1. Wide Area Networks

2. Copyright 2005 John Wiley & Sons, Inc
Introduction
Metropolitan area networks (MANs)
Span from 3 to 30 miles and connect backbone networks (BNs) and LANs
Wide area networks (WANs)
Connect BNs and MANs across longer distances, often hundreds of miles or more
Typically built by using leased circuits from common carriers such as AT&T
Most organizations cannot afford to build their own MANs and WANs,
Copyright 2005 John Wiley & Sons, Inc

3. Services Used by MANs/WANs
Circuit Switched Network Services
Dedicated Circuit Networks Services
Packet Switched Networks Services
Virtual Private Networks Services
Copyright 2005 John Wiley & Sons, Inc

4. Circuit Switched Services
Oldest and simplest MAN/WAN approach
Uses the Public Switched Telephone Network (PSTN)
i.e., telephone networks
Provided by common carriers like AT&T and Ameritech
Basic types in use today:
POTS (Plain Old Telephone Service)
Via use of modems to dial-up and connect to ISPs
ISDN (Integrated Services Digital Network )
Copyright 2005 John Wiley & Sons, Inc

5. Basic Architecture of Circuit Switched Services
“Cloud” architecture
Simpler design:
What happens inside of network is hidden from the user
A computer using modem dials the number of a another computer and creates a temporary circuit
Can be expensive (connection and traffic based payment)
When session is completed, circuit is disconnected.
Copyright 2005 John Wiley & Sons, Inc

6. POTS based Circuit Switched Services
Use regular dial-up phone lines and a modem
Modem used to call another modem
Once a connection is made, data transfer begins
Commonly used to connect to the Internet by calling an ISP’s access point
Wide Area Telephone Services (WATS)
Wholesale long distance services used for both voice and data
Users buy so many hours of call time per month (e.g., 100 hours per month) for one fixed rate
Copyright 2005 John Wiley & Sons, Inc

7. ISDN based Circuit Switched Services
Combines voice, video, and data over the same digital circuit
Sometimes called narrowband ISDN
Provides digital dial-up lines (each requires):
An “ISDN modem” which sends digital transmissions is used
Also called: Terminal Adapter (TA)
An ISDN Network Terminator (NT-1 or NT-2)
Each NT needs a unique Service Profile Identifier (SPID)
Acceptance has been slow
Lack of standardization, different interpretations. and relatively high cost
ISDN: I Still Don’t Know
Copyright 2005 John Wiley & Sons, Inc

8. Copyright 2005 John Wiley & Sons, Inc
Types of ISDN Services
Basic rate interface (BRI)
Basic access service or 2B+D
Two 64 Kbps bearer ‘B’ channels (for voice or data)
One 16 Kbps control signaling ‘D’ channel
Can be installed over existing telephones lines (if less than 3.5 miles)
Requires BRI specific end connections
Primary rate interface (PRI)
Primary access service or 23B+D
Twenty three 64 Kbps ‘B’ channels
One 64 Kbps ‘D’ channel (basically T-1 service)
Requires T1 like special circuit
Copyright 2005 John Wiley & Sons, Inc

9. Copyright 2005 John Wiley & Sons, Inc
Broadband ISDN
A circuit-switched service but it uses ATM to move data
Backwardly compatible with ISDN.
B-ISDN services offered:
Full duplex channel at Mbps
Full duplex channel at Mbps
Asymmetrical service with two simplex channels (Upstream: Mbps, downstream: Mbps)
Copyright 2005 John Wiley & Sons, Inc

10. Circuit Switched Services
Simple, flexible, and inexpensive
When not used intensively
Main problems
Varying quality
Each connection goes through the regular telephone network on a different circuit,
Low Data transmission rates
Up to 56 Kbps for POTS, and up to 1.5 Mbps for ISDN
An alternative
Use a private dedicated circuit
Leased from a common carrier for the user’s exclusive use 24 hrs/day, 7 days/week
Copyright 2005 John Wiley & Sons, Inc

11. Copyright 2005 John Wiley & Sons, Inc
Dedicated Circuits
Leased full duplex circuits from common carriers
Used to create point to point links between organizational locations
Routers and switches used to connect these locations together to form a network
Billed at a flat fee per month (with unlimited use of the circuit)
Require more care in network design
Basic dedicated circuit architectures
Ring, star, and mesh
Dedicated Circuit Services
T carrier services
Synchronous Optical Network (SONET) services
Copyright 2005 John Wiley & Sons, Inc

