Module 3. Wide Area Networking (WAN)

Transcription

1 Module 3 Wide Area Networking (WAN) When many nodes or LAN s are connected together in a large scale, we have a WAN. This may be world wide. Because of the separation of the nodes, the Public Data Network (PDN) is used. There are two main approaches to the problem. Packet switching Circuit switching CNN: Lecture Notes Module 3 Computer Comm. & Networking 1

2 Packet switching Use Packet Switched PDN (PSPDN) Data is assembled into a packet and addressed to destination Packet is sent to Packet Switching Exchange (PSE) Packet is transmitted to destination via PSPDN Connection is purely logical not physical Nodes may have different data rates etc. Two services are provided Datagram: Like sending letter. One packet is addressed and sent at a time with each packet being independent of the others. Virtual call: Like a phone call. When many packets are to be sent to the same address a virtual circuit is established. CNN: Lecture Notes Module 3 Computer Comm. & Networking 2

3 Circuit switching Use Circuit Switched PDN (CSPDN) Physical connection is established by CSE Nodes must all obey standard rates etc. CNN: Lecture Notes Module 3 Computer Comm. & Networking 3

4 Packet switched Networks Whole system based around the DTE to PSDN connection. The original protocol used was X.25. Now see variations, eg Frame Relay etc. Look at X.25 (CCITT): Interface between DTE and Data Circuit-Terminating Equipment (DCE) for terminals operation in the Packet mode on Public Data Networks X.25 referred to OSI s first three layers CNN: Lecture Notes Module 3 Computer Comm. & Networking 4

5 Packet switched Networks X.25 addresses the interface between user s equipment (DTE) and the network. DTE comprises a computer, modem or line interfaces X.25 provide a standard for accessing the WAN CNN: Lecture Notes Module 3 Computer Comm. & Networking 5

6 X.25 s Physical Layer X.25 physical layer specifies the interface between the DTE and DCE, such as physical and electrical characteristics of the media, the types of connectors, etc. It references the X.21 interface standard. Support RS-232C CNN: Lecture Notes Module 3 Computer Comm. & Networking 6

7 X.25 s Link Access Layer X.25 layer 2 is defined by the Link Access Protocol-Balanced (LAPB) It provides link access specifications for Frame composition Flow-control procedures Error-checking method CNN: Lecture Notes Module 3 Computer Comm. & Networking 7

8 X.25 s Packet Layer X.25 packet layer specifies a virtual circuit service for transporting packets across the packet-switched network Two services: Permanent virtual circuit Virtual call CNN: Lecture Notes Module 3 Computer Comm. & Networking 8

9 Circuit Switched Networks This type of connection is concerned only with the X.21 protocol which is used from end to end over a physical connection. Call set up, data transfer and call clearing is handled by this protocol. HDLC is used once again. CNN: Lecture Notes Module 3 Computer Comm. & Networking 9

10 Integrated Services Digital Network (ISDN) ISDN was conceived as a way to combine many of our communication needs into a system using the same standard techniques for each service. The common usage of digitised analogue waveform paved the way for ISDN. E. g., telephone cables carrying time division multiplexed digitised speech, ie, Pulse Code Modulation (PCM) This concept has been brought right back to the subscribers premises. If all information out of a subscribers premises could be put into a standard form, the it could be neatly packaged together and piped to its destination. CNN: Lecture Notes Module 3 Computer Comm. & Networking 10

11 ISDN (Cont d) ISDN could supply a path for Telephone Fax Computer/Terminal connection Fire alarm Video phone HiFi music Video images Appliance control Meter reading LAN access. Let s look at the adopted standard CNN: Lecture Notes Module 3 Computer Comm. & Networking 11

12 ISDN Channel Types (basic rate interface) ISDN uses bearer or B channels for transmitting data, and a signaling or D channel for transmitting signaling and control information B Channel 64kbps Digital voice, computer data(text and Graphics), digitised video D Channel -- 16kbps Signaling, or packet switched user data such as telemetry, meter reading (low speed) CNN: Lecture Notes Module 3 Computer Comm. & Networking 12

13 ISDN Channel Types A normal subscribers premises will be given a Basic Service 2 B channels 1 D channel 48 bits of overhead for framing and synchronisation Total 192 kbps Normally shielded pair to premises Premises may convert to a shielded 4 wire bus CNN: Lecture Notes Module 3 Computer Comm. & Networking 13

