Lecture (05) Network interface Layer media & switching II

Transcription

1 Lecture (05) Network interface Layer media & switching II By: ElShafee ١ Agenda Circuit switching technology (cont,..) Packet switching technique Telephone network ٢

2 Circuit switching technology (cont,..) Blocking and non blocking characteristic : Blocking occurs when the network is unable to connect two stations because all possible paths between them are already in use. nonblocking network permits all stations to be connected (in pairs) at once and grants all possible connection requests as long as the called party is free. Voice network point of view: When a network is supporting only voice traffic, a blocking configuration is generally acceptable, because it is expected that most phone calls are of short duration and that therefore only a fraction of the telephones will be engaged at any time. Circuit switching technology (cont,..) ٣ Data network point of view: Not accepted, as data entry application, a terminal may be continuously connected to a computer for hours at a time. Specially if some sensitive operation is performed like money transfer (ATM machine and bank server) for data applications, there is a requirement for a no blocking or "nearly no blocking" (very low probability of blocking) configuration. ٤

3 Circuit switching technology (cont,..) Space Division Switch ٥ Circuit switching technology (cont,..) Space division switching was originally developed for the analog environment and has been carried over into the digital one. Each connection requires the establishment of a physical path through the switch that is dedicated to the transfer of signals between the two endpoints. The basic building block of the switch is a metallic cross point or semiconductor gate that can be enabled and disabled by a control unit. Figure shows simple crossbar matrix with 10 full duplex I/O lines. ٦

4 ٧ Circuit switching technology (cont,..) The matrix has 10 inputs and 10 outputs; each station attaches to the matrix via one input and one output line. Interconnection is possible between any two lines by enabling the appropriate cross point. Note that a total of 100 cross points is required. ٨

5 Circuit switching technology (cont,..) Advantages Non blocking system Simple control scheme ٩ Circuit switching technology (cont,..) Disadvantages The number of cross points grows with the square of the number of attached stations. This is costly for a large switch. The loss of a cross point prevents connection between the two devices whose lines intersect at that cross point. The cross points are inefficiently utilized; even when all of the attached devices are active, only a small fraction of the cross points are engaged. ١٠

6 Circuit switching technology (cont,..) multiple stage switches To overcome these limitations, multiple stage switches are employed. ١١ ١٢

7 Circuit switching technology (cont,..) Advantages 1. The number of cross points is reduced, 2. increasing crossbar utilization. 3. Multiple paths/connections In the example, the total number of cross points for 10 stations is reduced from 100 to 48. There is more than one path through the network to connect two endpoints, increasing reliability. ١٣ Circuit switching technology (cont,..) Disadvantages: System have some blocking probability. requires a more complex control scheme (free path through the stages must be determined and the appropriate gates enabled). ١٤

8 Circuit switching technology (cont,..) The heavier lines indicate the lines that are already in use. In this state, input line 10, for example, cannot be connected to output line 3, 4, or 5, even though all of these output lines are available. ١٥ A multiplestage switch can be made non blocking by increasing the number or size of the intermediate switches, but of course this increases the cost. ١٦

9 Circuit switching technology (cont,..) Time Division Switching Instead of relatively dumb space division systems, modern digital systems rely on intelligent control of space and time division elements.. ١٧ Virtually all modern circuit switches use virtual time division techniques for establishing and maintaining "circuits. Time division switching involves the partitioning of a lowerspeed bit stream into pieces that share a higher speed stream with other bit streams. ١٨

10 The individual pieces, or slots, are manipulated by control logic to route data from input to output ١٩ Circuit switching technology (cont,..) Soft switches: A soft switch is a central device in a telecommunications network which connects telephone calls from one phone line to another phone line or terminal PC, typically via the internet, entirely by means of software running on a computer system. This work was formerly carried out by hardware, with physical switchboards to route the calls. generally it handles IP to IP phone calls, ٢٠

11 Circuit switching technology (cont,..) ٢١ Circuit switching technology (cont,..) Soft switch consists of two components: Media gateway: which is the equivalent software version of physical/ virtual connection matrix. Media gateway controller: which is the equivalent software version of switch controller. ٢٢