12. Copyright 2005 John Wiley & Sons, Inc
Ring Architecture
Reliability
Messages can be rerouted around the failed link (Data can flow in both directions (full-duplex circuits))
With the expense of dramatically reduced performance
Performance
Messages need to travel through many nodes before reaching their destination
Copyright 2005 John Wiley & Sons, Inc

13. central routing computer Copyright 2005 John Wiley & Sons, Inc
Star Architecture
Easy to manage
Central computer routes all messages in the network
Reliability
Failure of central computer brings the network down
Failure of any circuit or computer affects one site only
Performance
Central computer becomes a bottleneck under high traffic
central routing computer
Copyright 2005 John Wiley & Sons, Inc

14. Copyright 2005 John Wiley & Sons, Inc
Mesh Architectures
Combine performance benefits of ring and star networks
Use decentralized routing, with each computer performing its own routing
Impact of losing a circuit is minimal (because of the alternate routes)
More expensive than setting up a star or ring network.
Setting up alternate routes between computers
Full mesh
Expensive, seldom used
Partial mesh
More practical
Copyright 2005 John Wiley & Sons, Inc

15. Copyright 2005 John Wiley & Sons, Inc
T-Carrier Services
Most commonly used dedicated digital circuits in North America
Units of the T-hierarchy
DS-0 (64 Kbps); Basic unit
T-1 (a.k.a. DS-1) (1.544 Mbps)
Allows 24 simultaneous 64 Kbps channels which transport data or voice messages using PCM
T-2 (6.312 Mbps) multiplexes 4 T-1 circuits
T-3 ( Mbps); 28 T-1 capacity
T-4 ( Mbps); 178 T-1 capacity (672 DS-0 channels)
Fractional T-1, (FT-1) offers a portion of a T-1
Copyright 2005 John Wiley & Sons, Inc

16. T-Carrier Digital Hierarchy
T-Carrier Designation DS Designation Data Rate
T-1
T-2
T-3
T-4
DS-0
DS-1
DS-2
DS-3
DS-4
64 kbps
1.544 Mbps
6.312 Mbps
Mbps
Mbps
Copyright 2005 John Wiley & Sons, Inc

17. Synchronous Optical Network (SONET)
ANSI standard for optical fiber transmission in Gbps range
Similar to ITU-T-based, synchronous digital hierarchy (SDH)
SDH and SONET can be easily interconnected
SONET hierarchy
Begins with OC-1 (optical carrier level 1) at Mbps
Each succeeding SONET hierarchy rate is defined as a multiple of OC-1
Copyright 2005 John Wiley & Sons, Inc

18. SONET Digital Hierarchy
SONET Designation SDH Designation Data Rate
OC-1
OC-3
OC-9
OC-12
OC-18
OC24
OC-36
OC-48
OC-192
STM-1
STM-3
STM-4
STM-6
STM-8
STM-12
STM-16
51.84 Mbps
Mbps
Mbps
Mbps
Mbps
1.244 Gbps
1.866 Gbps
2.488 Gbps
9.952 Gbps
Copyright 2005 John Wiley & Sons, Inc

19. Packet Switched Services
In both circuit switched and dedicated services
A circuit established between two computers
Solely assigned for use only between these two computers
Data transmission provided only between these two computers
No other transmission possible until the circuit is closed
Packet switched services
Enable multiple connections to exist simultaneously between computers over the same physical circuits
User pays a fixed fee for the connection to the network plus charges for packets transmitted
Copyright 2005 John Wiley & Sons, Inc

20. Basic Architecture of Packet Switched Services
Packet assembly/ disassembly device (PAD).
Owned by the customer or the common carrier
Users buy a connection into the common carrier network, and connect via a PAD
Point-of-Presence (POP)
leased dedicated circuits
Copyright 2005 John Wiley & Sons, Inc

21. Copyright 2005 John Wiley & Sons, Inc
Packet Switching
Interleave packets from separate messages for transmission
Most data communications consists of short burst of data
Packet switching takes advantage of this burstiness
Interleaving bursts from many users to maximize the use of the shared network
Copyright 2005 John Wiley & Sons, Inc

22. Packet Routing Methods
Describe which intermediate devices the data is routed through
Connectionless (Datagram)
Adds a destination and sequence number to each packet
Individual packets can follow different routes
Packets reassembled at destination (by using their sequence numbers)
Connection Oriented (Virtual Circuit (VC))
Establishes an end-to-end circuit between the sender and receiver (before the packets sent)
All packets for that transmission take the same route over the virtual circuit established
Same physical circuit can carry many VCs
Copyright 2005 John Wiley & Sons, Inc

