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

Asynchronous Transfer Mode 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