1. WANs

2. Table of Contents WAN Technology & Devices WANs & The OSI Model
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WAN Technology & Devices
Go There!
WANs & The OSI Model
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WAN Encapsulation Formats
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WAN Link Options

3. WAN Technology & Devices
Table of Contents

4. WAN Services
A WAN is the communications network that connects LANs through a WAN Service Provider (e.g. RBOC)
WANs operate at the first three layers of the OSI, but focus mainly on the physical and data link layers.
Examples of WAN Services include….
Call-setup
SS7 used for telephone calls
ISDN
A circuit-switched service using TDM, which allocates bandwidth to different sources on the same media
Frame Relay
A packet-switched service using statistical multiplexing

5. WAN Physical Structure
Toll Network – collection of switches/trunks in WAN cloud
CO Switch – nearest point of presence for the WSP’s service
Local loop – extends from the CPE (at the demarc) to the CO
CPE – devices located on premises, either owned or leased

6. DTE/DCE Interface
The transfer of responsibility for service between the ISP and the customer occurs at the DTE/DCE interface.
The DTE generates the data and passes them, along with any necessary control characters to a DCE.
The DCE converts the signal to a format appropriate to the transmission medium and introduces it into the network link.
(Reverse process at the receiving end)

7. WAN Virtual Circuits
Switched Virtual Circuits (SVCs) are WAN paths to the destination established and terminated on demand.
Three phases:
Circuit establishment – creates the virtual circuit
Data transfer – sending and receiving user data
Circuit termination – tearing down the virtual circuit
Telephone service and ATM use SVCs
Increased use of bandwidth but decreased cost
Permanent Virtual Circuits (PVCs) are permanently established circuits with one mode: data transfer
X.25 and Frame Relay use PVCs
Decreased use of bandwidth but increased cost

8. Link Types & Bandwidth

9. WAN Devices
In order to connect to a leased line, the customer must have…
Access to the service provider’s circuit
An appropriate router port available
An a CSU/DSU, modem or ISDN Terminal Adapter

10. Routers Routers offer many services including: Internetworking
WAN serial interfaces
Routers can operate as...
Internal Routers
Backbone Routers
Area Border Routers
Autonomous System Boundary Routers

11. Modems
Also called CSU/DSUs (channel service units/digital service units)
Interface with voice-grade connection in order to convert analog signal to digital.
Represents the DCE side of the DTE/DCE connection.
Leased line
Router
CSU/DSU

12. WANs & The OSI Model
Table of Contents

13. WAN Standards What layers of the OSI model do WAN standards describe?
Physical and Data Link Layers

14. WAN Physical Layer
Protocols that describe how to provide electrical, mechanical, operational, and functional connections for WAN services.
These services are most often obtained from WAN service providers such as RBOCs, alternate carriers, post, telephone and telegraph (PTT) agencies.
Describes the interface between the data terminal equipment (DTE) and the data circuit-terminating equipment (DCE).

15. WAN Physical Layer
Typically, the DCE is the service provider and the DTE is the attached device.
In this model, the services offered to the DTE are made available through a modem or a CSU/DSU.

16. WAN Physical Layer
Several physical layer standards specifying this interface between the DTE & DCE are...
EIA/TIA-232
EIA/TIA-449
V.24
V.35
X.21
G.703
EIA-530

17. WAN Data-Link Layer
WAN data link protocols describe how frames are carried between systems on a single data link.
They include protocols designed to operate over dedicated point-to-point, multipoint, and multi-access switched services.
WAN standards are defined and managed by a number of recognized authorities, including the following agencies: ITU-T, ISO, IETF, & EIA

18. WAN Encapsulation Formats
Table of Contents

19. WAN Data-Link Encapsulations
PPP (Point-to-Point Protocol)
Developed by IETF; replacing SLIP
Contains a field to identify the network layer protocol
PPP can check for link quality during connection establishment
Supports PAP (Password Authentication Protocol) & CHAP (Challenge Handshake Authentication Protocol)

20. WAN Data-Link Encapsulations
High-Level Data Link Control (HDLC)
Cisco default encapsulation; typically used between routers running Cisco IOS; replacing SDLC
Streamlined: no windowing or flow control
may not be compatible with different vendors because of the way each vendor has chosen to implement it.
HDLC supports both point-to-point and multipoint configurations with minimal overhead