12 Circuit switching technology (cont,..) ٢٣ Packet switching technique Introduction In contrast a packet switching network is designed for data use. Data are transmitted in short packets. Fixed packet length say 1000 octets (bytes). If a source has a longer message to send, the message is broken up into a series of packets. Each packet contains a portion (or all for a short message) of the user's data plus some control information. ٢٤

13 The control information, at a minimum, includes the information that the network requires to be able to route the packet through the network and deliver it to the intended destination. At each node, the packet is received, stored briefly, and passed on to the next node. ٢٥ Advantages 1. Line efficiency is greater, because a single node to node link can be dynamically shared by many packets over time. The packets are queued up and transmitted as rapidly as possible over the link. By contrast, with circuit switching, time on a node to node link is pre allocated using synchronous time division multiplexing. Much of the time, such a link may be idle because a portion of its time is dedicated to a connection that is idle (considered to disadvantage) ٢٦

14 2. A packet switching network can perform data rate conversion. Two stations of different data rates can exchange packets because each connects to its node at its proper data rate. 3. Decrease blocking, When traffic becomes heavy on a circuit switching network, some calls are blocked; that is, the network refuses to accept additional connection requests until the load on the network decreases. On a packet switching network, packets are still accepted, but delivery delay increases. ٢٧ 4. Priorities can be used. If a node has a number of packets queued for transmission, it can transmit the higher priority packets first. These packets will therefore experience less delay than lower priority packets. ٢٨

15 Switching technique Two approaches are used in packet switching networks: datagram and virtual circuit. ٢٩ datagram 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, 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. Exit node, or destination rearrange the data grams to restore the original squence. ٣٠

16 Virtual circuit a preplanned route is established before any packets are sent. Once the route is established, all the packets between a pair of communicating parties follow this same route through the network. Because the route is fixed for the duration of the logical connection, it is somewhat similar to a circuit in a circuits switching network and is referred to as a virtual circuit. 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. ٣١ At any time, each station can have more than one virtual circuit to any other station. The virtual circuit is established before data being transferred, even, it s not a dedicated path. A transmitted packet is buffered at each node, and queued for output over a line, while other packets on other virtual circuits may share the use of the line. Nods do no routing decisions, as virtual path is already established. ٣٢

17 Comparisons between datagrame, and Virtual circuits Datagram No sequencing or error control, destination or exit node should solve these problems Packets are slow, need routing decision at each node. no call setup phase, immediate transmission More reliable, any available path, if node fail, forward to any nearby node more flexible Virtual circuit network can provide sequencing and error control, as all packets follow the same route packets are forwarded more quickly, no routing decisions Call setup, wait before start less reliable, only predefined path, if node fails, path down Less fixable ٣٣ Packet size and transmission time Assume that a virtual circuit exists from station X through nodes a and b to station Y. The message to be sent comprises 40 octets, with 3 octets of control information at the beginning of each packet in the header. ٣٤

18 If the entire message is sent as a single packet of 43 octets (3 octets of header plus 40 octets of data), then the packet is first transmitted from station X to node a When the entire packet is received, it can then be transmitted from a to b. ٣٥ When the entire packet is received at node b, it is then transferred to station Y. Ignoring switching time, total transmission time is 129 octet times (43 octets 3 packet transmissions). ٣٦

19 If we break the message into two packets with 20 octets of message and 3 octets of header each. node a can begin transmitting the first packet as soon as it has arrived from X. Because of this overlap in transmission, Tx time=23x4=92 octet time ٣٧ the total transmission time drops to 92 octet-times. breaking the message into five packets (40/5)+3=11 octet, each intermediate node can begin transmission even sooner, with a total of = 11x7 = 77 octet times for transmission. Dividing message to 10 packets each of 4 octet + 3 control=7 octet/ packet Total Tx time = 12x7=84 octet time ٣٨

20 Conclusions: each packet contains a fixed amount of header, and more packets Also, processing and queuing delays at each node are greater when more packets are handled for a single message. extremely small packet size (53 octets) can result in an efficient network design. ٣٩ Packet switching delays Propagation delay: The time it takes a signal to propagate from one node to the next. The speed of electromagnetic signals through a wire medium, for example, is typically m/s, delay is neglectable. Transmission time: The time it takes for a transmitter to send out a block of data. For example, it takes 1 s to transmit a 10,000 bit block of data onto a 10 kbps line. Node delay: The time it takes for a node to perform the necessary processing as it switches data. ٤٠

