WANs and Routers

1
WANs and Routers

• Shawn Shuoh-Ren Tsai
• Cisco Academy Training Center
• David Ta-Wei Shih
• CGUIC

2
WANs introduction

• Operates at the physical layer and the data link
  layer of the OSI reference model
• Interconnects LANs, usually separated by large
  geographic areas
• Use serial connections of various types to access
  network bandwidth

3
WAN Devices
CO
CPE
CPE
CO
CO
NIU
DTE
DCE
DTE
DCE
NIU
LAN
LAN
router
router
demarc
demarc
CSU/DSU
CSU/DSU
Local loop
Local loop
CO
PSE
4
WAN Devices

• CPE (Customer premises equipment)
• DTE (data terminal equipment)
• servers , switches, routers etc.
• DCE (data circuit-terminating equipment) /
  Interface
• Modems, CSU/DSU (channel service unit/data
  service unit), TAs etc.
• Demarc NIU (network interface unit)
• Local loop
• CO (central office) POP (point of presence)
• PSE (packet switching exchange)

5
WAN Standards

• WAN standards describe both physical layer and
  data link layer

6
WAN physical layer standards

• The WAN physical layer describes the interface
  between the DTE and the DCE
• EIA/TIA-232
• EIA/TIA-449
• V.24
• V.35
• X.21
• G.703
• EIA-530

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WAN data link encapsulation

• The common data link encapsulations are
  associated with serial lines
• High-Level Data Link Control (HDLC)
• Point-to-Point Protocol (PPP)
• Frame Relay
• Other serial lines
• Simple Data Link Control Protocol (SDLC)
• Serial Line Interface Protocol (SLIP)
• Link Access Procedure Balanced (LAPB)
• Link Access Procedure D-channel (LAPD)

9
WAN data link layer standards

• High-Level Data Link Control (HDLC) -- an IEEE
  standard
• may not be compatible with different vendors
• supports both point-to-point and multipoint
  configurations with minimal overhead
• Cisco router default

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WAN data link layer standards (cont)

• Point-to-Point Protocol (PPP)
• Much like HDLC, with an added field to specify
  the upper-layer protocol
• Default
• connection-oriented
• bit-oriented mode that is reliable
• Can also operate in
• byte-oriented mode that is unreliable

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WAN data link layer standards (cont)

• Frame Relay -- uses high-quality digital
  facilities
• uses simplified framing with no error correction
  mechanisms,
• send Layer 2 information much more rapidly than
  other WAN protocols

12
WAN data link layer standards (cont)

• Simple Data Link Control Protocol (SDLC)
• an IBM-designed WAN data link protocol
• Serial Line Interface Protocol (SLIP)
• an extremely popular WAN data link protocol for
  carrying IP packets
• being replaced in many applications by the more
  versatile PPP

13
WAN data link layer standards (cont)

• Link Access Procedure Balanced (LAPB)
• a data link protocol used by X.25
• has extensive error checking capabilities
• Link Access Procedure D-channel (LAPD)
• the WAN data link protocol used for signaling and
  call setup on an ISDN D-channel.

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WAN Technologies

• Dedicated Digital Services
• Circuit-Switched Services
• Packet-Switched Services
• Cell-Switched Services

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WAN Technologies

• Dedicated Digital Services
• T1, T3, E1, E3 -- the T series of services in the
  U.S. and the E series of services in Europe
• bandwidth
• T1 -- 1.544 Mbps
• T3 -- 44.736 Mbps
• E1 -- 2.048 Mbps
• E3 -- 34.368 Mbps

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WAN Technologies (cont)

• Dedicated Digital Services
• xDSL (DSL for Digital Subscriber Line and x for a
  family of technologies)
• a new and developing WAN technology intended for
  home use
• has a bandwidth which decreases with increasing
  distance from the phone companies equipment

18
WAN Technologies (cont)

• Circuit-Switched Services
• POTS (Plain Old Telephone Service) / PSTN (public
  switched telephone network)
• not a computer data service
• Narrowband ISDN (Integrated Services Digital
  Network
• 128 kbps for the lower cost BRI (Basic Rate
  Interface)
• about 3 Mbps for the PRI (Primary Rate Interface)

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WAN Technologies (cont)

• Packet-Switched Services
• X.25
• usage is fairly extensive
• bandwidth may be as high as 2 Mbps
• cost is moderate
• typical medium is twisted-pair copper wire

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WAN Technologies (cont)

• Packet-Switched Services
• Frame Relay
• has become an extremely popular WAN technology
• more efficient than X.25, but with similar
  services
• maximum bandwidth is 44.736 Mbps 56kbps and
  384kbps are extremely popular in the U.S.
• cost is moderate to low
• typical media include twisted-pair copper wire
  and optical fiber

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WAN Technologies (cont)

• Cell-Switched Services
• ATM (Asynchronous Transfer Mode)
• closely related to broadband ISDN
• becoming an increasingly important WAN (and even
  LAN) technology
• uses small, fixed length (53 byte) frames to
  carry data
• maximum bandwidth is currently 622 Mbps
• SMDS (Switched Multimegabit Data Service)
• closely related to ATM, and typically used in
  MANs maximum bandwidth is 44.736 Mbps

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Router Basics

• Internal configuration components of a router are
  as follows
• ROM -- contains power-on diagnostics, a bootstrap
  program, and operating system software
• Flash -- erasable, reprogrammable ROM holds the
  operating system image and microcode

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Router Basics (cont)

• NVRAM -- nonvolatile RAM
• stores a routers backup/startup configuration
  file content remains when you power down or
  restart.
• RAM/DRAM
• Stores routing tables, ARP cache, fast-switching
  cache, packet buffering (shared RAM), and packet
  hold queues.
• interface

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The Startup Routines (cont.)

• power-on self test (POST)
• bootstrap
• Load IOS from flash
• Load configuration file from NVRAM
• If no configuration file, then setup dialog will
  be executed.