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Chap31-Lan

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  • The original local area networks in the late seventies were intended to interconnect computers in a single building. As electronic equipment and especially memory was still fairly expensive, a passive system with no memory was the obvious choice. A full mesh interconnection was unpractical because the number of interfaces in all computers has to be changed whenever new computers are added in the network. A broadcast system with a passive communications medium, such as a bus cable is the simplest solution, even if it requires a medium access control protocol to ensure that at any moment only one of the computers is allowed to put data on the shared medium.
  • In the seven layers communications model, the medium access control is logically located between the physical and the data link layer. As medium access control is intimately related to the physical communications, it would be quite logical to consider it as an upper sublayer of the physical layer. However, as, at the time such decision had to be made, a large part of the medium access control protocols were implemented by software, they ended up in an optional sublayer at the bottom side of layer 2.
  • Ethernet is the commercial name of the local area network that has remained dominant in the market for the last 25 years, even if its technology has evolved.
  • To understand the medium access protocol called Aloha, one of its first large scale applications, developed, as was the protocol itself, at the University of Hawaii is a good starting point. The computer center of the University of Hawaii had many terminals located in various islands in the pacific ocean. These terminals were connected through a geostationary satellite which behaved as a shared passive communications medium. At that time, when several terminals were connected to a single computer through a passive shared medium, access control was done through “polling”, which means that the computer asks successively each terminal if it has something to transmit and eventually allows the terminal to use the shared medium to transmit. In normal systems, each terminal was polled approximately once a second to give users the illusion that they were permanently interacting with the computer. With a geostationary satellite and tens of terminals the round trip delay would cause unacceptable polling intervals and an other medium access control is needed. With Aloha, each station is allowed to transmit whenever it wants to do so. Of course it is possible that two stations transmit simultaneously and destroy each others messages. This can be detected by each station by verifying whether the message returned by the satellite is the one it just transmitted. If not, and to avoid that the two messages that collided should again collide, each station waits a random delay before retransmitting the damaged message.
  • A simple mathematical model can be used to estimate the cost in transmission capacity of the retransmissions eventually caused by the Aloha medium access control protocol. One supposes that fixed length data blocks are transmitted and that the undisturbed transmission of one block takes t seconds. G is the number of blocks that should be transmitted in t seconds, if the transmission capacity were unlimited (proposed traffic) S is the number of blocks transmitted and received correctly in t seconds (throughput) p is the probability that a transmitted block avoids collisions and is received correctly.
  • If one supposes that transmissions in all stations occur at uniformly distributed random and uncorrelated moments (even for retransmissions this hypothesis is quite acceptable due to the random delays inserted), the probability of a message being transmitted can be modeled by means of a Poisson distribution. If one supposes that station A starts transmitting a block at t 0 , Any station starting a transmission in the time interval going from t 0 – t to t 0 + t will cause a collision. Knowing that in a time interval of duration t, there are G blocks to be transmitted, theory of Poisson distributions states that the probability of no transmission starting in an interval with a duration of 2t is given by p = e -2G This means that one can relate the throughput S to the total proposed traffic G
  • The relationship between S and G shows that the throughput has a maximum for G = 0.5. For higher values of G, the number of collisions causes a net reduction of the throughput. The maximum possible throughput is 0.184, showing that more than 80% of the total transmission capacity is the price to pay for this very simple medium access control protocol. By having a master station that broadcasts every t seconds a synchronization signal and obliging all stations to start their transmission at the moment they receive the synchronization signal, the time interval used to compute the probability of a collision is halved and the expression of the probability of avoiding a collision becomes p = e -G Which means that the maximum throughput corresponds to G = 1 and has a value of 36 %. This improved version of Aloha is called slotted Aloha and is used in the GSM network on the signaling channel.
  • On a cable with a length of a few hundred meters the Aloha protocol can be further improved considerably, because the long propagation delays no longer exist. Such an improved version called CSMA/CD is used in Ethernet. Its operation is as follows: Before starting to transmit, a station listens on the cable and does not start transmission unless nobody else is transmitting. Several stations could be waiting to transmit and notice almost simultaneously the availability of the cable and start to transmit, causing a collision. By listening while transmitting, a station can quickly detect if a collision has occurred. Once this has happened there is no reason to continue transmitting as the entire frame needs retransmission. The Ethernet standard specifies that a collision should be detected in less than the time needed to transmit the shortest possible frame. This allows receivers to discard any damaged messages resulting from collisions just by checking their length and also ensure that a transmitter can always discover a collision before the end of the transmission of a frame. To avoid overloading with retransmissions an already overloaded network the average value of the random delay preceding a retransmission is incremented whenever a frame needs to be retransmitted more than one time.
  • This slide and the following one show how collisions do occur and when the transmitting stations can become aware of them.
  • In order to always detect a collision before the end of the transmission of a frame, the duration of the transmission of the shortest allowed frame should exceed twice the propagation time from one end of the cable to the other. The minimal frame length being specified (64 bytes) one can compute the maximal length of an Ethernet, at a given bit rate on the cable. The duration of transmission on a 10 MB/s Ethernet of a 64 bytes frame is 51.2 μ S. The time necessary to detect a collision is in the order of 20 μ S, leaving 30 μ S for the round trip propagation delay. As the propagation speed on a coaxial cable is approximately 2 10 5 Km/S, the maximal length of the cable is 3 Km. On a 100 Mb/s Ethernet, this maximum distance will be in the order of 300m and on a 1 Gb/s Ethernet it would be as short as 30m. The above limitations result from the CSMA/CD protocol and have nothing to do with the electrical attenuation of the signal along the cable. The latter can be avoid by amplifying the signal by means of repeaters, but this will not extend the maximum length imposed by the protocol, on the contrary, as repeaters introduce themselves an additional delay, the acceptable length will be reduced by the insertion of repeaters.
