network devices, types of delay
- 1. ASSIGNMENT # 2 SUBMITTED TO: MISS SARA SARWAR SUBMITTED BY: RABIA ZAFAR 17581556-045 BS IT (5TH SEMESTER) SECTION ‘A’ INTERNET ARCHITECTURE & PROTOCOL
- 2. Q1: What are uses of networking devices (Hubs, Bridges, Switches, Router)? HUB: Hub is the most basic networking device that connects multiple computers or other network devices together. Most hubs can detect basic network errors such as collisions, but having all information broadcast to multiple ports can be a security risk. In the past, network hubs were popular because they were cheaper than a switch or router. Today, switches do not cost much more than a hub and are a much better solution for any network. A hub is basically a multiport repeater. A hub connects multiple wires coming from different branches, for example, the connector in star topology which connects different stations. Hubs cannot filter data, so data packets are sent to all connected devices. In other words, collision domain of all hosts connected through Hub remains one. Also, they do not have intelligence to find out best path for data packets which leads to inefficiencies and wastage. Bridge:
- 3. A bridge operates at data link layer. A bridge is a repeater, with add on the functionality of filtering content by reading the MAC addresses of source and destination. It is also used for interconnecting two LANs working on the same protocol. It has a single input and single output port, thus making it a 2 port device. Switch: A switch is a multiport bridge with a buffer and a design that can boost its efficiency (a large number of ports imply less traffic) and performance. A switch is a data link layer device. The switch can perform error checking before forwarding data that makes it very efficient as it does not forward packets that have errors and forward good packets selectively to correct port only. In other words, switch divides collision domain of hosts, but broadcast domain remains same. Routers: A router is a device like a switch that routes data packets based on their IP addresses. Router is mainly a Network Layer device. Routers normally connect LANs and WANs together and have a dynamically updating routing table based on which they make decisions on routing the data packets. Router divide broadcast domains of hosts connected through it.
- 4. Question 3: Delay in Packet-Switched Networks: As a packet travels from one node (host or router) to the subsequent node (host or router) along this path, the packet suffers from several types of delays at each node along the path. The most important of these delays are the nodal processing delay, queuing delay, transmission delay, and propagation delay; together, these delays accumulate to give a total nodal delay. Types of Delay Processing Delay: The time required to examine the packet’s header and determine where to direct the packet is part of the processing delay. The processing delay can also include other facts, such as the time needed to check for bit-level errors in the packet that occurred in transmitting the packet’s bits from the upstream node to router A. Processing delays in high-speed routers are typically on the order of microseconds or less. After this nodal processing, the router directs the packet to the queue that precedes the link to router B. Queuing Delay At the queue, the packet experiences a queuing delay as it waits to be transmitted onto the link. The length of the queuing delay of a specific packet will depend on the number of earlier-arriving packets that are queued and waiting for transmission onto the link. If the queue is empty and no other packet is currently being transmitted, then our packet’s queuing delay will be zero. On the other hand, if the traffic is heavy and many other packets are also waiting to be transmitted, the queuing delay will be long. We will see shortly that the number of packets that an arriving packet might expect to find is a
- 5. function of the intensity and nature of the traffic arriving at the queue. Queuing delays can be on the order of microseconds to milliseconds in practice. Transmission Delay: Assuming that packets are transmitted in a first-come first-serve manner, as is common in packet switched networks, our packet can be transmitted only after all the packets that have arrive before it have been transmitted. Denote the length of the packet by L bits, and denote the transmission rate of the link from router A to router B by R bits/sec. For example, for a 10 Mbps Ethernet link, the rate R = 100 Mbps. The transmission delay is L/R. This is the amount of time required to push (that is, transmit) all of the packet’s bits into the link. Transmission delays are typically on the order of microseconds to milliseconds in practice. Propagation Delay: Propagation delay is the time required for a digital signal to travel from the input(s) of a logic gate to the output. Once a bit is pushed into the link, it needs to propagate to router B. The time required to propagate from the beginning of the link to router B is the propagation delay. The bit propagates at the propagation speed of the link. The propagation speed depends on the physical medium of the link (that is, fiber optics, twisted-pair copper wire, and so on). Which is equal to, or a little less than, the speed of light. The propagation delay is the distance between two routers divided by the propagation speed. That is, the propagation delay is d/s, where d is the distance between router A and router B and s is the propagation speed of the link. Once the last bit of the packet propagates to node B, it and all the preceding bits of the packet are stored in router B. The whole process then continues with router B now performing the forwarding. In wide-area networks, propagation delays are on the order of milliseconds. Packet Loss and its causes: There are many reasons of data loss. Link Congestion: One of the major causes of packet loss is link congestion.