21. WAN Data-Link Encapsulations
Frame Relay
uses high-quality digital facilities;
uses simplified framing with no error correction mechanisms (connectionless!!);
it can send Layer 2 information much more rapidly than other WAN protocols

22. WAN Link Options
Table of Contents

23. WAN Link Options Dedicated Switched T1, E1, T3, E3 xDSL SONET
Circuit Switched
Packet/Cell Switched
POTS
ISDN
X.25
Frame Relay
ATM (cell)
SMDS (cell)

24. WAN Link Options Dedicated Switched T1, E1, T3, E3 xDSL SONET
Circuit Switched
Packet/Cell Switched
POTS
ISDN
X.25
Frame Relay
ATM (cell)
SMDS (cell)

25. Dedicated Digital Services
Dedicated Digital Services provide full-time connectivity through a point-to-point link
T series in U.S. and E series in Europe
Uses time division multiplexing to “slice up” data and assign time slots for transmissions
T1 = Mbps
T3 = Mbps
E1 = Mbps
E3 = Mbps
Uses twisted pair & fiber
Extremely popular
Moderate cost

26. Dedicated Digital Services
Digital Subscriber Lines (xDSL); the x stands for a family of technologies
WAN Technology for home use; decreasing bandwidth with increasing distance from the phone companies CO.
Data rates as high as Mbps but more common to be in the 100s of Kbps
Varieties include HDSL, SDSL, ADSL, & VDSL
Moderate expense and getting cheaper

27. Dedicated Digital Services
Synchronous Optical Network (SONET)
Specialized high bandwidth technology for use at various Optical Carrier speeds (OC) ranging from Mbps (OC-1) to 9,952 Mbps (OC-192)
Uses lasers to divide the wavelength of the light into sections that can carry large amounts of data (Wave Division Multiplexing)
Very expensive; used by large ISPs and other Internet backbone entities.

28. WAN Link Options Dedicated Switched T1, E1, T3, E3 xDSL SONET
Circuit Switched
Packet/Cell Switched
POTS
ISDN
X.25
Frame Relay
ATM (cell)
SMDS (cell)

29. Circuit Switched Services
Plain Old Telephone System (POTS)
Not a computer data service but...
POTS is an important component of our communication infrastructure and
It is still the standard for designing reliable networks

30. Circuit Switched Services
Integrated Services Digital Network (ISDN)
Historically important--first dial-up digital service
Provides both voice & data transmission over existing phone lines
Cost is moderate; max. bandwidth = 128 kbps for BRI (Basic Rate Interface)
2 B 64kps and 1 D 16kps
B channels are voice/data channels; D for signaling B D

31. WAN Link Options Dedicated Switched T1, E1, T3, E3 xDSL SONET
Circuit Switched
Packet/Cell Switched
POTS
ISDN
X.25
Frame Relay
ATM (cell)
SMDS (cell)

32. Packet Switched Services
X.25 (Connection-oriented)
Older WAN technology developed in 1970s
Reliable--X.25 has been extensively debugged and is now very stable--literally no errors in modern X.25 networks
Store & Forward--Since X.25 stores the whole frame to error check it before forwarding it on to the destination, it has an inherent delay (unlike Frame Relay) and requires large, expensive memory buffering capabilities.

33. Packet Switched Services
Frame Relay (Connectionless)
More efficient and much faster than X.25
Packet switched version of ISDN (which is circuit switched); data rates up to Mbps with 56kbps and 384kbps being the most popular
Used mostly to forward LAN IP and IPX packets but can be used to forward other types of traffic
Primary competitive advantage is its low cost

34. Cell Switched Services
Asynchronous Transfer Mode (ATM)
Relatively new WAN Technology related to broadband ISDN; max. bandwidth = 622 Mbps
Developed in order to provide one technology for both WANs and LANs to transport data, video, and voice. (High Cost)
Key Benefits:
One network for all traffic--voice, data, video
Compatible with current wiring infrastructure (cable plant)
Very flexible and scalable
Simplifies network management

35. Cell Switched Services
Switched Multimegabit Data Service (SMDS)
Closely related to ATM; SMDS is the MAN (Metropolitan Area Network) implementation of ATM
High Cost with max. bandwidth Mbps

36. WAN Link Options Review

37. Table of Contents
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