  • CSMA/CD considerably improves the throughput with respect to Aloha. Nevertheless the useful throughput remains lower than the nominal channel capacity. The above graph shows the ratio between throughput and channel capacity as a function of the number of stations and the frame length.
  • This scheme gives the structure of an Ethernet frame. The first eight bytes allow the receiver to synchronize its clock with the transmitter. The next two fields give the 48 bits Ethernet addresses of source and destination. These 48 bit addresses are hardwired into Ethernet interfaces and are guaranteed to be unique. The payload (the application information carried by the frame) has a variable length, The actual length (in bytes) is given in two bytes, located in front of the actual payload. In order to satisfy the minimal length requirement of 64 bytes, the payload field can not be shorter than 38 bytes. If the actual payload is shorter than 38 bytes, meaningless bytes are added to reach the minimal frame length. The last four bytes of a frame are used to store a frame check sequence which allows detection of transmission errors in the frame.
  • The first Ethernets used a thick (usually yellow) 50 Ω coaxial cable. This cable was quite rigid and therefore the interfaces were split into two units, the transceiver directly mounted on the cable and the actual interface inserted in a computer IO slot. The two were interconnected by means of a specific cable with a length of a few meters. Thick Ethernets have practically disappeared except in some older buildings.
  • Even with the high quality thick cables, the maximum length of an Ethernet cable can not exceed 500m due to the electrical attenuation of the signal along the cable. This can be solved by inserting, amplifiers between cables. Such amplifiers are called repeaters. To interconnect Ethernets in different buildings half repeaters interconnected by means of optical fibers can be used. They can be useful when the Ethernets to be interconnected are separated by distances exceeding the acceptable cable length. Moreover they provide electrical insulation between buildings which is a good prevention against lightning damages. When using repeaters, and especially interconnected half repeaters, one should however never forget the maximum distance between stations imposed by the collision detection protocol.
  • In most office applications the 500m segments allowed by thick Ethernet were not needed. If the total distance did not exceed 185m thin, fairly flexible, 50 Ω coaxial cable could be used. It was much cheaper and could directly be connected to the computer interfaces by means of small and cost effective BNC connectors.
  • As thin Ethernet cables were quite fragile, especially when they were lying on the floor, repeaters were used to separate from each other relatively short sections of cable, ensuring that in case of an electrical defect in one section the other sections should not be affected. Ethernet started to evolve from a physical bus line interconnecting all computers towards a star of interconnected bus lines with a small number of computers on each.
  • Progress in transceiver technology and in the quality of twisted pairs similar to those used for telephony allowed to carry digital signals at several Mb/s over point to point links with a length of several tens of meters. On such links, one separate pair is used for each direction of data flow. Sometimes, in buildings extensively wired with twisted pairs of insufficient quality to carry data at the desired rate, two pairs per direction are used, each carrying half of the bits, through special interfaces that are expensive , but often cheaper than the replacement of the wiring.
  • Point to point links over twisted pairs do not fit very well in the original Ethernet bus topology, but can be used, provided that a repeater is used for every single connected station. Sets of 8, 16 or more repeaters with appropriate connectors for twisted pairs are widely available and constitute one of the major building blocks of modern Ethernets. Such sets of repeaters are called hubs.
  • In its early days Ethernet had many competitors. Some, like the Cambridge Ring and the Liu Ring were academic designs which were proven on the campus where they were conceived but which never evolved into widespread commercial products. The Token Ring was designed by IBM because that company did not want to depend on licenses from a competitor for strategic products like local area networks. For some years it was the only local area network supported by IBM, but under customers pressure IBM also adopted Ethernet and recently gave up the Token Ring. The token bus was a local area network designed to interconnect computer aided manufacturing equipments. It was more reliable and better adapted to industrial environments than Ethernet, but also much more costly. Improvements in Ethernet technology have made obsolete the token bus.
  • In the early eighties, the IEEE undertook standardization of local area networks under the project number 802. It was not possible nor opportune to select one type of LAN and to reject all others. So the IEEE 802 committee choose to define a common framework, with a single service specification, in which different physical media and the associated MAC protocols could be plugged. The standard has two common parts, called IEEE 802.1 and IEE802.2 and an unlimited number of plug-ins ranging from IEEE 802,3 to, in 2005, IEEE 802.16. IEEE 802.1 specifies what services a LAN should provide, how different conformant LANs can be interconnected and how they can be managed. IEEE802.2 specifies the common link control protocol to be used above specific LANs such as Ethernet. IEEE 802.3 specifies various CSMA/CD networks commercially known as Ethernet, There are however some small differences between the specifications of IEEE 802.3 and those of the original Ethernet specified by Xerox. Many of the original IEEE 802 standards are by now obsolete, while others such as IEEE 802.11, specifying wireless local area networks have become very important.
