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Chapter 3

Chapter 3. Underlying Technologies. Underlying Technologies. 3.1 DTRANSMISSION MEDIA Transmission media can be divided into two broad categories; guided and unguided . Guided Media. Figure 3-1 Guided media. Twisted-Pair Cable

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Chapter 3

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  1. Chapter 3 Underlying Technologies

  2. Underlying Technologies 3.1 DTRANSMISSION MEDIA Transmission media can be divided into two broad categories; guided and unguided. Guided Media Figure 3-1 Guided media

  3. Twisted-Pair Cable Twist pair-cable comes in two forms: unshielded and shielded. Unshielded Twisted-Pair(UTP) Cable The most common type of telecommunication medium in use today. Its frequency range is suitable for transmitting both data and voice. UTP is cheap, flexible, and easy to install. Underlying Technologies Figure 3-2 Unshielded twisted-pair cable

  4. Twisted-Pair Cable The EIA(Electronic Industries Association)has developed standards to grad UTP cable by quality.(1 as lowest and 5 as highest) Category 1. Category 2. Category 3. Category 4. Category 5. Underlying Technologies Figure 3-3 Cable with 5 unshielded twisted-pairs

  5. Shielded Twisted-Pair(STP) Cable STP installation is more difficult than UTP because the shield bust be connected to a ground. Materials and manufacturing requirements make STP more expensive than UTP but less susceptible no noise. Underlying Technologies Figure 3-4 Shielded Twisted-Pair(STP) cable

  6. Coaxial Cable High frequency ranges than twisted-pair cable. Underlying Technologies Figure 3-5 Coaxial cable

  7. Optical Fiber 1 Optical fiber is made of glass or plastic and transmits signals formof light. Refraction Refection Critical angle Underlying Technologies Figure 3-6 Refraction and reflection

  8. Optical Fiber 2 Optical fibers use reflection to guide light through a channel. The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead of being refracted into it. Information is encoded onto a beam of light that represents 1 and 0 bits. Underlying Technologies Figure 3-7 Fiber-optic cable

  9. Optical Fiber / major advantages The major advantages : noise resistance. Less signal attenuation Higher bandwidth Underlying Technologies

  10. Uuguided Media Unguided media transport electromagnetic wave without using a physical conductor. The section of the electromagnetic spectrum defined as radio communication is divided into eight ranges, called bands, each regulated by governmental authorities. These bands are rated from very low frequency(VLF) to extremely high frequency(EHF). Underlying Technologies

  11. Figure 3-8 Radio and infrared frequencies

  12. Underlying Technologies • 3.2 LOCAL AREA NETWORKS(LANS) • LANs are dominated by three architectures: Ethernet, token ring, and fiber distributed data interface(FDDI). • Ethernet • Access method: Carrier Sense Multiple Access with Collision Detection(CSMA/CD) • In CSMA/CD, before a station transmits data, it “listens” to the medium to check if another station is currently using the medium. • If no other station is transmitting, the station can send its data. • It can happen that two or more stations send data at the same time, resulting in a collision. • If a collision occurs, all stations ignore the data received. • The sending stations wait for a period of time before resending data. • To reduce the possibility of a second collision, the sending station each generate a random number that determines how long the station should wait before resending.

  13. Figure 3-9 CSMA/CD access method

  14. Underlying Technologies • Addressing • In each station on an Ethernet network has its own NIC. The NIC usually fits inside the station and provides the station with six-byte physical address • Data Rate • Ethernet LANs can support data rates between 1 and 10 Mbps(Fast Ethernet supports 100 M bps) • Frame Format • Ethernet does not provide any mechanism for acknowledging received frames, making it what is • known as an unreliable medium. • Acknowledgments must be implement at the higher layers.