14 ISDN Channel Types A B channel connection may be Circuit switched Data is sent over B channels Call setup and control on D channel Packet switched X.25 connection Semi-permanent Fixed connection to another user What used to be leased line The first anticipated utilisation of the B channel was the telephone (history has shown the internet has changed this) 4kHz speech BW Sampled at 8kHz 8 bits per sample (256 levels) 8k 8=64kbps same as PCM used now CNN: Lecture Notes Module 3 Computer Comm. & Networking 14

15 ISDN Channel Types (primary rate interface) BRI is for home and small office, PRI is for organisations. ISDN Device B Channel 64 kbps B Channel 64 kbps. B Channel 64 kbps D Channel 64 kbps D Channel 64 kbps Data and Voice Data and Voice U.S. = 23 Europe = 30 Data and Voice Signaling U.S. = 1 Signaling Europe = 2 North American DS-1 format: Rate: 1.544Mbps 23B + D European E-1 format: Rate: 2.048Mbps 30B + 2D CNN: Lecture Notes Module 3 Computer Comm. & Networking 15

16 How does the user obtain access to the system The Network Terminating Equipment (NTE) supplied will have a number of access points. CNN: Lecture Notes Module 3 Computer Comm. & Networking 16

17 How is control of the channels performed All control procedures are performed on the D channel, A B channel will then be assigned to a device, thus no contention will occur. Contention may occur on the D channel as all of the devices may try to access this channel. Before transmitting, a device must listen for a long string of consecutive 1 s (idle channel). When busy, 0 s are inserted into long 1 strings. Collision may still occur so a protocol similar to CSMA is adopted. CNN: Lecture Notes Module 3 Computer Comm. & Networking 17

18 Connection overview The interface protocol is broken into 2 planes C-plane Control and signalling (D channel) U-plane User, voice & data (B channel) The amount of intelligence required in the NTE depends on the type of connection. C frame complexity is the same for all connections but U frame alters. CNN: Lecture Notes Module 3 Computer Comm. & Networking 18

19 (a) Circuit switched CNN: Lecture Notes Module 3 Computer Comm. & Networking 19

20 (a) Frame relay/frame switching CNN: Lecture Notes Module 3 Computer Comm. & Networking 20

21 (a) Packet switching X.25 CNN: Lecture Notes Module 3 Computer Comm. & Networking 21

22 With X.25, multiplexing of multiple virtual circuits is handled by the packet layer and the link layer handles only error correction. This makes these combined layers complex which limits the throughput. With frame relay, multiplexing and routing are done at the link layer. This simplifies the protocol which allows greater throughput. Using this technique many calls may be in progress ar any time. Each virtual connection is allocated an identifier whichi is included in the header. CNN: Lecture Notes Module 3 Computer Comm. & Networking 22

23 Example connection digital telephone call Circuit mode connection control CNN: Lecture Notes Module 3 Computer Comm. & Networking 23

24 ISDN Fax At present our FAX system operates predominantly around the Group 3standard. This allows up to 9600bps of modulated data over a PSTN line. This group will transmit a page in approximately 30 secs Group 4 FAX is intended for ISDN usage and is purely a digital system operating at 64kbps. The source is encoded with Huffman codes Resolution is 200dpi with options for 240, 300 and 400 dpi. These are laser printer type resolutions. Group 4 can send a page in approximate 3-4 secs. CNN: Lecture Notes Module 3 Computer Comm. & Networking 24

25 Broadband ISDN (B-ISDN) Data hungry users require more and more bps for many applications To serve this need a faster service called B-ISDN was introduced. Ordinary ISDN is now often called Narrowband ISDN (N-ISDN) Services are broken into two types, Interactive and Distributive. CNN: Lecture Notes Module 3 Computer Comm. & Networking 25

26 B-ISDN Interactive Conversational: Real time services such as telephone calls, video telephony etc. Messaging: Non real time services such as voic , data mail, video mail etc. Retrieval: Services requiring action from both end, eg. Retrieving video from a videotex provider. Distributive Services from providers to subscribers issuing a request. User control: Service where subscriber needs to enable acceptance of service eg. Pay TV where programs are sent in defined time slots. No user control: Data is supplied at providers discretion without control of subscriber, eg. TV broadcast to premises. CNN: Lecture Notes Module 3 Computer Comm. & Networking 26

