Download WAN - ULB

Wide Area Networks
WAN vs LAN
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Span
BW
Delay
Different protocols
Usually you don’t own the WAN
infrastructure
Point to point link
• That’s what you “see”
• Ex: leased line
• Usually simulated by a circuit or packet
switched network
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
Packet Switching
• Data streams segmented in packets
• Statistical Multiplexing (FIFO or QoS
techniques)
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
Connection oriented vs
Connectionless
• Circuit: CO
• Data: CL => need addressing
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
VC multiplexing
Synchronous Data Link Control
SDLC
SDLC
• Developped by IBM for use w/ SNA
• Most of L2 protocols are based on the
SDLC format (HDLC, LAPB, 802.2,
etc…)
SDLC Frame Format
X.25
X.25
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1970s
Data Terminal Equipment (DTE)
Data Circuit-terminating Equipment (DCE)
Packet Switching Exchange (PSE)
DCE provides clock
X.25 topology
Packet Assembler/Disassembler
X.25 Stack
LAPB Frame
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)
X.121 address
X.121 address
• Data Network Identification Code (DNIC)
• National Terminal Number (NTN)
Packet Level Protocol
• Several circuits multiplexed
• Sliding window error and congestion
control for every VC
• Call restriction, charging, QoS, ...
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
Frame Relay
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”
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
Frame Relay Topology
DLCI
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Data Link Connection Identifier
Uniquely identify circuits
Assigned by service provider
Local significance only (except with LMI)
DLCI
Frame Format
Discard Eligibility
• One bit in the address field
• Identifies lower importance traffic that will
be dropped first if congestion occurs
• Set by DTE equipment
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
Congestion Control: BECN
• BECN: Backward Explicit Congestion
Notification
• Same as FECN but set on the return flow
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
LMI (2)
• Global addressing: DLCIs become global
addresses
• Virtual-circuit status messages
• Multicasting
LMI Frame Format
CIR
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What you buy with a FR connection
Committed Information Rate
CIR= Committed Burst/Committed Time
Also Maximum Rate
ATM
Asynchronous Transfer Mode
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)
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
ATM Interfaces
• UNI: User to Network Interface
• NNI: Network to Network Interface
ATM Interfaces
UNI and NNI cell formats
UNI and NNI differences
• NNI has bigger VPI range
• UNI has Generic Flow Control field
• GFC used to identify different end stations
VPI and VCI
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Used to determine paths
VPI: Virtual Path Identifier
VCI: Virtual Channel Identifier
VPI identifies a bundle of VCIs
VPI and VCI (2)
ATM Switching
• Table look up
• Incoming interface/VPI/VCI is mapped to
an outgoing interface/VPI/VCI
ATM Reference Model
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
ATM Sources
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)
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
Path Establishment
LAN Emulation (LANE)
• Purpose: emulate a LAN over an ATM
network
• Ethernet or Token Ring
• Resolves MAC addresses to ATM addresses
LANE Equivalent
LANE Components
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LEC: LAN Emulation Client
LES: LAN Emulation Server
BUS: Broadcast and Unknown Server
LECS: LAN Emulation Configuration
Server
LANE Components
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)
Communication
• LE ARP Request sent to LES
• If LES doesn’t know, it floods the request