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Wide Area Networks.

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Presentation on theme: "Wide Area Networks."— Presentation transcript:

1 Wide Area Networks

2 WAN vs LAN Span BW Delay Different protocols
Usually you don’t own the WAN infrastructure

3 Point to point link That’s what you “see” Ex: leased line
Usually simulated by a circuit or packet switched network

4 Circuit Switching Based on the PSTN (Public Switched Telephone Network) Analog: modems up to 56K Digital: 64K circuits - SDH w/ TDM cf Bocq Designated circuits

5 Packet Switching Data streams segmented in packets
Statistical Multiplexing (FIFO or QoS techniques)

6 Circuit vs Packet switching
Circuit: Sum of peak data rates < transmission capacity Packet: Sum of average data rates < transmission capacity Circuit: waste of BW Packet: delay => unacceptable for voice

7 Connection oriented vs Connectionless
Circuit: CO Data: CL => need addressing

8 Virtual Circuits Connection Oriented: encapsulation includes a “flow” identifier Best of two worlds? Switched VCs - 3 phases: circuit setup, data transfer, circuit termination Permanent VCs - more expensive as need to be constantly up, use less BW

9 VC multiplexing

10 Synchronous Data Link Control

11 SDLC Developped by IBM for use w/ SNA
Most of L2 protocols are based on the SDLC format (HDLC, LAPB, 802.2, etc…)

12 SDLC Frame Format

13 X.25

14 X.25 1970s Data Terminal Equipment (DTE)
Data Circuit-terminating Equipment (DCE) Packet Switching Exchange (PSE) DCE provides clock

15 X.25 topology

16 Packet Assembler/Disassembler

17 X.25 Stack

18 LAPB Frame

19 X.25 Data Link Control Point to point full duplex data links
Correction of errors and congestion control Encapsulation of data in variable length frames delimited by flags Redundant error correction bits Sliding window (8 or 128 frames)

20 X.121 address

21 X.121 address Data Network Identification Code (DNIC)
National Terminal Number (NTN)

22 Packet Level Protocol Several circuits multiplexed
Sliding window error and congestion control for every VC Call restriction, charging, QoS, ...

23 VC Setup PVC: permanent entry in “routing” table (static), substitute to leased lines SVC: dynamic entry in “routing” table triggered by an “open” packet and torn down by “close” packet

24 Frame Relay

25 Characteristics Introduced in 1984 but only (significantly) deployed in the late 1980s L1 and 2 Packet Switched technology: PVCs and SVCs Connection-oriented data link layer communication X.25 “lite”

26 Differences with X.25 Less robust Assumes more reliable medium =>
No retransmission of lost data No windowing Error control handled by higher layers Higher performance and transmission efficiency

27 Frame Relay Topology

28 DLCI Data Link Connection Identifier Uniquely identify circuits
Assigned by service provider Local significance only (except with LMI)


30 Frame Format

31 Discard Eligibility One bit in the address field
Identifies lower importance traffic that will be dropped first if congestion occurs Set by DTE equipment

32 Congestion Control: FECN
FECN: Forward Explicit Congestion Notification DCE sets FECN bit to 1 When received by DTE, it indicates that frame experienced congestion Sent to higher layers or ignored

33 Congestion Control: BECN
BECN: Backward Explicit Congestion Notification Same as FECN but set on the return flow

34 LMI Local Management Interface Frame Relay “extension”
Introduced in 1990 by the “gang of four” (Cisco, DEC, Nortel and Stratacom) Additional capabilities for complex internetworking environments Later Standardized by CCITT

35 LMI (2) Global addressing: DLCIs become global addresses
Virtual-circuit status messages Multicasting

36 LMI Frame Format

37 CIR What you buy with a FR connection Committed Information Rate
CIR= Committed Burst/Committed Time Also Maximum Rate

38 Asynchronous Transfer Mode
ATM Asynchronous Transfer Mode

39 Characteristics Originally designed to transmit voice, video and data over the same network Cell switching Each communication is assigned a timeslot Timeslots are assigned on a demand-basis => asynchronous (as opposed to TDM)

40 Cells 53 bytes: 5 byte header + 48 byte payload
Tradeoff between voice world and data world: Voice needs small payloads and low delay Data needs big payload and less overhead

41 ATM Interfaces UNI: User to Network Interface
NNI: Network to Network Interface

42 ATM Interfaces

43 UNI and NNI cell formats

44 UNI and NNI differences
NNI has bigger VPI range UNI has Generic Flow Control field GFC used to identify different end stations

45 VPI and VCI Used to determine paths VPI: Virtual Path Identifier
VCI: Virtual Channel Identifier VPI identifies a bundle of VCIs

46 VPI and VCI (2)

47 ATM Switching Table look up
Incoming interface/VPI/VCI is mapped to an outgoing interface/VPI/VCI

48 ATM Reference Model

49 ATM Adaptation Layer (AAL)
Together with ATM layer, equivalent to Data Link layer in OSI model AAL1: Connection Oriented => Voice and Video AAL 3,4: Connection Oriented and Connectionless (similar to SMDS) AAL 5: Connection Oriented and Connectionless for CLIP and LANE

50 ATM Sources

51 ATM Addresses ITU-T Standard: E.164 (Telephone #)
ATM Forum defined 20-byte NSAP Addresses for use in private networks E.164 address used as prefix on NSAP Mapped to IP addresses by ATM ARP (in CLIP)

52 ATM QoS Traffic Contract: peak bandwidth, average sustained bandwidth, burst size , … Similar to FR Traffic Shaping (end device): Queuing, Buffering Traffic Policing (switches): Enforces contract

53 Path Establishment

54 LAN Emulation (LANE) Purpose: emulate a LAN over an ATM network
Ethernet or Token Ring Resolves MAC addresses to ATM addresses

55 LANE Equivalent

56 LANE Components LEC: LAN Emulation Client LES: LAN Emulation Server
BUS: Broadcast and Unknown Server LECS: LAN Emulation Configuration Server

57 LANE Components

58 Initialization LEC finds LECS via pre-established ILMI procedure or through well-known circuit LECS returns: ATM address of the LES, type of LAN being emulated, maximum packet size on the ELAN, and ELAN name LEC registers to its LES (LES checks with LECS) LES assigns LECID (LE Client ID)

59 Communication LE ARP Request sent to LES
If LES doesn’t know, it floods the request

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