  • The first local area networks to be standardized by the IEEE were 802.3 Random access bus (~ Ethernet) 802.4 Token controlled bus 802.5 Token controlled ring (~ IBM Token Ring) 802.6 Dual Bus Distributed Control 802.7 Broadband coaxial networks 802.11 Wireless local networks
  • At the MAC layer, LANs offer a connectionless service, defined between any pair of MAC addresses. By adding the LLC layer above the MAC layer, the service presented to the network layer can be selected among three different classes of services, going from purely connectionless to fully connection oriented with error correction. In addition, as the LLC layer also features 16 bit sender and receiver addresses, it is possible to define for a single MAC address up to 2 16 subadresses allowing to dedicate specific subaddresses for specific data links and to specify for each the LLC protocol to be applied.
  • LLC frames are variable length. They are delimited by flags, which are unique 8 bit patterns (01111110) that can not appear in the body of the frame. To make sure that flags do not appear by accident in the body of a frame, a bit 0 is inserted after every sequence of five consecutive ones just before the frame is passed to the MAC layer. At the receiver side, the zero following a sequence of 5 zeros is automatically removed when the frame is passed from the MAC layer to the LLC layer.
  • According to the IEEE 802.2 standard three different kind of frames can be used for the LLC protocol. The unnumbered frames are the only ones used in the classes 1 and 3 as these classes are connectionless and numbering of frames would require state variables. Unnumbered frames are used in class 2 when a connection is to be established. The information frames are used to carry the payload in class 2. They are only used when a connection exists and are numbered. The supervisory frames are also numbered and are used to manage the sliding window protocol and to close a connection
  • Networks can be interconnected at different layers with important consequences for the properties of the interconnection. The different layers where an interconnection can be made will be reviewed here to provide a global image of the issues involved. Thereafter interconnection at the MAC layer wil be discussed in somewhat more detail because this kind of interconnections plays a major role in the evolution of local area networks.
  • Interconnection at the physical layer by means of repeaters has already been introduced.
  • The role of repeaters is limited to the physical layer, they have no significant influence on the higher layers with the exception that they can introduce additional delays.
  • The minimal influence repeaters have on the higher layers is illustrated by the fact that the maximum distance imposed by the MAC protocol (3 Km at 10 Mb/s) remains, whatever the cable length that can be made by means of repeaters.
  • Networks can also be interconnected through specific applications.
  • For interconnections at the application layer, networks don’t have to have anything in common, they are completely independent, the only requirement is that they run similar applications and that transferring data from one application to the other is meaningful.
  • A caricature of an application level gateway which shows quite well what it is and how restrictive it can be made would be a secretary with two terminals, one connected to each network, who transfers emails from one network to the other by typing on one terminal what she reads on the other. Firewalls
  • Application gateways constitute the core of some of the most secure firewalls. For instance, on the Intranet side, the firewall is a web server, on the Internet side it is a web browser. The requests made on the intranet side are relayed on the Internet side by the browser and the answers, if they do not contain any dangerous feature are made available by the web server on the intranet. Such a firewall gives access to the web from the intranet, without any direct link between the intranet and the Internet and can easily filter the exchanges with the web both on addresses and on content. However, clever applications such as Skype and Casa succeed in encapsulating most of their data in innocent looking web traffic.
  • The most logical level to interconnect networks is at the network layer, and more specifically, at the internet layer. Towards the higher (application) layers the interconnected networks offer the services of a single global network.
  • The lower layers of networks interconnected at the internet level don’t need to be identical as they are completely independent. Similar service offerings are however desirable, as the global network will only be capable of guaranteeing a level of service corresponding to the least performing of the underlying networks.
  • For LANs, an interesting alternative to interconnection at the internet level can be an interconnection at the MAC level. It allows to include in a single global LAN different LANs using different MAC layers. This is, for instance, useful when several local wireless “WiFi hot spots” are interconnected by means of an Ethernet. As the interconnected LANs have their own MAC layers, distance restrictions resulting from the MAC layer are to be applied to each of the LANs separately, allowing for composite LANs with significantly longer distances than what is possible with a single LAN. As all devices on LANs have distinct MAC layer addresses, it is also possible to assign a filtering task to the bridges to restrict traffic to those LANs where it is useful. Towards the data link layer and the higher layers, interconnected LANs appear as just one LAN.
  • When in an Ethernet environment different network protocols, such as IP and IPX (Novell PC networks) are used, interconnecting LANs by means of bridges allows to use the different protocols over the global LAN while interconnections by means of routers would require special routers capable of handling both protocols or separate LANs.
  • Among many other topics, IEEE802.1 specifies how LAN’s can be interconnected by means of filtering and learning bridges.
  • A bridge contains for each direction a receiver, a fifo buffer and a transmitter. If the bridge has no filtering function, each frame received is retransmitted on the other side. The fifo buffer is needed when a frame needs to be retransmitted after a collision and the next frame has already been received. To add a filtering function to the bridge a database containing MAC addresses located at the left and at the right side of the bridge is added. Whenever a received frame does not need to be transmitted to the other side because its destination address is, according to the database, on the side it came from, the frame is discarded by the filter device between the receiver and the fifo buffer. Provided that a large fraction of the frames have their source and destination on the same LAN, filtering bridges allow large global networks with a reasonable traffic on the different interconnected LAN’s. Addresses can be introduced manually by a systems manager in the database, but this is labor intensive and hardly feasible when people have laptops that are continuously carried from one wireless network to another
  • IEEE802.1 specifies that bridges should be capable of learning which addresses are on what side. The bridge transmits all frames that have a destination address that is not yet known in the database. Initially the database is empty and all frames are transmitted. By observing the source addresses of the transmitted frames, the bridge can learn which addresses are located on its left and right sides. Provided there are no loops in the set of bridged networks, this learning algorithm can quickly discover the most active addresses on both sides of the bridge. Inactive addresses will not be discovered but they do not contribute significantly to total the traffic. To avoid corruption of the database by laptops with frequently changing locations, all entries in the database are time-stamped and discarded after a few minutes.
