Networks

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A communication network is a set of equipment and facilities that provides a service : the transfer of information between users located at various geographical points. Telegraph – message switching Telephone – circuit switching Internet – packet switching IP was developed to provide for the connectionless transfer of packets across an internetwork. IP uses a hierarchial address space that has location info embedded in the structure.( a network id and a host id).

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Layers Application : to provide services that are frequently required by applications that involve communications. Presentation : Provide the application layer with independence from differences in the representation of data. Session layer enhances a reliable transfer service provided by the transport layer by providing dialogue control. Transport layer is responsible for the end – to – end transfer of messages from a session entity in the source machine to a session entity in the destination machine.it can be responsible for setting up and releasing connections across the network.

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Network layer provides for the transfer of data in the form of packets across the communication network. By routing we mean the procedure that is used to select a path across the network. Data link layer provides for the transfer of frames across a transmission link that directly connects two nodes. Physical layer deals with the transfer of bits over a communication channel.The layer is concerned with the particular choice of system parameters such as voltage levels and signal durations. Telnet : is based on the concept of a network virtual terminal (NVT), which is an imaginary device that represents a lowest common denominator terminal.

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FTP: Telnet requires one TCP and FTP requires two TCP connections.1)control 2)data connection. Traceroute: Makes use of both ICMP and UDP. The sender first sends a UDP datagram with TTL=1 as well as an invalid port number to the specified destination host. NETSTAT: Queries a host about its TCP/IP status.

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ARQ Protocols: Stop and wait ARQ Go back N ARQ Selective repeat ARQ PPP Protocol: It provides a method for encapsulating IP packets over point – to – point links. It can be used as a data link control to connect two routers or can be used to connect a personal computer to an internet service procider using a telephone line and a modem. Frame format:0 6 1’s 0(flag),8 1’s(adr), 6 0’s 11(control),protocol,info,CRC,flag.

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Flow control refers to the procedures that prevent a transmitter from overrunning a receiver’s buffer.

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The primary function of the MAC is to eliminate or minimize the incidence of collisions to achieve a reasonable utilization of the medium. Random access Scheduling

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Throughput is defined as the actual rate at which information is transmitted through the shared medium.

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MAC layers provides for the connectionless transfer of datagrams.

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Polling in which systems take turns in accessing the medium.

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Token passing rings(pg no.372)

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FDMA(Frequency Division Multiple Access): In frequency division multiple access, the transmission medium is divided into M separate frequency heads. Each station transmits its info continuously on an assigned band.

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TDMA: Stations take turns making use of the entire transmission channel. Overhead:Preamble signal.

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In CDMA the transmission from different stations occupy the entire frequency band at the same time. The transmissions are separated by the fact that different codes are used to produce the signals that are transmitted by the different stations. The receivers use these codes to recover the signal.

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FAST ETHERNET: The IEEE 802.3u approves it. Used to aggregate traffic from shared 10 Mbps LANs. Provide greater bandwidth to a server Provide greater bandwidth to individual users.

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Gigabit Ethernet: Increase in speed by another factor of 10 put a focus on the limitations of the CSMA – CD MAC algorithm.

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Physical layer standards of Gigabit Ethernet have been defined for multimode fiber with maximum length of 550m.

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The ring latency is defined as the time that it takes for a bit to travel around the ring.

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Fiber Distributed Data Interface is a token – based LAN standard developed by ANSI. FDDI uses a ring – topology network in which station interfaces are interconnected by optical fiber transmission links operating at 100 Mbps in a ring that spans up to 200 kms and accommodates upto 500 stations. Uses 4B5B binary line code and NRZ inverted signaling that requires a symbol rate of 125 Msymbols/s.

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Each station uses 10 – bit elastic buffer to overcome the delay.

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Ad hoc networks are typically temporary in nature.

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The Distributed Co – ordination function provides support for the asynchronous data transfer of MAC SDUs on a best – effort service.

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Point coordination function to support connection – oriented time – bounded transfer of MAC SDUs.

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When two or more networks are interconnected at Physical layer – repeater.

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MAC or data – link layer – bridge. Network layer – router Higher layers – gateway. Gateways perform some protocol conversions and security functions. Switch/Router: Line card(I/O) Inter connection fabric Control Virtual – circuit packet switching: Involves the establishment of a fixed path between a source and a destination

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Prior to the transfer of packets. Switching, buffering, and transmission resources need not be dedicated explicitly for the use of the connection, but the number of flows admitted may be limited to control the load on certain links. Routing Static(Pre computed paths are then manually loaded to the routing table and remain fixed for a relatively long period of time) Dynamic(each router continuously learns the state of the network by communicating with its neighbors. Shortest path algorithms: Bellman ford algorithm Di = ∞ ∀i≠d Dd=0

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Updating for each I ≠ d, Di =min {Cij + Dj}, ∀j ≠I Repeat step 2 until no more changes occur in the iteration. Dijkstra’s algorithm: Initialization: N={s} Dj=Csj, ∀ ≠s Ds=0 Finding the next closest node Update the minimum cost: Dj=min {Dj,Di+Cij}

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ATM is a connection – oriented packet switching technique that generalizes the notion of a virtual connection to one that provides QoS guarantees. ARP converts the logical IP addresses into specific physical addresses. IPv4 Header: Version, IHL(Internet Header Length(32 bit words)), TOS(Type of Service), Total length,Id, flags, fragment offset,TTL, Protocol, Header Checksum, Source and Destination IP address,options(RSVP,2113) and padding. IP Address – 32 bits. Class A –{ 0 | 0.0.0 – 127|255.255.255} Class B – { 128.0|.0.0 – 191.255|.255.255} Class C – { 192.0.0.|0 – 223.255.255.|255} Class D – { 224.0.0.0 – 239.255.255.255} Multi cast Address Class E – { 240.255.255.255 – 255.255.255.255}Experimental

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TCP= 6,UDP=17,FTP=21,ICMP=1. A host id that contains all 1s is meant to broadcast the packet to all host on the network specified by the network id. A host ID that contains all 0s refers to the network specified by the network ID, rather than the host. 127.X.Y.Z loopback.