  15. Figure 3-10 Ethernet frame. • Preamble. • This field alert the receiving system to the coming frame and enable it to synchronize its • timer.(56bits) • Start frame delimiter(SFD). • This one-byte field(10101011)is used as a flag and signals the beginning of the frame. • Destination address. This six-byte field contains the physical address of the next station. • Source address. This six-byte field contains the physical address of the previous station. • Type. This field defines the type of the data encapsulated in the frame. • Data. This field contains the data from the upper layer(46 ~ 1,500 byte) • Data length < 46 byte: padding should be added to make it 46 bytes • Data length > 1,500 byte: : The upper layer should fragment the data • Cyclic redundancy check(CRC): This is a four-byte field for error detection.

  16. Implementation • 10BASE5: Thick Ethernet: • Thick Ethernet is a bus topology LAN that uses baseband signaling and has a maximum segment length of 500 meters. Figure 3-11 Thick Ethernet

  17. 10BASE2: Thin Ethernet: • It is a Bus topology LAN, The advantages of thin Ethernet are reduced cost and ease of installation . Figure 3-12 Thin Ether net

  18. 10BASE-T: Twisted-pair Ethernet: • It is a star topology LAN using UTP cable. Data rate 10 Mbps, Maximum length 100 meters. Figure 3-13 Twisted-pair Ethernet 100 meters

  19. Underlying Technologies • Token Ring • Access method: Token ring • In token passing, the token is passed from station to station in sequence until it encounters a • station with data to send. • Addressing • Token ring uses a six byte address, which is implemented on the NIC similar to Ethernet address. • Data Rate • Token ring support two data rates: 4 and 16 Mbps. • Frame Format • The token ring protocol specifies three types of frames: data frame, token frame , and abort frame.

  20. Figure 3-14 Token passing access method

  21. Figure 3-15 Token ring frames

  22. Data Frame • In token ring, the data frame is the only type of frame that can carry data. • Start delimiter(SD). • This one-byte field(10101011)is used to alert the receiving station to the the arrival of a frame. • Access control(AC) • This one-byte field contains information about priority and reservation. • Frame control(FC) • This one byte field defines the type of information contained in the data field. • Destination address(DA). This variable-length(2 to 6 bytes) field contains the physical address of the next station. • Source address.(SA) This variable-length(2 to 6 bytes) field contains the physical address of the previous station.

  23. Data. This field contains the data. Data can be up to 4,500 bytes. • CRC: This field is four bytes long and contains a CRC-32 error-detection sequence. • End delimiter(ED). • This one-bytes field indicates the end of the sender’s data and contains more control information. • Frame status(FS). • The FS field is set by the receiver to indicate that the frame has been read, or by the monitor station to indicate that the frame has already been around the ring. • Token Frame • The token frame includes only three fields: SD, AC, and ED. • Abort Frame • The abort frame includes only two fields: SD, and ED. It can be used by the monitor station to abort the token passing mechanism when problem occur.

  24. Implementation • Configuring the network as a ring introduces a potential problem: One disabled or disconnected • node could stop the flow of traffic around the entire network. • To solve this problem, each station is connected to an automatic switch. • This switch can bypass an inactive station. While a station is disabled, the switch closed the ring • to bypass it. • When the station comes on, a signal sent by the NIC moves the switch and brings the station into • the ring. • For practical purposes, individual automatic switches are combined into a hub called a • multistation access unit(MAU) .

  25. Figure 3-16 Token ring implementation

  26. Underlying Technologies • Fiber Distributed Data Interface(FDDI) • FDDI is a LAN protocol standardized by ANSI and the ITU-T. • It supports data rates of 100 Mbps and provides a high-speed alternative to Ethernet and token ring. • Access method: Token ring • The access method in FDDI is also called token passing. • In token ring network, a station can send only one frame each time it captures the token. • In FDDI, access is limited by time. • Each station receives the token earlier than the designated time, it can keep the token and send data until the scheduled leaving time. • On the other hand, if a station recives the token at or later than this time, it should let the token pass to the next station and wait for its next turn. • Addressing • FDDI uses a two-to six-byte address. • Data Rate • FDDI supports data rates of 100 Mbps.

