Networks Lecture - 5 PDF

Objectives  To discuss about the wireless LANs.  To differentiate between the CSMA/CD and CSMA/CA  To describe how the technologies can be used to build a NW that spans a large area  To discuss about the various switching networks.  To describes the basic components used to build...

Objectives  To discuss about the wireless LANs.  To differentiate between the CSMA/CD and CSMA/CA  To describe how the technologies can be used to build a NW that spans a large area  To discuss about the various switching networks.  To describes the basic components used to build a packet switching (PS) system that can span a large area  To explains the fundamental concept of routing and shows how routing is used in such network (NW). CCN.1 Topics WirelessLAN  Hidden and exposed terminals IEEE 802.11 CSMA/CA RTS – discovery packets BS, AP Switched Networks CCN.2 Wireless LAN Protocols Wireless has complications compared to wired. Nodes may have different coverage regions Leads to hidden and exposed terminals Nodes can’t detect collisions, i.e., sense while sending Makes collisions expensive and to be avoided CCN.3 Wireless LANs – Hidden terminals Senders that cannot sense each other but nonetheless collide at intended receiver A and C are hidden terminals when sending to B A B C C A’s signal C’s signal B strength strength A space CCN.4 IEEE 802.11  802.11: CSMA - sense before transmitting don’t collide with ongoing transmission by another node goal: avoid collisions: CSMA/C(ollision)A(voidance) A B C C A’s signal C’s signal B strength strength A space CCN.5 CSMA/CA CSMA/CA inserts backoff slots to avoid collisions MAC uses ACKs/retransmissions for wireless errors CCN.6 Wireless LANs 802.11b and CSMA/CA  Collision Detection does not work because a transmission from one computer may only be received by its immediate neighbors  Solution: Wireless LANs uses Collision Avoidance. CCN.7 Collision Avoidance - Before sending the frame, sender transmit a brief control message to receiver - Allow sender to “reserve” channel rather than random access  sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but they’re short) Receiver responds by sending another control message “clear to send (CTS)” to indicate that it is ready to receive a transmission Once a response from its intended recipient arrives  sender begins transmitting the frame CCN.8  Collision Avoidance: RTS-CTS exchange A B AP RTS(A) RTS(B) reservation collision RTS(A) CTS(A) CTS(A) DATA (A) defer time ACK(A) ACK(A) CCN.9 WAN Technologies and Routing CCN.10 Wide Area Network technology  The key issue that separates WAN technologies from LAN technologies is Scalability A WAN must be able to grow as needed to connect many sites spread across large geographic distances. CCN.11 Switched Network CCN.12 Switched Networks Packet switching circuit packet VC Datagram CCN.13 Circuit Switching  Communication via circuit switching implies that there is a dedicated communication path between two stations.  The path is a connected sequence of links between network nodes. Communication via circuit switching involves three phases: Circuit establishment Data transfer Circuit disconnect CCN.14 Packet Switch (PS)  Basic electronic switch used in a WAN.  It moves complete packets from one connection to another.  Each PS is a small computer, special HW has a processor, memory, I/O devices used to send and receive Forming a WAN CCN.15 Principles of congestion control Congestion: informally: “too many sources sending too much data too fast for network to handle” manifestations: long delays (queueing in router buffers) packet loss (buffer overflow at routers) differentfrom flow control! congestion  a top-10 problem! control: too many senders, sending too fast flow control: one sender too fast for one receiver CCN.16 Packet Switching  Hosts send packets into the network; packets are forwarded by routers  Originally designed to handle voice traffic, and resources within the network.  Data are transmitted in short packets (packet length is 1000 bytes).  Station breaks long message into packets. Packets sent one at a time to the network  Each packet contains a portion of the user's data plus some control information.  At each node enroute, the packet is received, stored briefly, and passed on to the next node. CCN.17 Advantages of packet-switching network  Line efficiency is greater.  The packets are queued up and transmitted as rapidly as possible over the link.  Much of the time, such a link may be idle because a portion of its time is dedicated to a connection that is idle.  A packet-switching network can perform data-rate conversion.  Even during heavy traffic - packets are still accepted, but delivery delay increases.  Priorities can be used. If a node has a number of packets queued for transmission, it can transmit the higher-priority packets first. CCN.18 Connectionless Service – Datagrams  Packet is forwarded using destination address inside it Different packets may take different paths ISP’s equipment CCN.19 Datagram approach  Each packet is treated independently(as datagram), with no reference to packets that have gone before.  Each node chooses the next node on a packet's path, taking into account information received from neighboring nodes on traffic, line failures, and so on.  So the packets, each with the same destination address, do not all follow the same route, and they may arrive out of sequence at the exit point.  If a packet-switching node crashes momentarily, all of its queued packets may be lost.  Again, it is up to either the exit node or the destination to detect the loss of a packet and decide how to recover it. CCN.20 Connection-Oriented – Virtual Circuits  Packet is forwarded along a virtual circuit using tag inside it Virtual circuit (VC) is set up ahead of time ISP’s equipment CCN.21 CCN.22 Virtual Circuit  A preplanned route is established before any packets are sent.  All the packets between a pair of communicating parties follow this same route through the network  The route is fixed for the duration of the logical connection.  Each packet contains a virtual circuit identifier as well as data.  Each node on the pre established route knows where to direct such packets; no routing decisions are required.  At any time, each station can have more than one virtual circuit to any other station and can have virtual circuits to more than one station.  Note that this does not mean that this is a dedicated path, as in circuit switching. CCN.23 Comparison of Virtual-Circuits & Datagrams CCN.24 References  Chapters 13, 15,16 and 18. Computer Networks and Internets, Douglas E. Comer, Prentice Hall. 5th Edition.  Chapter 10. Data and Computer Communications 8th Edition by William Stallings CCN.25