333: DISCUSS THE FUNDAMENTALS OF NETWORKING

 1. Discuss networking concepts (20 hrs)  2. Discuss hardware & software requirement to setup a Local Area Network (20 hrs)

 PERFORMANCE STANDARD ◦ Given a scenario, identify correctly the hardware and services required for a Network  Objectives: ◦ Define a Node/Host ◦ Discuss functional & geographical relationship among nodes ◦ Discuss services provided by a common carrier such as  Leased line  Switched line ◦ Describe switching techniques  Circuit  Message  Packet ◦ Discuss ISO/OSl reference model ◦ Explain network communication protocols

Describe switching techniques

 Networks are used to interconnect many devices.  We have checked with Local Area Networks.  Now, wide area networks ◦ Since the invention of the telephone, circuit switching has been the dominant technology for voice communications. ◦ Since 1970, packet switching has evolved substantially for digital data communications. It was designed to provide a more efficient facility than circuit switching for bursty data traffic.  Two types of packet switching:  Datagram (such as today’s Internet)  Virtual circuit (such as Frame Relay, ATM)

 The figure shows that if the data from station A intended for station F are sent to node 4.  They may then be routed via nodes 5 and 6 or nodes 7 and 6 to the destination.  Some nodes connect only to other nodes (e.g., 5 and 7).  Their sole task is the internal (to the network) switching of data. Other nodes have one or more stations attached as well.

 Node-station links are generally dedicated point- to-point links.  Node-node links are usually multiplexed, using either frequency division multiplexing (FDM) or time division multiplexing (TDM).  Usually, the network is not fully connected; that is, there is not a direct link between every possible pair of nodes.  However, it is always desirable to have more than one possible path through the network for each pair of stations.  This enhances the reliability of the network.

 In large networks there might be multiple paths linking sender and receiver. Information may be switched as it travels through various communication channels. There are three typical switching techniques available for digital traffic. Circuit Switching Message Switching Packet Switching

Switching Techniques

 Circuit switching is a technique that directly connects the sender and the receiver in an unbroken path.  Telephone switching equipment, for example, establishes a path that connects the caller's telephone to the receiver's telephone by making a physical connection.  With this type of switching technique, once a connection is established, a dedicated path exists between both ends until the connection is terminated.  Routing decisions must be made when the circuit is first established, but there are no decisions made after that time

 Circuit switching in a network operates almost the same way as the telephone system works.  A complete end-to-end path must exist before communication can take place.  The computer initiating the data transfer must ask for a connection to the destination.  Once the connection has been initiated and completed to the destination device, the destination device must acknowledge that it is ready and willing to carry on a transfer.

 Circuit switching: ◦ There is a dedicated communication path between two stations (end-to-end) ◦ The path is a connected sequence of links between network nodes. On each physical link, a logical channel is dedicated to the connection.  Communication via circuit switching has three phases: ◦ Circuit establishment (link by link)  Routing & resource allocation (FDM or TDM) ◦ Data transfer ◦ Circuit disconnect  Deallocate the dedicated resources  The switches must know how to find the route to the destination and how to allocate bandwidth (channel) to establish a connection.

 Inefficiency ◦ Channel capacity is dedicated for the whole duration of a connection ◦ If no data, capacity is wasted  Delay ◦ Long initial delay: circuit establishment takes time ◦ Low data delay: after the circuit establishment, information is transmitted at a fixed data rate with no delay other than the propagation delay. The delay at each node is negligible.  Developed for voice traffic (public telephone network) but can also applied to data traffic. ◦ For voice connections, the resulting circuit will enjoy a high percentage of utilization because most of the time one party or the other is talking. ◦ But how about data connections?

 Advantages: ◦ The communication channel (once established) is dedicated.  Disadvantages: ◦ Possible long wait to establish a connection, (10 seconds, ◦ more on long- distance or international calls.) during which no data can be transmitted. ◦ More expensive than any other switching techniques, because a dedicated path is required for each connection. ◦ Inefficient use of the communication channel, because the channel is not used when the connected systems are not using it.

16 Subscribers: the devices that attach to the network. Subscriber loop: the link between the subscriber and the network. Exchanges: the switching centers in the network. End office: the switching center that directly supports subscribers. Trunks: the branches between exchanges. They carry multiple voice- frequency circuits using either FDM or synchronous TDM.

A circuit-switched network consists of a set of switches connected by physical links. A connection between two stations is a dedicated path made of one or more links. However, each connection uses only one dedicated channel on each link. Each link is normally divided into n channels by using FDM or TDM.