  • In large global networks build with many LAN’s interconnected by a large number of bridges loops can exist and could even be introduced voluntarily for ensuring alternative paths in case of defects. Such loops would make the learning algorithm ineffective as a frame could reach a specific bridge from both sides. To avoid this problem IEEE802.1 specifies the so called “spanning tree algorithm” to discover bridges that close loops and to temporarily disable them. Periodically the bridges communicate with each other to find the bridge in the network that has the lowest serial number. This bridge sends periodically broadcast messages. Whenever a bridge receives such broadcast message on both of its receivers, it knows that it is closing a loop and stops transferring frames from one side to the other, but continues to listen to messages from the root. If these messages stop appearing on both sides, the loop has been opened at an other location and the bridge can resume its normal operation.
  • Nothing in the concept of a bridge requires that both sides are located next to each other. By building half bridges connected through a data link than can carry the frames from one half to the other, one can obtain LAN’s with parts that can be separated by thousands of kilometers.
  • The links which interconnect half bridges don’t need to be point to point links. Any network capable of carrying MAC frames from one point to an other can be used to interconnect LAN’s, creating virtual global LAN’s. The network that interconnects the different parts of such a virtual LAN is commonly called “backbone”. The backbone doesn’t need to use the same protocols as the interconnected LAN’s because its only role is to carry frames from one half bridge to an other half bridge.
  • The backbone must be able to carry frames from one specific half bridge to an other specific half bridge, which means that the half bridges must have addresses on the backbone, that the frames need to be encapsulated in frames specific to the backbone network and that the backbone network needs to have the mechanisms needed to route the payload frames from source to destination. These backbone frames can be MAC frames, LLC frames or network frames.
  • Providing shared backbones for companies wanting to have interconnected LAN’s in their different offices is one of the services proposed by all major telecommunications operators. Of course, the encapsulation and routing mechanism on the backbone must ensure that frames belonging to a specific customer can only reach LAN’s belonging to that customer.
  • The first widespread Ethernets operated at 10 Mb/s. Much higher throughputs were quickly desirable.
  • To increase the throughput of an Ethernet, two approaches are possible: - The bit rate can be increased, but the collision detection protocol automatically imposes a total length that is roughly inversely proportional to the bit rate (10 Mb/s : 3 Km, 100 Mb/s : 400 m, 1 Gb/s : 50 m!) By reducing the number of stations on a single Ethernet, the probability of collisions can be reduced, which has a positive effect on the throughput. Reducing the number of stations on a single Ethernet implies the use of half bridges between different Ethernets and a, possibly very short, backbone. An extreme evolution of this approach consists in putting only one station that can send on each Ethernet (this implies two separate Ethernets for connecting a station with the backbone). This extreme evolution has a dramatic consequence: as with a single transmitter, collisions can no longer occur, it is no longer necessary to respect the maximum distance imposed by the collision detection algorithm.
  • An Ethernet build around a collapsed backbone with as many half bridges attached to it as there are station connected to the Ethernet is called a “switched Ethernet”. The collection of interconnected half bridges is called an “Ethernet switch” Stations are most often connected with the switch through unshielded twisted pairs, or, at data rates in the Gb/s range, optical fibers. The cable length between the switch and the connected stations is only restricted by the attenuation of the signal, the propagation delay does no longer play a significant role. One could object that a switched Ethernet has very little in common with the original CSMA/CD Ethernet. This is true, but as in mass produced Ethernet interfaces the hardware implementation of the full Ethernet MAC protocol does no longer represent a significant component of the total interface cost, so there is really nothing against just using only a very small subset of the original
  • Not only the traditional telephone infrastructure can be used for domestic access to the Internet, but the cable TV infrastructure also offers an attractive alternative. Such networks are often called “Metropolitan Area Networks”.
  • The original cable TV networks consisted in a set of coaxial cables leaving a central office (“Head-end”) where the different TV programs where multiplexed with frequency domain multiplexing on the coaxial distribution cables. Approximately every Km the attenuation on the distribution cables is compensated by amplifiers. A fairly high signal level is used on the cable in order to allow a strong attenuation between the distribution cable and individual subscribers. This attenuation avoids that a misbehaving user could send disturbing signals on the distribution cable.
  • The frequency domain multiplexing uses a bandwidth of 7 or 8 MHz per TV channel. The frequency bands and the multiplexing scheme used on the cable are approximately the same as those used for wireless TV broadcasting.
  • Using such a cable TV network for bidirectional Internet access or for digital telephony is technically quite challenging. First of all a frequency band has to be selected for the return traffic. The frequency band bellow the frequencies normally used for TV is assigned for that purpose. Then all unidirectional amplifiers have to be replaced by double amplifiers, in one direction for the lowest frequencies and one in the other direction for the higher frequencies. Finally, one or several TV channels need to be reserved for data traffic. When all this has been done, it becomes possible to use the cable infrastructure for bidirectional data transmission by means of so called “cable modems”. A last fundamental problem that needs to be solved is the medium access control on the data channels, as these are shared among all the users. Access to the downstream channel is no problem as no direct communication between stations is allowed and only the head-end can put data on the downstream channels. Access to the upstream channel is more complex as any station must be able to transmit data and as CSMA/CD is not applicable, because of the amplifiers that hide collisions occurring in upstream segments. More complex MAC protocols, involving medium reservation, such as CSMA/CA (see wireless LAN’s) are needed.
