. Sets up a dedicated path between two stations. On each physical link, an unshared channel is allocated. Most common example: Telephone network. Three phases 1. Circuit establishment - Source sends a request for a channel to be allocated. - A free channel must be allocated on every link on the path. - Must have routing intelligence to find these. - If a channel cannot be found: - Another path is sought. - Or the call/connection is blocked. 2. Information Transfer - May be analog voice, digitized voice, or binary data. - May be sent in bit streams or in packets. - Analog voice is not used any more. - There is no congestion with other users, since the channel is not shared. - This means no significant delay for data. - But the channels typically have low bandwidth kbps for digitized voice, for example. 3. Circuit disconnect - When finished, the channels are released. Why can circuit switching be quite inefficient? Dedicated circuits will not be used up to 100% capacity. Voice may be up to 40% utilized. Lecture 2, Page 5 of 11

6 In both directions, one talking at a time, plus 20% silence. Terminal to computer is usually even less utilized. But why might circuit switching still be used? No interference from congestion. Low delay. Low variation in delay (jitter). Examples of circuit switching Public Switched Telephone Network (PSTN) Private Branch Exchange (PBX) - Interconnects phones in a building or office. Data switch - Interconnects data circuits. The textbook has details on components of the telephone network. Packet Switching Dedicated data connections have low percentage utilization. Much of the time these connections are idle. It is not efficient to dedicate a circuit for data traffic. Packet switching addresses this problem. Data is grouped into blocks called. A typical maximum packet length is 1500 octets. - Octet = 8 bits. - Use octets instead of bytes, because bytes may or many not be 8 bits. Lecture 2, Page 6 of 11

7 Two parts of a packet - Data area - Header control information on how to route and handle a packet. - Addresses - Packet handling instructions - Error detection codes - Packet length indicator - Type of data Routing - Each packet is sent independently. - Nodes receive and send packets on to their destination. - First receive the whole packet. - Store briefly (for processing or in a queue because other packets might need to go first). - Then send to the next node on the path. See Figure 3.7 Lecture 2, Page 7 of 11

8 Benefits of Packet Switching Greater line efficiency - Many packets can share the same line or medium (like wireless). - The term is used to say that the random nature of packet arrivals allows the medium to be shared (multiplexed) based simply on that randomness. - This is one reason why 3G cellular wireless is going to packet switching instead of circuit switching. Data-rate conversion - Packets do not need to come in and go out at the same rates. Better response to heavy traffic - With circuit switching, calls are blocked when traffic gets heavy. - With packet switching, packets are just queued and experience more delay. Packets can be prioritized. Disadvantages of packet switching More delay - Entire packets have to be received at a node before they can be sent to the next node, so packets might need to be kept small. - In circuit switching, the data stream goes from end-to-end with no pauses along the way. - Then there may be queueing delay if lots of packets are waiting to be sent. - Packet processing time can also add delay. Packets may follow different paths. - So, packet delays can be highly variable. - Called. - And the packets can arrive out of order. - So there must be a way to store packets as they arrive, put everything together, and retrieve missing pieces. Packets require lots of header information. - There must be enough information for the packet to be sent based only on the information in the header. - This takes away bandwidth (10% or more) from the data. Lecture 2, Page 8 of 11

9 Routing of packets can require a lot of processing. - Each and every packet needs to be processed. - Circuit switching just sends data along the path already established. Most packets that go through a network have relationships with other packets. Why? Come from the same file. One large file will not be one packet, but will be split into several packets Part of a video or audio stream. What types of packets do not usually have relationships with other ones? Control packets. Two approaches for packet switching 1. Each packet treated independently. - Packets are processed with no knowledge of the other packets. - Even when packets come from the same stream or file. - This was illustrated above in Figure The destination must put packets back in order. - Packets can be lost due to packet errors. - Called a approach. Lecture 2, Page 9 of 11

10 2. Virtual Circuit - A preplanned route is established ahead of time. - Then a group of packets all follow that route. - No need to make routing decisions for each packet. - Except just to correlate the packet with its path. - Note this does not mean that dedicated bandwidth is given to a route. - Other packets can follow that route and share the links. - So this is not actual circuit switching. - But it is virtual circuit switching, since a path is established. - Packets all stay in order. - Packets can transit the network faster. - A virtual circuit can be used to exert better control over the performance of a group of packets. - Also used in 3G wireless. - 3G (1x-EVDO Rev. A) Packets plus virtual circuits. Lecture 2, Page 10 of 11

11 - A disadvantage is that the path must be established before packets are sent. - Which is only useful if several packets are to be sent. - Certainly wasteful for only one packet. - If a node fails, the path must be reestablished for a different path. See pages and homework for a discussion of the effects of packet sizes. Asynchronous Transfer Mode (ATM) technology is discussed in the book. But this is an older technology being phased out and is not used for wireless networks. We will not look at this. Next lecture: Chapter 13 on Wireless LAN Technology. Lecture 2, Page 11 of 11