Wide Area Networks

WAN vs LAN

• 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

• 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

• 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

• 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

• 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

• 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