  • As strong attenuation between the distribution cable and the users is not possible in bidirectional MAN’s, the shared cable based networks are poorly protected against (accidental) misbehavior of one station that transmits uncontrolled data on the cable. Moreover, as the data channels are shared among all the users on the same cable, the available bandwidth to each user depends on the activity of the other users. For these reasons, when new TV distribution networks have to be build, more and more operators do consider replacing the shared cable by a switched Ethernet operating at 10 Gb/s over optical fibers.
  • The fastest growing subdivision in LAN’s is wireless LAN’s. They have two main objectives: Providing true mobility for portable devices laptop computers connected to the internet through WiFi networks mobile phones connected with hands-free equipment in cars remote controls for TV sets, video recorders, … etc. Avoiding the expensive, messy and unpractical wiring between electronic devices in the office and the home. Although proprietary protocols are common, mainly in the consumer electronics, many of the more professional wireless communications have been standardized and the most common of these standards is part of the IEEE 802 set of standards.
  • In the 802 standards, all LAN’s share a common LLC layer (IEEE802.2) and common standards for management, interconnection and services definition. The physical layer and the corresponding MAC layer are standardized by means of specific modules. One of these modules (802.11) handles wireless LAN’s.
  • As wireless LAN’s cover a large range of needs and technologies, it was not possible to make a single standard for all LAN’s. The same trick as the one used to fit the different MAC protocols in the 802 standards was used to fit the different wireless communications technologies into one of the 802 MAC standards. They all share the same medium access protocol, but specific physical layers, using different wireless technologies are specified. The more important of those are IEEE802.11 a :
  • The CSMA/CD protocol is inadequate as a MAC layer for wireless LAN’s, because of the hidden station problem. In a wireless LAN, because of particular propagation conditions it is perfectly possible that a station B can communicate with both stations A and C, while A and C can not communicate directly. If A and C would transmit simultaneously a frame to B, these frames would collide but A nor C would notice it because they can not receive the signal from the other station. This excludes protocols based on collision detection and even makes carrier sense more or less useless. Instead, reservation scheme, called “collision avoidance” is used. Whenever a station (A for instance) wants to send a message to an other station (B), it sends a “request to send” short message containing the length of the frame to be sent. If B receives correctly that message, it broadcasts a “Clear To Send” message, allowing A to use the radio channel for the requested duration. Ass all stations that can interfere with the transmission from A to B will hear the CTS message, they know that they are not allowed to transmit in the specified interval. If, by accident, A and C would simultaneously transmit a RTS message to B, these messages would collide, B would not be able to decode them and would send no CTS message. Both A and C, not receiving the expected CTS will wait a random time interval and try again. Ä similar protocol is used for accessing the return channel on cable TV.

Chap31-Lan Chap31-Lan Presentation Transcript

  • Telecommunications Concepts
    • Chapter 3.1
    • Packet Switched
    • Local Area Networks
  • Contents
    • Original Local Area Networks
      • Ethernet
      • Token Ring, Token bus, Cambridge ring, Liu ring, …
    • The IEEE 802 standards
      • IEEE 802.3, IEEE 802.4, IEEE 802.5, ...
      • IEEE 802.2 : Logical Link Control
      • IEEE 802.1 : Learning bridges
    • High Performance LAN’s and MAN's
      • High performance Ethernet
      • Metropolitan Networks
      • Wireless LAN's
    • Side Tracks
      • Interconnection of data communication systems.
  • Contents
    • Original Local Area Networks
      • Ethernet
      • Token Ring, Token bus, Cambridge ring, Liu ring, …
    • The IEEE 802 standards
      • IEEE 802.3, IEEE 802.4, IEEE 802.5, ...
      • IEEE 802.2 : Logical Link Control
      • IEEE 802.1 : Learning bridges
    • High Performance LAN’s and MAN's
      • High performance Ethernet
      • Metropolitan Networks
      • Wireless LAN's
    • Side Tracks
      • Interconnection of data communication systems.
  • Shared Media Networks Need for Medium Access Control Protocols Commonly used in Local Area Networks The network itself has practically no storage capacity
  • The 7 OSI layers Applications Layer Networks Layer Internet Sublayer 1 : Physical 2 : Data Link 3 : Network 4 : Transport 5 : Session 6 : Presentation 7 : Application Transport Layer Connectivity Interoperability
  • The 3 lower OSI layers For Shared Medium Networks Network Internet 3 Physical Medium Access Control 2 1 Data Link Control
  • Contents
    • Original Local Area Networks
      • Ethernet
      • Token Ring, Token bus, Cambridge ring, Liu ring, …
    • The IEEE 802 standards
      • IEEE 802.3, IEEE 802.4, IEEE 802.5, ...
      • IEEE 802.2 : Logical Link Control
      • IEEE 802.1 : Learning bridges
    • High Performance LAN’s and MAN's
      • High performance Ethernet
      • Metropolitan Networks
      • Wireless LAN's
    • Side Track
      • Interconnection of data communication systems.