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Class B – 64000 hosts. It is difficult to manage all 64k hosts. The basic idea of subnetting is to add another hierarchial level called the “subnet”.

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The subnet – addressing scheme is that it is oblivious to the network outside the organization.

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Reverse Address Resolution: The host broadcasts its IP address, the host first broadcasts an RARP request packet containing its MAC address on the network. All hosts on the network receive the packet, but only the server replies to the host by sending an RARP response packet containing the host’s MAC and IP address. IPv6: Longer address fields(128 – bit) Simplified Header Format: Flow label capability Security Fragmentation at source only.No check sum field.

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IPv6 Header

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Version Traffic class Flow label Payload Length Next Header Hop Limit SRc and dest. IP Address

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UDP: Source and destination port. UDP length and checksum Data DHCP: When a host wishes to obtain an IP address, the host broadcasts a DHCP Discover message in its physical network. The server in the nt may respond with a DHCP offer message that provides an IP address and other configuration information. Automatically configures host that connect to a TCP/IP network.

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Mobile IP allows portable devices called mobile hosts(MHs) to roam from one area to another while maintaining the communication sessions.

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An autonomous system is loosely defined as a set of routers or networks that are technically administered by a single organization such as an ISP.

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Border Gateway Protocol is an interAS routing protocol that is used to exchange network reachability information among BGP routers.

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Multi – protocol Label Switching(MPLS) working group to standardize a label – switching paradigm that integrates layer 2 switching with layer 3 routing. Device – Label switching router(LSR).

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A label is a form of shorthand for the packet header that simplifies the forwarding decision a node would make for the packet.

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The label at the top of the stack(depth 1) is the only one that determines the forwarding decision.

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A packet with an empty stack(depth 0) is called an unlabeled packet.

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RSVP:can be used by a host to request a specific QoS resource for a particular flow and by a router to provide the request QoS along the path by setting up appropriate states.

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Topologies: Star,Ring,Bus,Tree(Star+Bus). Repeater: Clean Signal Pulses. Boost Signal power. Pass al signals between attached segments Bridges: Algorithm:Spanning tree protocol.

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Ethernet uses carrier sense, multiple access with collision detection(CSMA/CD).

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Types of Ethernet: Standard Ethernet Thin wire Ethernet Twisted – pair Ethernet Optical Fiber Ethernet Fast and Gigabit

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Switch is a very fast, low – latency, multiport bridge that is used to segment LANs.

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Store – and – forward switching Involves reading the entire Ethernet Frame, before forwarding it, with the required protocol and at the correct speed, to the destination port. Advantages: Improved error check Protocol filtering Speed matching Disadvantage System delay

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A Token Ring network circulates an electronic token around a closed electronic loop. Each node on the network reads the token and repeats it to the next node. The control token circulates around the ring even when there is no data being transmitted.

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Nodes must await a token. When they get it, they fill a frame with data and add the source and destination addresses then send it to the next node.

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FDDI frame format: PA, SD, FC, DA,SA, INFO, C\FCS,ED,FS. Frame control Frame status

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Firewall is the routing computer which isolates the intranet from the outside world. Firewall or security gateway protects a network against intrusion from outside sources. Network level firewalls.(address filtering) Application level firewalls.(uses proxy to act as an interim system) Circuit – level firewalls. Proxy connects to a number of clients. Acts as a client when communicating with a server and as a server when communicating with a client.

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Used for security purposes where the client and server are separated by a firewall. The server must authenticate itself to the firewall before a connetion can be made with the proxy.

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Extranets allows two or more companies to share parts of their intranets related to joint projects.

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SONET(Synchronous Optical Network) as a bit way implementation providing end – to – end transport of bit streams.

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SONET sources are synchronized to a common master clock. Different streams are multiplexed by byte interleaving.

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SONET Layers: Layer - Function Path – services; end – to – end error detection Line – Multiplexing , with frame and frequency alignment; Section – Framing,scrambling and datala links Photonic – Electrical to optical and vice versa conversion. Digital Subscriber line is the generic term for a set of technologies that use the bandwidth for digital transmission. Asymmetric DSL:A modem technology that uses existing twisted pairs to create three channels:high – speed downstream, a medium – speed upstream or duplex channel and Plain Old Telephone System or an ISDN.

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ATM Bearer Service:

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The service is connection – oriented, with data transferred over a virtual circuit. The data is transferred in 53 – byte octets. Cells from different VCs that occupy the same channel or link are statistically multiplexing. Treats the cell streams in different VC connections unequally over the same channel in order to provide different QoS

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ATM address is 20 bytes long. GFC, VPI,VCI,PT,CLP,HEC

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