21 Circuit switching and packet switching comparison Circuit switching Transparent service, once connection is established fixed data rate in provided No overhead required for routing packets Not achieving 100% line utilization Blocking till free up resources no traffic priority expansive Fixed data rate Analog and digital Wait till establishment ٤١ Packet switching Variable delay is introduced to packets, which need some sequencing and error recovery Add header to each packet contains routing information Almost achieves 100% line utilization No Blocking till solving congestion Traffic Priority Cheap Equalizing data rates Digital only Immediate start(datagram) or wait for establishment (Virtual circuit) Event timing ٤٢

22 For circuit switching, there is a delay before the message is sent. First, a Call Request signal is sent. If the destination station is not busy, a Call Accepted signal returns. Note a processing delay is incurred at each node during the call request. On return, this processing is not needed because the connection is already set up. After the connection s set up, the message is sent as a single block, with no noticeable delay at the switching nodes. ٤٣ Virtual circuit packet switching appears similar to circuit switching. A virtual circuit is requested using a Call Request packet, which incurs a delay at each node, and is accepted with a Call Accept packet. In contrast to the circuit switching case, the call acceptance also experiences node delays, since each packet is queued at each node and must wait its turn for transmission. ٤٤

23 Once the virtual circuit is established, the message is transmitted in packets. Packet switching involves some delay at each node in the path. Worse, this delay is variable and will increase with increased load. ٤٥ Datagram packet switching does not require a call setup. Thus, for short messages, it will be faster than virtual circuit and perhaps circuit switching. However, because each individual datagram is routed independently, the processing at each node may be longer than for virtual circuit packets. For long messages virtual circuits may be superior. ٤٦

24 Telephone network Telephone networks use circuit switching. The telephone network had its beginnings in the late 1800s. The entire network, which is referred to as the plain old telephone system (POTS), was originally an analog system using analog signals to transmit voice. Local loop Trunk Trunk End offices Tandem offices Regional offices ٤٧ A telephone system LATA (Local access and transport area) : were grouped into regions within which one particular telecomm service provider (like telecomm Egypt) that allowed by government or regime authority to provide services. The LATAs in each of these regions are numbered beginning with the same digit. ٤٨

25 A class 4, or tandem (toll) office is a national telephone company central office telephone exchange used to interconnect local exchange carrier (LXC) offices for long distance communications in the public switched telephone network (PSTN). ٤٩ ٥٠

26 The tasks of data transfer and signaling are separated in modern telephone networks: data transfer is done by one network, signaling by another. ٥١ Dial up modem Traditional telephone lines can carry frequencies between 300 and 3300 Hz, giving them a bandwidth of 3000 Hz. All this range is used for transmitting voice, where a great deal of interference and distortion can be accepted without loss of intelligibility. ٥٢

27 Modem stands for modulator/demodulator. Modulation/demodulation ٥٣ ٥٤ Uploading and ElShafee, downloading ACU in Networks 56K I modems

28 Digital Subscriber line After traditional modems reached their peak data rate, telephone companies developed another technology, DSL, to provide higher speed access to the Internet. Digital subscriber line (DSL) technology is one of the most promising for supporting high speed digital communication over the existing local loops. ADSL is an asymmetric communication technology designed for residential users; it is not suitable for businesses. The existing local loops can handle bandwidths up to 1.1 MHz. ADSL is an adaptive technology. The system uses a data rate based on the condition of the local loop line. ٥٥ ٥٦ Bandwidth division in Networks ADSLI

29 Discrete multi tone simulation ٥٧ ٥٨

30 ٥٩ Discrete multitone technique ٦٠ ElShafee, ADSL ACU : modem Fall 2015,

31 ٦١ ElShafee, DSLAM ACU ٦٢ Summary ElShafee, of ACU DSL : Fall technologies 2015,

32 ATM Adaptation Layer 5 (AAL5) Optical Carrier 3 (OCC3) ٦٣ Thanks,.. See you next week (ISA), ٦٤