  • Ethernet
    • Developed in the seventies at Xerox Parc
      • Invented by Lamport
      • Commercially supported by Digital and Intel
      • Still has a dominant market position
    • Originally based on coaxial bus
      • Passive broadcast medium
      • Length limited by electrical properties of cable
        • Thick coaxial : 500 m
        • Thin coaxial : 185 m
      • Repeaters ( = electronic amplifiers ) allow to
        • extend cable length
        • replace coaxial bus by twisted pairs star
    • Medium access control inspired by Aloha
  • Aloha (N.Abramson, F.Kuo, 1970) Communication between terminals and mainframe via geostationary satellite h = 36 000 Km Propagation delay = 240 mS REPEAT Transmit data block B; Receive same block Br; ok := B = Br; IF NOT ok THEN Wait Random Delay END UNTIL ok
  • ALOHA Performance
    • t : Transmission time for one data block.
    • G : Total proposed traffic ( # of blocks in time t )
    • S : Throughput ( # of successful blocks in time t )
    • 0 <= S <= 1
    • p : Probability that a block will avoid collision
    • S = G . p
  • ALOHA Collision Window Station A Station B 2t Blocks transmitted according to Poisson Distribution p = e - 2G S = G . e - 2G
  • ALOHA Throughput 0 0.1 0.2 0 0.5 1 1.5 0.184 G ) S ( G
  • CSMA/CD operation
    • Carrier Sense
      • Listen before sending
    • Multiple Access
      • Everybody noticing silence can start sending
    • Collision Detect
      • Stop sending when noticing that others sending
      • Collision fragments characterized by insufficient length and therefore discarded by MAC entity.
    Graceful degradation due to average repetition delay proportional to number of retransmissions
  • Carrier Sense A A A B B B Listen before talking (carrier sense) Send data Listen and wait No carrier sensed, transmission starts
  • Collision Enforcement A A B B Collision Collided signals propagate B detects collision and transmits jam A B Collided signals reach A Station A should detect the collision BEFORE it has send its last databit
  • Minimum packet size A B A B No carrier sensed, transmission starts Min. packet time > max. round trip delay Collided signals reach A Station A should detect the collision BEFORE it has send its last databit
  • Efficiency of channel
  • Ethernet Data Frame Preamble (7 bytes) Start Frame (1 byte) Destination Address (6 bytes) Source Address (6 bytes) Length payload (2 bytes) Payload (evt. + Padding) Frame Check Sequence (4 bytes) Min : 64 bytes Max : 1518 bytes
  • Thick Ethernet Thick cable Total Length <= 500 m
  • Ethernets with repeaters Repeaters Distance <= 3000 m Half Repeaters Optical Link
  • Thin Ethernets Thin cable Total Length <= 185 m
  • Thin Ethernets Repeaters Thin cable Distance <= 3000 m Segment Length <= 185 m
  • Repeaters for thin Ethernet
  • Ethernet over twisted pairs One or two pairs per direction
  • Twisted pair Ethernet (10 Mb/s) Repeaters (= hubs) Twisted pairs Distance <= 3000 m Segment Length <= 100 m Class 3 utp
  • Cascade of Ethernet hubs A HUB = Set of repeaters > all frames broadcasted B B B B B B B B
  • High-performance Ethernet (100 Mb/s) Repeaters Twisted pairs Distance <= 400m Segment Length <= 100 m 4 twisted pairs (class 3) 2 twisted pairs (class 5)
  • Ethernet in office buildings
  • Contents
    • Original Local Area Networks
      • Ethernet
      • Token Ring, Token bus, Cambridge ring, Liu ring, …
    • The IEEE 802 standards
      • IEEE 802.3, IEEE 802.4, IEEE 802.5, ...
      • IEEE 802.2 : Logical Link Control
      • IEEE 802.1 : Learning bridges
    • High Performance LAN’s and MAN's
      • High performance Ethernet
      • Metropolitan Networks
      • Wireless LAN's
    • Side Track
      • Interconnection of data communication systems.
  • Contents
    • Original Local Area Networks
      • Ethernet
      • Token Ring
    • The IEEE 802 standards
      • IEEE 802.3, IEEE 802.4, IEEE 802.5, ...
      • IEEE 802.2 : Logical Link Control
      • IEEE 802.1 : Learning bridges
    • High Performance LAN’s and MAN's
      • High performance Ethernet
      • Metropolitan Networks
      • Wireless LAN's
    • Side Track
      • Interconnection of data communication systems.
  • IEEE 802 Standards in the OSI model IEEE 802.2 Logical Link Control IEEE 802.1 Management, interconnection , Upper layers interface 2 1 MAC LLC Physical 3 802.7 Broadband 802.3 ~ Ethernet 802.4 ~ Token Bus 802.5 ~ Token Ring 802.6 Metropolitan 802.? ???
  • Contents
    • Original Local Area Networks
      • Ethernet
      • Token Ring
    • The IEEE 802 standards
      • IEEE 802.3, IEEE 802.4, IEEE 802.5, ...
      • IEEE 802.2 : Logical Link Control
      • IEEE 802.1 : Learning bridges
    • High Performance LAN’s and MAN's
      • High performance Ethernet
      • Metropolitan Networks
      • Wireless LAN's
    • Side Track
      • Interconnection of data communication systems.
  • Contents
    • Original Local Area Networks
      • Ethernet
      • Token Ring
    • The IEEE 802 standards
      • IEEE 802.3, IEEE 802.4, IEEE 802.5, ...