23. Types of Virtual Circuits
Permanent Virtual Circuit (PVCs)
Established for long duration (days or weeks)
Changed only by the network manager
More commonly used
Packet switched networks using PVCs behave like a dedicated circuit networks
Switched Virtual Circuit (SVC)
Established dynamically on a per call basis
Disconnected when the call ends
Copyright 2005 John Wiley & Sons, Inc

24. Data Rates of Virtual Circuits
Users specify the rates per PVC via negotiations
Committed information rate (CIR)
Guaranteed by the service provider
Packets sent at rates exceeding the CIR are marked discard eligible (DE),
Discarded if the network becomes overloaded
Maximum allowable rate (MAR)
Sends data only when the extra capacity is available
Copyright 2005 John Wiley & Sons, Inc

25. Packet Switched Service Protocols
X.25
Asynchronous Transfer Mode (ATM)
Frame Relay
Switched Multimegabit Data Service (SMDS)
Ethernet/IP packet networks
Copyright 2005 John Wiley & Sons, Inc

26. Copyright 2005 John Wiley & Sons, Inc
X.25
Oldest packet switched service
A standard developed by ITU-T
Offers SVC and PVC services
Uses LAPB and PLP protocols at the data link and network layers, respectively
Requires protocol translations at PADs (for those users who use different protocols at their LANs)
A reliable protocol (it performs error control and retransmits bad packets)
Widely used in Europe
Not in widespread use in North America
Low data rates (64 Kbps) (available now at Mbps)
Copyright 2005 John Wiley & Sons, Inc

27. Asynchronous Transfer Mode (ATM)
Newer than X.25; also standardized
ATM in MAN/WAN similar to ATM technology discussed for BNs
Similar to X.25
Provides packet switching service
Different than X.25: Operating characteristics
Performs encapsulation (no translation) of packets
Provides no error control (an unreliable protocol)
Provides extensive QoS information
Scaleable (easy to multiplex ATM circuits onto much faster ones)
Copyright 2005 John Wiley & Sons, Inc

28. Error Control in X.25 vs. ATM
Error control in ATM is handled typically the transport layer (providing end-to-end communications)
ACKs sent immediately by each node
ACKs sent by final destination
Copyright 2005 John Wiley & Sons, Inc

29. Copyright 2005 John Wiley & Sons, Inc
ATM Features
Uses fixed length, 53 byte “cells”
5 bytes of overhead and 48 bytes of user data
More suitable for real time transmissions.
Provides extensive QoS information
Enables setting of precise priorities among different types of transmissions (i.e. voice, video & )
Data Rates
Same rates as SONET: 51.8, 466.5, Mpbs
New versions: T1 ATM (1.5 Mbps), T3 ATM (45 Mbps)
Copyright 2005 John Wiley & Sons, Inc

30. Copyright 2005 John Wiley & Sons, Inc
Frame Relay
Another standardized technology
Faster than X.25 but slower than ATM
Encapsulates packets
Packets delivered unchanged through the network
Unreliable, like ATM
Up to the end-points to control the errors
NO QoS support (under development)
Common CIR speeds:
56, 128, 256, 384 Kbps, 1.5, 2, and 45 Mbps
Copyright 2005 John Wiley & Sons, Inc

31. Copyright 2005 John Wiley & Sons, Inc
SMDS
A non-standardized technology
Developed by Telcordia for local phone companies
Unreliable, like ATM
Encapsulates packets
Originally developed for MANs, but could be used for WANs as well
Transmission speeds offered:
56 Kbps to 45 Mbps
Uncertain future
Not standardized; competition from FR, ATM, and others
Copyright 2005 John Wiley & Sons, Inc

32. Ethernet/IP Packet Networks
Offer Ethernet/IP packet services for building MAN/WAN networks
Gigabit Ethernet fiber optic networks (bypassing common carrier network)
Currently offer CIR speeds from 1 Mbps to 1 Gbps at 1/4 the cost of more traditional services
No need to translate LAN protocol (Ethernet/IP) to the protocol used in MAN/WAN services
X.25, ATM, Frame Relay and SMDS use different protocols requiring translation from/to LAN protocols
Emerging technology; expect changes
Copyright 2005 John Wiley & Sons, Inc