- 6. Example: A simple analogy is rush hour traffic when there are more cars on the road than the road can sufficiently handle. Another analogy is a 4-lane road merging into 2 lanes. What happens is that there are more packets arriving on a link than that link is designed to handle. Over-utilized devices Another cause of packet loss similar to network congestion is Over-Utilized devices. This means that a device is operating at a capacity it was not designed for. In a network, packets may arrive faster than they can be processed/sent out. To handle this type of situation, many devices have buffers where they hold packets temporarily until they are able to be processed and sent out. However, in the case of an Over-Utilized device, the buffer will probably fill up quickly, resulting in excess packets being dropped. For example, a Cisco ASA 5506-X is designed to handle up to 750 Mbps of throughput traffic. If you use such a device at the network edge of an organization pushing more than that maximum throughput, you will definitely have an issue. Faulty Hardware and/or Software: Another cause of Packet loss is Faulty hardware. This could be a component of a device or the whole device itself. Closely related to faulty hardware is a buggy software running on the network device. As with any other software, it is usually impossible for the development team to catch all the bugs in the software running on network devices, and one of such bugs may result in packet loss. Wireless versus Wired networks:
- 7. The type of network medium can also be a cause of packet loss. Generally speaking, Wireless networks suffer more setbacks than their wired counterparts. For example, radio frequency interference can be a major issue on wireless networks resulting in packet loss. Other challenges on a wireless network that can result in packet loss include weak signal, distance limitations etc. In the case of wired networks, faulty cables can result in packet loss. This could result from the fact that the cable is not properly terminated or that the cable is damaged, causing issues for the electrical signal meant to flow through the cable. Effects of Packet Loss: The effects of packet loss vary depending on the protocol/application concerned. TCP is generally designed to handle packet loss because of the acknowledgment and re- transmission of packets – if a packet gets lost (i.e. no acknowledgment is received for that packet), it will usually be re-transmitted. UDP, on the other hand, does not have inbuilt re-transmission capability and may not handle packet loss as well. However, irrespective of the protocol/application, too much loss of packets is definitely a problem. Question 2: Circuit switching: It is a transmission mode that involves setting up a dedicated end to end connection. Example: Circuit switching is commonly used in telephone system.
- 8. When a call is made from one telephone to another, switches within the telephone exchanges create a continuous wire circuit between the two telephones, for as long as the call lasts. It is connection oriented. No delay in data flow. Circuit switching principal: A connection between the ingoing and outgoing segments of the transmission path is established on demand, for the exclusive use of a pair of end users - until explicitly released. Packet switching: Packet switching is a transmission process in which data is broken into perfect- sized blocks for fast and efficient transfer via different network devices. In which message is broken into packets for transmission. Individual packets take different router to reach the destination. Follow the store and forward packet scheme. Pipelining of packet transmission reduces transmission delay.
- 9. Two scheme for packet switching is 1) Datagram 2) Virtual circuit switching Datagram: Each packet treated independently. Packets can take any practical rout. Packets may arrive out of order. Packets may go missing. Up to receiver to re-order packets and recover from missing packets. Don’t set up a connection, just make sure each packet contains enough information to get it to destination. Processing data: Switch creates a table, mapping destinations to output port (ignores input ports) – when a packet with a destination address in the table arrives, it pushes it out on the appropriate output port – when a packet with a destination address not in the table arrives, something clever has to be done ( a different problem!) Virtual Circuit: Logical connection is build between sending and receiving connection. All packets are travel through the logical connection. In which bandwidth is high and transmission delay rate is low. Large amount of RAM is required. And it required more power.
- 10. Switched virtual circuit: Switched Virtual Circuits (SVCs) are temporary connections created for the purpose of information transfer. There are four steps to establish SVC connection viz. call setup, data transfer, Idle and call termination. Once the virtual circuit is cleared, it has to be re-establish in order to transmit any further data. As the circuit is not fixed (i.e. not open) all the time, the cost to use the SVC service is less. It is established on need basis in order to transmit data as required. PVC-PermanentVirtualCircuit Permanent Virtual Circuits (PVCs) are permanent connections established for the sole purpose of frequent as well as consistent data transfer. It is like leased line, PVC connection do not require to be established using call setup or termination states. PVC will be either in data transfer mode or in 1IDle mode. In data transfer mode, data is transmitted between two devices over virtual circuit path. In Idle mode, connection between devices is available but data transfer is not progressing. Unlike Switched Virtual Circuit, PVCs are not terminated during idle state. As this type of virtual circuit connection is permanent, data transfer can take place as soon as it is ready to transmit.