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Transcript
Chapter Objectives At the end of this chapter you will be able to: Describe standardisation in the data communications area Describe the following technologies: X.25, Frame Relay, ISDN, PPP, Modem, XDSL and ATM Analyse the protocols of the X.25, Frame Relay and ISDN technologies Leased Line and Dial-Up Leased Line – With a leased line connection, a data user has a permanent dedicated transmission path which can be end to end across the network, locally, nationally or internationally. Dial-up – This method is used for modem to modem data communication over the Public Switched Telephone Network (PSTN). Both the source and destination must have compatible modems Circuit Switching and Packet Switching Circuit Switching – In a circuit switched network, a dedicated communications path is established between two terminals through the nodes of the network and for information transfer Packet Switching – In this case it is not necessary to dedicate transmission capacity along a path through the network. Rather, data is sent out in a sequence of small chunks, called packets. Each packet is passed through the network from node to node along some path leading from the source to the destination HDLC Variations HDLC Subset Uses NRM Multipoint networks that typically use SDLC LAP Early X.25 implementations LAPB Current X.25 implementations LAPD ISDN D channel and frame relay LAPM Error-correcting modems (specified as part of V.42) X.25 Network Modem Modem X.28 Port Modem Modem X.25 Port Modem Modem Modem Modem X.25 Port X.75 port to other packet switching networks PSE PSE X.28 Port Dumb Terminal X.25 Interface OSI Layers Layer 3 User Process Multi-channel Logical Interface Packet Layer 2 Link Access Layer 1 Physical User Process X.25 Packet Packet LAPB Link Level Logical Interface X.21 Physical Interface User Data Link Access Physical Layer 3 Header User Data LAPB Frame LAPB Layer 3 Header Header User Data LAPB Header LAPB Commands and Responses Command Name Content of Control Octet Bit Number 876 5 432 1 I (Information) N(R) P N(S) 0 RR (Receiver Ready) N(R) P 000 1 RNR (Receiver Not Ready) N(R) P 010 1 REJ (Reject) N(R) P 100 1 SABM (Set Asynchronous Balanced Mode) 001 P 111 1 DISC (Disconnect) 010 P 001 1 RR (Receiver Ready) N(R) F 000 1 RNR (Receiver Not Ready) N(R) F 010 1 REJ (Reject) N(R) F 100 1 UA (Unanswered Acknowledgement) 011 F 001 1 DM (Disconnect Mode) 000 F 111 1 FRMR (Frame Rejected) 100 F 011 1 Response Name N(S) is a three bit counter for packets sent from one end of the link N(R) indicates the next value of N(S) expected to be returned from the other end of the link X.25 Packet Header Bits 8 7 6 5 4 3 2 1 Octet #1 General Format Logical Channel Identifier Group Number Octet #1 Logical Channel Number Octet #1 P(R) M P(S) User Data User Data 0 X.121 Addressing Data Network Identification Code (DNIC) Network Terminal Number (NTN) (Max. 10 Digits) Data Country Code Network Digit Private Network Private Network Identification Code Terminal Number (PNIC) (PNTN) 3 Digits 1 Digit 4 Digits 3 Digits 2 Digits Max. 6 Digits Max. 7 Digits Max. 8 Digits Example: Example: 7 7 7 Examples: 2500 564 88 Examples: 321654 3217654 32197654 505 505 505 X.25 Operation User System A User Network Interface A Initiates a virtual call to B Call Request User System B Network User Network Interface Incoming Call When A is informed that the call is connected it can begin to send data packets Call Accepted Call Connected Data R=0 S=0 Data R=0 S=1 Data R=0 S=0 Data R=0 S=1 Data R=2 S=0 Data R=1 S=3 Receive Ready R=3 Data R=4 S=0 A Initiates clearing of the virtual call Data R=2 S=0 Receive Ready R=1 Data R=1 S=3 Data R=4 S=1 Clear Request Clear Indication Clear Confirmation Clear Confirmation The packets are delivered in sequence B has no data packet with which to acknowledge packet S-1, so it sends a supervisory packet Frame Relay and X.25 Protocol Stacks X.25 Packet Level Implemented by end system and network Implemented by end system but not network LAPF Control LAPB LAPF Core Physical Layer X.25 Implemented by end system and network Physical Layer Frame Relay LAPF Formats Flag Address Information FCS Flag 1 2-4 Variable < Frame Format > 2 1 Upper DLCI Lower DLCI 8 7 6 5 C/R EA 0 FECN BECN DE EA 1 4 3 2 1 C/R EA 0 DE EA 0 D/C EA 1 Address Field (2 Octets – default) Upper DLCI DLCI FECN BECN Lower DLCI or DL-Core Control Address Field (3 Octets) EA Address field extension bit C/R Command/response bit FECN Forward explicit congestion notification