Cisco Semester 4 Chapter 2

WANS

Technologies Not Coveredin Semester 4

• DSL

• Cable Modem

• ATM

• SONET

• SMDS

WAN Services

• Layer 1– CSU/DSU connects to WAN to CSU/DSU to

other route

• Service Providers– POTS, X.25/Frame Relay, TDM or T1/E1, Call

setup, e.g. SS7– Frame Relay LMI

• Allows DLCI to become unique network addresses• DLCI value placed in frame header

DCE/DTE

• CPE – customer premises equipment– On customer premises – owned and leased

• Demaraction – point where CPE ends and local loop service begins, e.g. POP

• Local Loop – last mile – CAT 5 – goes from demarc to WAN provider central office

• CO – nearest point of presence for provider’s WAN• Toll Network – inside WAN provider cloud• DTE – typically router DCE – converts user

data into WAN acceptable format

Virtual Circuits

• Pathway through a packet-switched network that appears to be dedicated physical circuit– Can be connection-oriented (like TCP)– PVC - Customer and carrier negotiate end

points and characteristics of virtual circuit• Constantly available

– SVC – virtual circuit availably only on demand

WAN Line Types and Bandwidth

• 56 DSO 56b kbps• 64 DSO 64 kbps• T1 DS1 1.544 Mbps• E1 ZM 2.048 Mbps• E3 M3 34.068 Mbps• J1 Yi 2.048 Mbps• T3 DS3 44.736 Mbps• OC 1-48 Sonet 51.84 to 2488.32 Mbps

WAN Devices

• Router – both LAN and WAN• WAN Switches – connect to WAN bandwidth

– Layer 2 (but distinct); mostly in WAN cloud

– Frame Relay, X.25, SMDS

• Modems - CSU/DSU and TA/NT1 devices that interface ISDN services

• Communication servers to concentrate dial-in and dial-out communications

Modems

• Interprets digital and analog signals – Enable data to be transmitted over voice-grade

telephone lines

Leased Lines

• Router Port

• CSU/DSU

• Service Provider Circuit

ISDN

• TA - device used to connect ISDN BRI connection to other interfaces

WAN and OSI Model

• WANS focus on physical and data link layers– Standards describe physical-layer delivery

methods and data-link layer requirements• Addressing, flow control, encapsulation

• Agencies– ITU-T, ISO, IETF, EIA, TIA

WAN Physical Layer Protocols

• EIA/TIA 232 - common standard• Supports up to 64 kbps; resembles V.24; was RS-232

• V.35 and X.21• HSSI• DTE-DCE• Modem• Standards

– EIA/TIA 449 up to 2 Mbps– EIA/TIA 612-3 HSSI– ITU-T V.24 and .35– X.21 – used in Europe and Japan

WAN Data Link Layer

• Encapsulation Protocols– HDLC high-level data link control

• Cisco has its own proprietary version

– PPP point to point – standard• Supports several router protocols• Replaced SLIP

• WAN Technologies– Frame Relay - simplified HDLC

• Packet Switched

– ISDN – data-link signal• Circuit Switched

WAN Data Link Layer

• Dedicated Point to Point– Cisco HDLC, PPP, and LAPB (ISDN)

• Packet Switched– Frame Relay (IETF)

• X.25, Frame Relay

• Circuit Switched– ISDN D channel LAPD– ISDN B channel

WAN Data Link Detail

• Frame Relay – simple encapsulation, no error correction, rapid data transmission

• PPP (RFC 1661) developed by IETF – contains field to identify network layer protocol

• ISDN – digital; uses existing telephone lines• LAPB- used to encapsulate packets in the X.25

stack; provides reliability and flow control• Cisco/IETF – encapsulates Frame Relay traffic• HDLC – implemented differently by vendors

WAN FORMATS

• Common Fields– Flag, Address, Control, Data, FCS , Flag

• Different Fields– PPP uses Protocol field; HDLC has proprietary

field

PPP

• Described in RFC 1661

• Specifies Layer 3 protocol type

• Is a link control protocol

• Provides Authentication– PAP Password Authentication Protocol– CHAP Challenge Handshake

Authentication Protocol

HDLC

• Derived from SDLC encapsulation (Synchronous Data Link Control)

• Cisco default for serial lines– No windowing or flow control– Allows only point to point connections– Address field set to 1s– 20byte type code – not interoperable

• NOTE: Use PPP with devices not running Cisco IOS software

Dedicated/Switched Lines

• Dedicated– Leased Lines

• Fractional T1/E1• T1/E1• T3/E3

• Switched– Circuit

• POTS, ISDN, Switched 56

– Packet/Cell Switched• X.25, Frame Relay, SMDS, ATM

Packet Switched Connections

• Network devices share PVC – permanent virtual circuit– Like a point-to-point link

• Frame Relay, SMDS, and X.25

Frame Relay

• Types of Service– DLCI (Data-Link Connections Identifier)

– CIR (Committed Information Rate)

– Access Rate 56, 65 kbps or 1.544 Mbps

– Two topologies• Fully meshed – every device has PVD to every other device

• Partially meshed – star topology

• LMI global addressing allows DLCIs to become unique address for DTE device

Circuit Switched Connections

• Connection on Demand

• Relatively low bandwidth

• Uses– Remote users– Mobile users– Backup lines

DDR –Dial on Demand Routing

• Router can dynamically initiate and close circuit-switched sessions– Used as backup link when primary fails– Source of extra bandwidth when traffic on

primary link reaches a threshhold– Substitute for dedicated lines when full-time

circuit availability is not required

ISDN

• Three channels – 2 64 kbps B (bearer) and 1 16 kbps D (signaling

• Connections– TE1 connects to NT (Type 1 or 2– TE2 designates device not compatible– TA converts standard signal to ISDN– NT1 Connects 4-wire ISDN wiring– NT2 Directs traffic to and from difference subscriber

devices and NT1– S/T defines TE1 and NT; R is TE2 and TA U is NT and

ISDN cloud