A circuit-switched network is made of a set of switches connected by physical links, in which each link is divided into n channels Note

A trivial circuit-switched network

In circuit switching, the resources need to be reserved during the setup phase; the resources remain dedicated for the entire duration of data transfer until the teardown phase. Note

As a trivial example, let us use a circuit-switched network to connect eight telephones in a small area. Communication is through 4-kHz voice channels. We assume that each link uses FDM to connect a maximum of two voice channels. The bandwidth of each link is then 8 kHz. Figure 8.4 shows the situation. Telephone 1 is connected to telephone 7; 2 to 5; 3 to 8; and 4 to 6. Of course the situation may change when new connections are made. The switch controls the connections. Example 8.1

8.22 Figure 8.4 Circuit-switched network used in Example 8.1

As another example, consider a circuit-switched network that connects computers in two remote offices of a private company. The offices are connected using a T-1 line leased from a communication service provider. There are two 4 × 8 (4 inputs and 8 outputs) switches in this network. For each switch, four output ports are folded into the input ports to allow communication between computers in the same office. Four other output ports allow communication between the two offices. Figure 8.5 shows the situation. Example 8.2

Figure 8.5 Circuit-switched network used in Example 8.2

8.25 Figure 8.6 Delay in a circuit-switched network

8.26 Note Switching at the physical layer in the traditional telephone network uses the circuit-switching approach.

 Problem of circuit switching ◦ designed for voice service ◦ Resources dedicated to a particular call ◦ For data transmission, much of the time the connection is idle (say, web browsing) ◦ Data rate is fixed ◦ Both ends must operate at the same rate during the entire period of connection  Packet switching is designed to address these problems.

 With message switching there is no need to establish a dedicated path between two stations.  When a station sends a message, the destination address is appended to the message.  The message is then transmitted through the network, in its entirety, from node to node.  Each node receives the entire message, stores it in its entirety on disk, and then transmits the message to the next node.  This type of network is called a store-and- forward network.

 A message-switching node is typically a general-purpose computer. The device needs sufficient secondary-storage capacity to store the incoming messages, which could be long. A time delay is introduced using this type of scheme due to store- and-forward time, plus the time required to find the next node in the transmission path.

 Advantages: ◦ Channel efficiency can be greater compared to circuit-switched systems, because more devices are sharing the channel. ◦ Traffic congestion can be reduced, because messages may be temporarily stored in route. ◦ Message priorities can be established due to store- and-forward technique. ◦ Message broadcasting can be achieved with the use of broadcast address appended in the message.

 Disadvantages ◦ Message switching is not compatible with interactive applications. ◦ Store-and-forward devices are expensive, because they must have large disks to hold potentially long messages.

 Packet switching can be seen as a solution that tries to combine the advantages of message and circuit switching and to minimize the disadvantages of both.  There are two methods of packet switching: Datagram and virtual circuit.

 Data are transmitted in short packets ◦ Typically at the order of 1000 bytes ◦ Longer messages are split into series of packets ◦ Each packet contains a portion of user data plus some control info  Control info contains at least ◦ Routing (addressing) info, so as to be routed to the intended destination ◦ Recall the content of an IP header!  store and forward ◦ On each switching node, packets are received, stored briefly (buffered) and passed on to the next node.

 Line efficiency ◦ Single node-to-node link can be dynamically shared by many packets over time ◦ Packets are queued up and transmitted as fast as possible  Data rate conversion ◦ Each station connects to the local node at its own speed  In circuit-switching, a connection could be blocked if there lacks free resources. On a packet-switching network, even with heavy traffic, packets are still accepted, by delivery delay increases.  Priorities can be used ◦ On each node, packets with higher priority can be forwarded first. They will experience less delay than lower- priority packets.

 A station breaks long message into packets  Packets are sent out to the network sequentially, one at a time  How will the network handle this stream of packets as it attempts to route them through the network and deliver them to the intended destination? ◦ Two approaches  Datagram approach  Virtual circuit approach

 Each packet is treated independently, with no reference to packets that have gone before. ◦ Each node chooses the next node on a packet’s path.  Packets can take any possible route.  Packets may arrive at the receiver out of order.  Packets may go missing.  It is up to the receiver to re-order packets and recover from missing packets.  Example: Internet

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 In virtual circuit, a preplanned route is established before any packets are sent, then all packets follow the same route.  Each packet contains a virtual circuit identifier instead of destination address, and each node on the preestablished route knows where to forward such packets. ◦ The node need not make a routing decision for each packet.  Example: X.25, Frame Relay, ATM

41 A route between stations is set up prior to data transfer. All the data packets then follow the same route. But there is no dedicated resources reserved for the virtual circuit! Packets need to be stored-and-forwarded.

 Virtual circuits ◦ Network can provide sequencing (packets arrive at the same order) and error control (retransmission between two nodes). ◦ Packets are forwarded more quickly  Based on the virtual circuit identifier  No routing decisions to make ◦ Less reliable  If a node fails, all virtual circuits that pass through that node fail.  Datagram ◦ No call setup phase  Good for bursty data, such as Web applications ◦ More flexible  If a node fails, packets may find an alternate route  Routing can be used to avoid congested parts of the network

Comparison of communicatio n switching techniques

 The size of the packet can vary from 180 bits, the size for the Datakit® virtual circuit switch designed by Bell Labs for communications and business applications; to 1,024 or 2,048 bits for the 1PSS® switch, also designed by Bell Labs for public data networking; to 53 bytes for ATM switching, such as Lucent Technologies' packet switches.

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