BECN Backward explicit congestion notification DLCI Data link congestion identifier D/C DLCI or Core control indicator DE Data link congestion identifier Frame Handler Operation Frame relay control point DLCI=0 TE A DLCI=0 DLCI=312 DLCI=334 DLCI=0 TE B TE D DLCI=342 DLCI=306 DLCI=0 TE E DLCI=0 TE C DLCI=322 DLCI=306 Frame Handler Frame Relay Network Access Rec. Speed (bit/s) PSTN 300 Transmission Mode Asynchronous (A) LL 4W Back-up via PSTN FD LL 2W FD Mod. Method FSK V.21 V.23 1200/600 A and S HD HD FD * FSK V.22 1200/600 A and S FD FD * DPSK V.22bis V.22f.bk V.26bis 2400/1200 A and S FD FD * QAM 2400/1200 Synchronous (S) HD HD * DPSK V.26ter 2400/1200 A and S FD FD * DPSK V.27ter V.26bisf.bk V.29 4800/2400 S HD HD * DPSK 9600/7200/4800 A and S V.32 9600/4800 A and S V.33 14400/12000 S V.34 28800 S FD TCM V.34bis 28800/31200/33600 S FD TCM Baseband 2400/1800/1200 7200/4800/3600 19200/14400/9600 56000 to the end user 33600 from the end user A and S V.90 S FD FD FD FD FD * FD HD Asymetric QAM QAM/TCM QAM/TCM FD PCM Basic AT Commands Command Function A Manually answer incoming call AT Appears at the beginning of every command line B_ ITU-T Mode (B0) Bell Mode (B1) D_ Pulse Dialling (DP) Touch-tone Dialling (DT) H_ Force modem on-hook, that is hang up (H0) Force modem off-hook, that is make busy (H1) M_ Internal speaker off (M0) Internal speaker on until carrier detected (M1) Applications of ISDN in Internetworking Dial Backup DDR ISDN Network SOHO ISDN BRI NT PRI ISDN Access Basic Rate Interface (BRI) Primary Rate Interface (PRI) ISDN Functional Devices and Physical Interfaces Customer Premises Local Exchange Terminal equipment ISDN interface Network termination equipment basic termination (DSU/CSU) Network termination equipment PBX or internetworking device TE1 TE2 Terminal equipment (Non-ISDN) NT1 TA Network termination Transmission equipment basic termination (DSU/CSU) Line ISDN Switch NT2 Terminal adapter S T U ITU-T Physical Interface Points U LAPD Frame Format Length in Octets 1 FLAG Service Access Point Identifier (SAPI) Terminal Endpoint Identifier (TEI) Control Information C/R O 1 1 1-2 0-128 or 0-260 Frame Check Sequence 2 FLAG 1 Messages for Circuit-mode Connection Control Call Establishment Messages Call Information Phase Messages Call Clearing Messages Miscellaneous Messages Alerting Resume Disconnect Information Call Proceeding Resume Acknowledge Release Notify Connect Resume Reject Release Complete Status Connect Acknowledge Suspend Progress Suspend Acknowledge Setup Suspend Reject Setup Acknowledge Status Enquiry Main Components of PPP Encapsulation Scheme Link Control Protocol Network Control Protocols PPP Encapsulation Maximum Receive Unit (MRU) Protocol 8 or 16 bits Information Padding LCP Management Functions Determine encapsulation format options Negotiate optimal packet size Terminate the link Authenticate the identity of the peer on the link (optional) Negotiate PPP Multilink Data Compression (optional) Link quality monitoring (optional) Network Control Protocol (NCP) NCPs are a series of independently-defined protocols that encapsulate network layer protocols such as TCP/IP, DECnet, AppleTalk, IPX, XNS, and OSI. PPP Link Phases In the process of configuring, maintaining and terminating the point-to-point link, the PPP link goes through several distinct phases: – – – – – Link dead (Physical layer not ready) Link establishment Authentication Network-layer protocol Link termination PPP Peer to Peer Connection Setting up a PPP connection between two peers involves four phases: – – – – LCP negotiation LCP steady state NCP negotiation Network layer protocol data flow ATM Network ATM Switch UNI UNI UNI Private UNI ATM Switch UNI Private UNI ATM Switch ATM Connections ATM supports two types of connections: – Point-to-point, which connects two ATM end systems and can be unidirectional (one-way communication) or bidirectional (two-way communication). – Point-to-multipoint connects a single-source end system (known as the root node) to multiple destination end systems (known as leaves). Such connections are unidirectional only. Root nodes can transmit to leaves, but leaves cannot transmit to the root or each other on the same connection. Cell replication is done within the ATM network by the ATM switches where the connection splits into two or more branches. XDSL Copper Access Ordinary POTS access: LE Copper POTS access supplemented LE C opper <3km Splitter ADSLunit Splitter with ADSL equipment: ADSLunit to BB Service Provider DMT Subchannels VDSL Connection HDSL