  27. Frame Format FDDI uses two types of frames: data frame, token frame. Figure 3-17 FDDI frame

  28. Underlying Technologies • SD. This one-byte field defines the starting flag. • FC. This one-byte field defines the type of the frame. • Destination address. This field contains the physical address of the next station. • Source address. This field contains the physical address of the previous station. • Data. Each data frame can carry up to 4,500 bytes of data. • CRC. FDDI use uses the standard IEEE four-byte cyclic redundancy check. • ED. This field consist of half a byte in the data frame or a full byte in the token frame. • It indicates the end of data and control information. • FS. This field is similar to that of token ring. It is included only in the data frame and consist of 1.5 bytes.

  29. Underlying Technologies • Implementation • FDDI is implemented as a dual ring. • Primary ring: Data transmission • Secondary ring: It is provided in case the primary ring. • When a problem occurs on the primary ring, It can be activated to • complete data circuits and maintain service. • MIC(media interface connector ) • Nodes connect to one or to both rings using a media interface connector(MIC) • Every MIC has two fiber port that allow it to connect to both ring cables. • NODE: FDDI defines three types of nodes; • DAS(dual attachment station): It has two MICs. • SAS(single attachment station) • DAC(dual attachment concentrator)

  30. Figure 3-18 FDDI implementation

  31. Underlying Technologies • 3.3 SWITCHING • Switches are hardware and/or software devices capable of creating temporary connections between two or more devices linked to the switch but not to each other. • Circuit Switching • Circuit switching creates a direct physical connection between two devices such as telephones • or computers. • In circuit switching, after a connection is made between two systems, the path is dedicated and • cannot be used by any other system.

  32. Figure 3-19 Circuit switching

  33. Underlying Technologies • Packet Switching • Datagram Approach • Each packet is treated independently of all others. • Even when one packet represents only a piece of a multipacket transmission, the network treats • it as through it existed alone. • Packets in this technology are referred to as datagram. • Destination: Datagram arrive out of order. • Transport layer: Reorder the datagrams before passing them on to the destination port. • The link joining each pair of nodes can contain multiple channels. • Each of these channels is capable, in turn, of carrying datagrams either from several different • sources simultaneously or from one source.

  34. Figure 3-20 Packet switching, datagram approach

  35. Underlying Technologies • Packet Switching • Virtual Circuit Approach • A single route is chosen between sender and receiver at the beginning of the session. • When the data are sent, all packets of the transmission travel one after another along that route. • Difference…. • In circuit switching, the path between the two end users is dedicated: it consist of only one channel. • In the virtual circuit, the line is not dedicated to only two users: the line can be divided into channel. • Message Switching • A node (usually a computer) receives a message, stores it until the appropriate route is free, then sends it along. It is not used today.

  36. Figure 3-21 Packet switching, virtual circuit approach

  37. Underlying Technologies • 3.3 WIDE AREA NETWORK(WANS) • Point-to-Point Protocol(PPP) • The PPP is designed to handle the transfer of data using asynchronous modem links or high-speed synchronous leased lines. Figure 3-22 Point-to-Point frame • Flag. Each frame starts with a one-byte flag, which has the value of 7E16(011111110). It is used for synchronization at the bit level between the sender and the receiver. • Address. The address field has the value of FF16. • Control. The control field has the value of 0316 • Protocol. This is a two-byte field used to define the protocol that uses the services of the line. For TCP/IP this value is 002116 • CRC. This is a two-byte cyclic redundancy check.