33. Virtual Private Networks
Provides equivalent of a private packet switched network over public Internet
Use PVCs (tunnels) that run over the Internet
Appear to the user as private networks
Encapsulate the packets sent over these tunnels
Using special protocols that also encrypt the IP packets they enclose
Provides low cost and flexibility
Uses Internet; Can be setup quickly
Disadvantages of VPNs:
Unpredictability of Internet traffic
Lack of standards for Internet-based VPNs, so that not all vendor equipment and services are compatible
Copyright 2005 John Wiley & Sons, Inc

34. Copyright 2005 John Wiley & Sons, Inc
VPN Architecture
ISP
Access
Server
VPN
Device
leased circuits
Office
Telephone
Line
VPN
Device
Employee’s
Home
Internet
Backbone
VPN Tunnel
VPN Tunnel
Office
VPN
Device
VPN is transparent to the users, ISP, and the Internet as a whole;
It appears to be simply a stream of packets moving across the Internet
Backbone
Copyright 2005 John Wiley & Sons, Inc

35. VPN Encapsulation of Packets
Packet from the client computer
Packet in transmission through the Internet
PPP IP
TCP
SMTP
ATM IP
L2TP
PPP IP
TCP
SMTP
ISP
L2TP: Layer 2 Tunneling Protocol
(An emerging VPN Layer-2 access protocol)
Telephone
Line
Access
Server
VPN
Device
Employee’s
Home
Packet from the VPN
VPN Tunnel
PPP IP
TCP
SMTP
Outgoing packets from the VPN are sent through specially designed routers or switches.
Internet
VPN
Device
Access
Server
VPN Encapsulation of Packets
Backbone
Copyright 2005 John Wiley & Sons, Inc

36. Copyright 2005 John Wiley & Sons, Inc
VPN Types
Intranet VPN
Provides virtual circuits between organization offices over the Internet
Extranet VPN
Same as an intranet VPN except that the VPN connects several different organizations, e.g., customers and suppliers, over the Internet
Access VPN
Enables employees to access an organization's networks from remote locations
Copyright 2005 John Wiley & Sons, Inc

37. MAN/WAN Design Practices
Difficult to recommend best practices
Services, not products, being bought
Fast changing environment with introduction of new technologies and services from non-traditional companies
Factors used
Effective data rates and cost
Reliability
Network integration
Design Practices
Start with flexible packet switched service
Move to dedicated circuit services, once stabilized
May use both: packet switched services as backup
Copyright 2005 John Wiley & Sons, Inc

38. Improving MAN/WAN Performance
Handled in the same way as improving LAN performance
By checking the devices in the network,
By upgrading the circuits between computers
By changing the demand placed on the network
Copyright 2005 John Wiley & Sons, Inc

39. Improving Device Performance
Upgrade the devices (routers) and computers that connect backbones to the WAN
Select devices with lower “latency”
Time it takes in converting input packets to output packets
Examine the routing protocol (static or dynamic)
Dynamic routing
Increases performance in networks with many possible routes from one computer to another
Better suited for “bursty” traffic
Imposes an overhead cost (additional traffic)
Reduces overall network capacity
Should not exceed 20%
Copyright 2005 John Wiley & Sons, Inc

40. Improving Circuit Capacity
Analyze the traffic to find the circuits approaching capacity
Upgrade overused circuits
Downgrade underused circuits to save cost
Examine why circuits are overused
Caused by traffic between certain locations
Add additional circuits between these locations
Capacity okay generally, but not meeting peak demand
Add a circuit switched or packet switched service that is only used when demand exceeds capacity
Caused by a faulty circuit somewhere in the network
Replace and/or repair the circuit
Make sure that circuits are operating properly
Copyright 2005 John Wiley & Sons, Inc

41. Reducing Network Demand
Determine impact on network
Require a network impact statement for all new application software
Use data compression of all data in the network
Shift network usage
From peak or high cost times to lower demand or lower cost times
e.g., transmit reports from retail stores to headquarters after the stores close
Redesign the network
Move data closer to applications and people who use them
Use distributed databases to spread traffic across
Copyright 2005 John Wiley & Sons, Inc

42. Implications for Management
Changing role of networking and telecom managers
Increased and mostly digitized data transmission causing the merger of these positions
Changing technology
Increasing dominance of VPNs, Frame Relay and Ethernet/IP
Decreasing cots of setting up MANs/WANs
Changing vendor profiles
From telecom vendors to vendors with Ethernet and Internet experiences
Copyright 2005 John Wiley & Sons, Inc