Wide Area Networks WAN vs LAN

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 56 K • Digital: 64 K circuits - SDH w/ TDM • cf Bocq • Designated circuits

Packet Switching • Data streams segmented in packets • Statistical Multiplexing (FIFO or Qo. S 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 L 2 protocols are based on the SDLC format (HDLC, LAPB, 802. 2, etc…)

SDLC Frame Format

X. 25

X. 25 • • • 1970 s 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, Qo. S, . . .

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 1980 s • L 1 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 • AAL 1: 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 Qo. S • 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