27 B-ISDN (Physical Specifications) Typically 3 types of service are available Symmetrical Mbps: For businesses requiring normal traffic in and out including video where a N-ISDN service in not suitable Mbps/ Mbps: For businesses requiring access to multiple services but who are not service providers. Symmetrical Mbps: For businesses who provide and receive services. Reference points for B-ISDN are similar to those for N-ISDN B-ISDN is the basis for ATM CNN: Lecture Notes Module 3 Computer Comm. & Networking 27

28 Asynchronous Transfer Mode (ATM) This is a further advancement over X.25 and frame relay especially designed for very high speed network segments. It is commonly used for high capacity backbones. Because of the way it operates, it is commonly called Cell Relay Like frame relay and X.25, data is sent in packets. The fundamental difference is that ATM uses fixed size packets called cells, which contains exactly 53bytes 48 bytes for user data and 5 bytes for overhead. This allows ATM switches to operate very quickly. A common implementation is 155 Mbps ATM over SONET (Synchronous Optical Network) ATM is often implemented on various physical layers SONET, FDDI, 155 MB fibre ATM does not map neatly into the ISO reference. CNN: Lecture Notes Module 3 Computer Comm. & Networking 28

29 ATM End nodes communicate with an ATM device via a userto-network interface (UNI) ATM devices communicate with each other via a network-to-network interface (NNI). CNN: Lecture Notes Module 3 Computer Comm. & Networking 29

30 ATM-- example of how ATM facilitates multiservice networking ATM is the only network that was built from the ground up to support data, voice, and video at the same time. Therefore, an ATM network can be used for almost any type of network environment in use today and in the future. CNN: Lecture Notes Module 3 Computer Comm. & Networking 30

31 ATM Protocol Architecture? Use Plan Control plane -- Info exchange, and flow and error control. -- Control and connection functions Management plane -- Management functions to provide coordination between layers. CNN: Lecture Notes Module 3 Computer Comm. & Networking 31

32 ATM Protocol Architecture The AAL is required to provide interface to transfer protocols that don t support fixed cell size transfer. ATM uses Virtual Channel Connections (VCC) and Virtual Path Connections (VPC) VCC -- Link a virtual circuit in X.25. The data is transferred over this circuit in fixed size cells at a variable rate. VPC -- VPCs are a bundle of VCCs that go between the same source and destination. This allows ATM to operate faster as it may switch all the cells together. CNN: Lecture Notes Module 3 Computer Comm. & Networking 32

33 ATM VPC Use of VPCs gives Network simplification can switch paths or connections Improved performance switching paths is quicker Improved connection setup and processing times -- new connections can be added to paths very quickly Improved network services users can define structure of connections and paths. CNN: Lecture Notes Module 3 Computer Comm. & Networking 33

34 ATM cells User-network interface CNN: Lecture Notes Module 3 Computer Comm. & Networking 34

35 ATM AAL The AAL is the layer which takes care of translating from another protocol to the fixed cells required by ATM. Historically there have been a number of AAL types developed AAL1 AAL2 AAL3/4 AAL5 AAL5 is now the version of choice for LANs and most other applications. The AAL is divided into 2 sub-layers Common Part Convergence Sublayer (CPCS) -- Talk to applications requiring the AAL Segmentation And Reassembly sublayer (SAR) Performs the breakup and assembly of frames to 48 octet cells and vice versa. A protocol data unit (PDU) is passed to the CPCS. Certain functions may be performed here before passing this CPCS-PDU to the SAR. The SAR breaks the CPCS-PDU into SAR-PDUs each of 48 octets which fit into an ATM cell CNN: Lecture Notes Module 3 Computer Comm. & Networking 35

36 ATM SAR CNN: Lecture Notes Module 3 Computer Comm. & Networking 36

37 ATM advantages ATM advantages Uses fixed length cells Gives much higher throughput Short cells have less chance of error Usually single bits errors (networks are now quite reliable) CRC can repair cells Cells can be handled mainly by hardware (ATM switches) Network protocol does not rely on retransmission Less work for network and more for nodes Can interface to most higher level protocols and most physical layers At present ATM is expected to be increasing in popularity throughout the world network. CNN: Lecture Notes Module 3 Computer Comm. & Networking 37