  38. Underlying Technologies • X.25 • X.25 is an interface between data terminal equipment(DTE) and data circuit terminating equipment(DCE) for terminal operation at the packet mode on public data network. • X.25 is a packet switching protocol used in a WAN. Figure 3-23 X.25

  39. Underlying Technologies • X.25 Layers • The X.25 protocol is organized into three layers: • Physical Layer : Physical layer of the OSI model. • LAPB(Link access procedure, balanced layer): Data link layer of the OSI model. • PLP(Packet Layer Protocol Layer). Network layer of the OSI model. Figure 3-24 X.25 packets and frames

  40. Physical Layer At the physical layer, X.25 specifies a protocol called X.21. X.21 is close enough to other physical layer protocols, such as EIA-232, that X.25 is able to support them as well. Data Link Layer(LAPB) X.25 provides data link controls using a bit-oriented protocol called link access procedure, balanced (LAPB). The fields in this layer are the same as those in the frame of LANs. The control field is used for flow and error control at the data link layer. The address field is used during connection establishment. The frame check sequence(FCS) is the CRC field defined in LAN frames. Underlying Technologies

  41. Network Layer(PLP) The network layer in X.25 is called packet layer protocol(PLP). This layer is responsible for establishing the connection, transferring the data, and terminating the connection. User and system data are passed down from the upper layers. At the PLP, a header containing control information is added to transform the data into a PLP packet. PLP packet are passed to the LAPB layer, where they are encapsulated into LAPB information frames and passed to the physical layer to be sent through the network. The following are fields of the network-layer packet: General format identifier(GFI). This field gives general information about the header. Logical channel group number(LCGN). This is the first part of the virtual circuit number. Underlying Technologies

  42. Logical channel number(LCN). This is the second part of the virtual circuit number. Control. This is used for flow control and error control. Frame Relay Frame relay is a WAN protocol designed in response to X.25 deficiencies. Frame relay does not provide error checking or require acknowledgment in the data link layer All error checking is left to the protocols at the network and transport layers, which use the services of frame relay.(Frame relay operates at only the physical and data link layers.) Underlying Technologies

  43. Figure 3-25 Comparison between X.25 and frame relay

  44. Frame Format Address fields. The first six bits of the first byte make up part 1 of the data link connection identifier(DLCI). The second part of the DLC1 uses the first four bits of the second byte. These bits are part of the 16-bit data link connection identifier defined by the standard. Command/response(C/R). The C/R bit is provided to allow upper layers to identify a frame as either a command or a response. Extended address(EA). The EA bit tells whether or not the current bytes is the final bytes of the address. EA 0 means that another address byte is to follow. EA 1 means that the current byte is the final one. Forward explicit congestion notification(FECN). The FECN bit indicates that traffic is congested in the direction in which the frame is traveling. It informs the destination that congestion may cause the present or future frames to arrive late. Backward explicit congestion notification(BECN). The BECN bit indicates a congestion problem in the direction opposite to the one in which the frame is traveling. It informs the receiver that data sent back to the sender may be delayed by congestion. Underlying Technologies

  45. Figure 3-26 Frame relay frame • Discard eligibility(DE). The DE bit indicates the priority level of the frame.

  46. Frame Relay Operation. Frame relay transmission is based on permanent virtual circuit(PVC) connection. Virtual circuits in other standards are implemented by the network layer. Frame relay uses DLCIs which identify a permanent virtual circuit that is set up when the system is put in place. All traffic between two given stations takes the same path. Underlying Technologies

  47. Figure 3-27 Frame relay operation

  48. Asynchronous Transfer Mode(ATM) ATM is designed to support the transmission of data, voice, and video through high data-rate transmission media such as fiber-optic cable. Cell ATM is a protocol for transferring cells. A cell is a small data unit of fixed size. It is 53 bytes long, made of a five-byte header and a 48-byte payload. The header contains, among other information, a virtual path identifier(VPI) and a virtual channel identifier(VCI). These two peaces of information are used to route the cell through the network to the final destination. Underlying Technologies

  49. Figure 3-28 Cell

  50. Cell Relay ATM network is a connection-oriented, cell relay(cell switching) network. To start up a connection, a system uses a 20-byte address. After the connection is established, the combination of VPI/VCI leads a cell from its final network. Underlying Technologies

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