      • IEEE 802.2 : Logical Link Control
      • IEEE 802.1 : Learning bridges
    • High Performance LAN’s and MAN's
      • High performance Ethernet
      • Metropolitan Networks
      • Wireless LAN's
    • Side Track
      • Interconnection of data communication systems.
  • IEEE 802.2 Logical Link Control
    • Purpose :
      • To fit LAN’s into the general OSI framework
      • Correct transmission errors
    • Services offered :
      • Data link multiplexing by means of LLC addresses
      • Data link error correction and flow control
        • Class 1 : Connectionless, unacknowledged service
        • Class 2 :
          • Connection oriented reliable link control
          • Sliding window protocol
          • Window size = 128 frames.
        • Class 3 : Connectionless, acknowledged.
  • LLC Frame Format
    • Flag : Unique bit pattern (01111110)
    • Address (2*16 bit) : allows to multiplex a single MAC address for different data links.
    • Control (16 bit) : Controls the sliding windows
    • Payload (variable length) : Data from layer 3
    • CRC (16 bit) : Redundant bits obtained by dividing the address, control and payload fields by a predefined polynomial.
    Flag CRC Address Control Flag Payload
  • LLC bit stuffing
    • The bit pattern 01111110 should never occur in the address, control, payload and CRC fields.
    • After 5 consecutive 1’s a 0 is inserted by the TX
    • A 0 preceded by 5 1’s is removed by the RX.
    0100110111110101101111111110111111000110000 010011011111 0 01011011111 0 1111011111 0 1000110000 Message to be transmitted : Message effectively transmitted :
  • LLC Frame Types
    • Unnumbered
      • used to set up connections
      • used to transfer data in connectionless mode
    • Supervisory
      • used for managing the sliding window
      • in connection oriented mode
    • Information
      • used to transfer data in connection oriented mode
  • Contents
    • Original Local Area Networks
      • Ethernet
      • Token Ring
    • The IEEE 802 standards
      • IEEE 802.3, IEEE 802.4, IEEE 802.5, ...
      • IEEE 802.2 : Logical Link Control
      • IEEE 802.1 : Learning bridges
    • High Performance LAN’s and MAN's
      • High performance Ethernet
      • Metropolitan Networks
      • Wireless LAN's
    • Side Track
      • Interconnection of data communication systems.
  • Interconnection of LAN’s
    • Layer 7 : Application level Gateways
    • Layer 3 : ROUTERS
      • Independent networks (lan or wan) interconnected by means of an internet protocol.
    • Layer 2 : BRIDGES
      • Independent MAC protocols on interconnected lan’s.
      • Distance restrictions apply to each lan individually
      • Half bridges can be interconnected by any lan or wan.
      • Traffic between lan’s can be filtered according to MAC level addresses.
    • Layer 1 : REPEATERS
      • No influence on MAC protocol
      • Same traffic in all interconnected lan’s
  • Repeaters Layer 1 Layer 2a Layer 2b Layer 3b Layer 3a Layer 1 Layer 1 Repeater Layer 2a Layer 2b Layer 3b Layer 3a Layer 1 Internet Data Link Medium Access Control Network
  • Practical Ethernets Repeaters Distance <= 3000 m Half Repeaters Optical Link
  • Interconnection of LAN’s
    • Layer 7 : Application level Gateways
    • Layer 3 : ROUTERS
      • Independent networks (lan or wan) interconnected by means of an internet protocol.
    • Layer 2 : BRIDGES
      • Independent MAC protocols on interconnected lan’s.
      • Distance restrictions apply to each lan individually
      • Half bridges can be interconnected by any lan or wan.
      • Traffic between lan’s can be filtered according to MAC level addresses.
    • Layer 1 : REPEATERS
      • No influence on MAC protocol
      • Same traffic in all interconnected lan’s
  • Application Level Gateways Application gateway Layer 2 Layer 3 Layer 1 Layer 5 Layer 4 Layer 7 Layer 6 Layer 2 Layer 3 Layer 1 Layer 5 Layer 4 Layer 7 Layer 6 Layer 2 Layer 3 Layer 1 Layer 5 Layer 4 Layer 7 Layer 6 Layer 2 Layer 3 Layer 1 Layer 5 Layer 4 Layer 7 Layer 6
  • Gateway example X400 - SMTP mail gateway X400 users SMTP users mail gateway
  • Firewalls
    • An application gateway between the Internet and an intranet is a fairly secure firewall.
    Intranet Internet Firewall
  • Interconnection of LAN’s
    • Layer 7 : Application level Gateways
    • Layer 3 : ROUTERS
      • Independent networks (lan or wan) interconnected by means of an internet protocol.
    • Layer 2 : BRIDGES
      • Independent MAC protocols on interconnected lan’s.
      • Distance restrictions apply to each lan individually
      • Half bridges can be interconnected by any lan or wan.
      • Traffic between lan’s can be filtered according to MAC level addresses.
    • Layer 1 : REPEATERS
      • No influence on MAC protocol
      • Same traffic in all interconnected lan’s
  • Routers Layer 1 Layer 2a Layer 2b Layer 3b Layer 3a Layer 1 Layer 1 Layer 2a Layer 2b Layer 3b Layer 3a Layer 1 Layer 2a Layer 2a Layer 3b Layer 2b Layer 3a Layer 3b Layer 2b Layer 3a Router
  • Interconnection of LAN’s
    • Layer 7 : Application level Gateways
    • Layer 3 : ROUTERS
      • Independent networks (lan or wan) interconnected by means of an internet protocol.
