Wide Area Networks

1
Wide Area Networks
2
WAN vs LAN

• Span
• BW
• Delay
• Different protocols
• Usually you dont own the WAN infrastructure

3
Point to point link

• Thats 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 lt transmission
  capacity
• Packet Sum of average data rates lt transmission
  capacity
• Circuit waste of BW
• Packet delay gt unacceptable for voice

7
Connection oriented vs Connectionless

• Circuit CO
• Data CL gt 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

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VC multiplexing
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Synchronous Data Link Control

• SDLC

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SDLC

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

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SDLC Frame Format
13
X.25
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X.25

• 1970s
• Data Terminal Equipment (DTE)
• Data Circuit-terminating Equipment (DCE)
• Packet Switching Exchange (PSE)
• DCE provides clock

15
X.25 topology
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Packet Assembler/Disassembler
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X.25 Stack
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LAPB Frame
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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)

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X.121 address
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X.121 address

• Data Network Identification Code (DNIC)
• National Terminal Number (NTN)

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Packet Level Protocol

• Several circuits multiplexed
• Sliding window error and congestion control for
  every VC
• Call restriction, charging, QoS, ...

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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

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Frame Relay
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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

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Differences with X.25

• Less robust
• Assumes more reliable medium gt
• No retransmission of lost data
• No windowing
• Error control handled by higher layers
• Higher performance and transmission efficiency

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Frame Relay Topology
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DLCI

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

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DLCI
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Frame Format
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Discard Eligibility

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

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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

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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
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CIR

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

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ATM

• Asynchronous Transfer Mode

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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 gt
  asynchronous (as opposed to TDM)

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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

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ATM Interfaces

• UNI User to Network Interface
• NNI Network to Network Interface

42
ATM Interfaces
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UNI and NNI cell formats
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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

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VPI and VCI (2)
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ATM Switching

• Table look up
• Incoming interface/VPI/VCI is mapped to an
  outgoing interface/VPI/VCI

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ATM Reference Model
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ATM Adaptation Layer (AAL)

• Together with ATM layer, equivalent to Data Link
  layer in OSI model
• AAL1 Connection Oriented gt Voice and Video
• AAL 3,4 Connection Oriented and Connectionless
  (similar to SMDS)
• AAL 5 Connection Oriented and Connectionless for
  CLIP and LANE

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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
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LAN Emulation (LANE)

• Purpose emulate a LAN over an ATM network
• Ethernet or Token Ring
• Resolves MAC addresses to ATM addresses

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LANE Equivalent
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LANE Components

• LEC LAN Emulation Client
• LES LAN Emulation Server
• BUS Broadcast and Unknown Server
• LECS LAN Emulation Configuration Server

57
LANE Components
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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)

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Communication

• LE ARP Request sent to LES
• If LES doesnt know, it floods the request