    • Layer 2 : BRIDGES
      • Independent MAC protocols on interconnected lan’s.
      • Distance restrictions apply to each lan individually
      • Half bridges can be interconnected by any lan or wan.
      • Traffic between lan’s can be filtered according to MAC level addresses.
    • Layer 1 : REPEATERS
      • No influence on MAC protocol
      • Same traffic in all interconnected lan’s
  • Bridges Layer 1 Layer 2a Layer 2b Layer 3b Layer 3a Layer 1 Layer 1 BRIDGE Layer 2a Layer 2b Layer 3b Layer 3a Layer 1 Internet Data Link Layer 2a Layer 2a Network
  • Bridges Bridge
  • Bridges Bridge WiFi
  • Bridges and multiple Network Protocols BRIDGE IP IPX
  • Contents
    • Original Local Area Networks
      • Ethernet
      • Token Ring
    • The IEEE 802 standards
      • IEEE 802.3, IEEE 802.4, IEEE 802.5, ...
      • IEEE 802.2 : Logical Link Control
      • IEEE 802.1 : Learning bridges
    • High Performance LAN’s and MAN's
      • High performance Ethernet
      • Metropolitan Networks
      • Wireless LAN's
    • Side Track
      • Interconnection of data communication systems.
  • Filtering Bridges Lan 1 Lan 2 RX RX Filter Filter Buffer Buffer TX TX Addresses Database 123 536 234 831 556 246 456
  • Learning Bridges
    • Frames with unknown destination are always forwarded (promiscuous mode)
    • Source addresses of all frames are monitored and added to the database, with a time stamp.
    • Frames with a destination address known to be on the same side as their source are not forwarded.
    • Addresses which have not been confirmed since some time are removed from the database.
    • The learning algorithm requires that there are no loops through the interconnected lan’s.
  • Spanning Tree Algorithm Root (Lowest serial Nr) Disabled
  • Half Bridges Ethernet frames are tunneled through the network interconnecting the half bridges Half Bridges Any Data Link
  • Backbones Half Bridges FDDI MAN ATM ISDN F.R. X25
  • Level 2a tunneling via level 2a intermediate network 2a 1 2a 1 2a 1 2a 1 Specific labels can be added to the frames to identify data flow 2a 2b 4 3 1 2a 2b 4 3 1
  • Level 2a tunneling via level 2b intermediate network 2a 1 2a 2b 1 2a 1 2a 2b 1 2a 2b 4 3 1 2a 2b 4 3 1
  • Level 2a tunneling via level 3 intermediate network 2a 1 2a 2b 3 1 2a 1 2a 2b 3 1 2a 2b 4 3 1 2a 2b 4 3 1
  • Virtual LAN’s
  • Contents
    • Original Local Area Networks
      • Ethernet
      • Token Ring
    • The IEEE 802 standards
      • IEEE 802.3, IEEE 802.4, IEEE 802.5, ...
      • IEEE 802.2 : Logical Link Control
      • IEEE 802.1 : Learning bridges
    • High Performance LAN’s and MAN's
      • High performance Ethernet
      • Metropolitan Networks
      • Wireless LAN's
    • Side Track
      • Interconnection of data communication systems.
  • High performance Ethernet less collisions Data rate 10 MB/s 100 MB/s 1 GB/s Single segment (broadcast) One station per segment = no collisions (switching) Throughput
  • Switched LAN’s
  • Contents
    • Original Local Area Networks
      • Ethernet
      • Token Ring
    • The IEEE 802 standards
      • IEEE 802.3, IEEE 802.4, IEEE 802.5, ...
      • IEEE 802.2 : Logical Link Control
      • IEEE 802.1 : Learning bridges
    • High Performance LAN’s and MAN's
      • High performance Ethernet
      • Metropolitan Networks
      • Wireless LAN's
    • Side Track
      • Interconnection of data communication systems.
  • Cable TV Networks Head-end Trunk cables Distribution cables
  • Cable TV Networks 7-8 MHz frequency VTM RTBF VRT RTL TF1
  • Data over Cable TV frequency VTM RTBF BRTN RTL Return 7-8 MHz Data Cable modem Cable modem
  • Gigabit Ethernet as MAN Head-end Distribution cables (optical fibers) Backbone (WDM optical fibers)
  • Contents
    • Original Local Area Networks
      • Ethernet
      • Token Ring
    • The IEEE 802 standards
      • IEEE 802.3, IEEE 802.4, IEEE 802.5, ...
      • IEEE 802.2 : Logical Link Control
      • IEEE 802.1 : Learning bridges
    • High Performance LAN’s and MAN's
      • High performance Ethernet
      • Metropolitan Networks
      • Wireless LAN's
    • Side Track
      • Interconnection of data communication systems.
  • IEEE 802 Standards in the OSI model IEEE 802.1 Management, interconnection, upper layers interface 2 1 MAC LLC Physical 3 802.3 ~ Ethernet 802.4 ~ Token Bus 802.5 ~ Token Ring 802.11 Wireless IEEE 802.2 Logical Link Control
  • IEEE 802.11 1 or 2 Mb/s (provisions for migration to higher speeds) One common MAC layer Provisions for a variety of physical layers Frequency hopping spread spectrum ??? Infrared IEEE 802.11 Medium Access Control Direct Sequence spread spectrum
  • 802.11 Hidden Station A B C RTS CTS CTS