A WAN is a network that covers a broad geographic area using multiple interconnected networks. The largest WAN is the Internet, which many organizations use to connect distributed sites. WANs transmit data over various technologies including telephone networks, wireless networks, fiber optic networks and protocols like Frame Relay and ATM. Error correction is important in WANs to ensure reliable data transmission over long distances between sites.

1. Chapter 11
Wide Area Network

2. What is a WAN?
• A wide area network (WAN) is one of the oldest kinds of data
communication networks
• A WAN is a distributed network that covers a broad geographic area;
a WAN typically consists of multiple networks at geographically
distributed locations that are interconnected
• Relative to LANs or MANs (see Figure 10-2) a WAN typically covers
a wider geographic area (see Figure 10-1) and operates at lower speeds
• The Internet is the largest WAN that has been created
– Many organizations leverage the Internet backbone to connect
geographically distributed sites
– Some links in the Internet backbone have transmission speeds higher than
those found in LANs and WANs

3. Figure 10-1
Figure 10-2

4. Table 10-1

5. Public Swathed telephone Network (PSTN)
• PSTNs were originally design exclusively for
telephone but have become highly sophisticated,
able to handle different kinds of data transmission,
including digital data transmission.
• PSTN consists following components:
– Subscriber wiring and equipment
– Demarcation point
– Local loops
– Central offices
– Switching offices
– Long-distance carriers
– Points of presence
– Data transmission services

6. Public Swathed telephone Network (PSTN)
• PSTN provides a number of options for data
communications, including services that routs
packets between different sites. Some available
services and transmission rates are given below:
Service Transmission Rate
– Switched 56 56Kpbs
– X.25 56Kpbs
– T1 1.544Mbps
– T3 44.736Mbps
– Frame Relay 1.544Mbps
– SMDS 1.544Mbps
– ISDN 1.544Mbps
– ATM 44.736Mbps

7. The Internet
• SLIP and PPP : Serial Line Protocol (SLIP) and Point-to-
Point Protocol (PPP) are two very common protocols used
to transmit IP packets over serial line and telephone
connections, most often as part of a dial-up Internet
connection.
• The TCP/IP protocol suite runs over a variety of network
media: IEEE802.3 (Ethernet), and 802.5 (Token Ring)
LAN X.25 line satellite links and serial lines.
• PPP is a multiprotocol transport mechanism. While SLIP is
design to handle one type of traffic (TCP/IP traffic) at a
time, PPP can transport TCP/IP traffic as well as IPX,
Apple Talk and other types of traffic simultaneously on the
same connection.

8. WAN Services Fundamentals
• There are two major categories of WAN
connections:
– Circuit-switched networks
– Packet-switched networks
• Switching is fundamental to both approaches
– Switching technologies establish paths across networks
from senders to receivers
– Switching allows connections to be established and
maintained between senders and receivers so that they
can exchange messages and information

9. Circuit-Switched Networks
• In circuit-switched networks, a switched dedicated circuit is created to
connect two (or more) parties
– To users, it is as if a direct physical point-to-point path is established
between sender and receiver
– Multiple-switches may be involved is establishing a switched connection
(see Figure 12-1)
• There are three phases to circuit-switched communications:
– Creation of the temporary circuit
– Information transmission
– Circuit termination
• Because there is a limit to the number of switched connections that can
be established at a particular point in time, circuit-switched network
users may not be able to initiate communication sessions during peak
usage times

10. Figure 12-1

11. Packet-Switched Networks
• In packet-switched networks (see Figure 12-2), data is
packetized prior to transmission
– Each packet is a group of bits organized in a predetermined
structure
– Each packet contains data bits as well as additional overhead
information to ensure error-free transmission to intended
recipients
– Packets may be called blocks, cells, datagrams, data units, or
frames
• Packet assembler/disassemblers (PADs) are responsible for
assembling outgoing data into packets for transmission over the
packet-switching network as well as for unpacking incoming
packets so that data can be delivered to intended recipients

12. Figure 12-2

13. Packet Formats
• Figure 12-3 illustrates the format of HDLC
packets used in X.25 packet-switching networks
Major overhead fields include:
– Flag: used to delimit the beginning and end of a packet
– Address: specifies the address of the intended packet
recipient
– Control: transports packet sequence numbers and
retransmission requests
– Frame check: used for error checking. CRC-16 or a 16-
bit checksum may be used with HDLC frames

14. Figure 12-3

15. Packet-Switching Advantages
and Disadvantages
• Relative to circuit-switching, packet-switching has a number of advantages
and disadvantages. Advantages include:
– A single-link between packet-switching nodes can be simultaneously
shared by multiple senders and receivers; senders are not denied access to
the network during peak usage periods
– Packet-priority systems can be established
– Subscribers to packet-switching services are often charged on the volume
of data (number of packets) transmitted rather than connection time
• Disadvantages include:
– Variable transmission delays caused by packet processing and packet
queues at packet switches
– Some packet-switching networks support variable packet sizes; this
contributes to longer packet processing times at packet switches
– The inclusion of overhead data in packets means that data transmission
efficiency and throughput is lower than that in circuit-switched networks

16. Switching Alternatives in Packet-
Switched Networks
• Two fundamental approaches are used to route packets from senders to
receivers:
– Datagram approach: individual packets, even those associated with a
single file, are routed independently
• Two packets (datagrams) from the same source can have two
different temporary circuits established to the same recipient
• This type of circuit allocation is called connectionless because a
dedicated connection is not established and because the packets that
make up a single file do not follow each other over the same circuit
from sender to receiver
– Virtual circuit approach: this is similar to establishing a dedicated circuit
in a circuit-switched network. Packets that comprise a single file (or
message) follow the same route in sequence from sender to receiver.
• This type of packet-switching is called connection-oriented
• It is not identical to circuit-switched connections because the route
segments in virtual circuits are shared, not dedicated

17. Virtual Circuits
• Call setup packets are used to establish virtual circuits; these are used
to identify the best path to the destination across the network.
• Virtual circuit details are stored in virtual circuit tables at packet
switches
• The paths followed by packets in virtual circuits are called logical
channels; each packet includes a logical channel number when created
by the PAD
• There are two major types of virtual circuits:
– Switched virtual circuits (SVCs): which are similar to temporary circuit-
switched connections
– Permanent virtual circuits (PVCs): which is similar to a leased, circuit-
switched connection
• Once a PVC is allocated, no call setup or call clearing is needed; the logical
circuit is permanently stored in virtual circuit tables

18. Packet Switching Protocol
• Packet-switching concepts
• A PDN is sometimes called an X.25 network or public data network.
– The X.25 designation stems from ITU’s recommendation X.25 which
defines the interface between DTE and DCE for public data networks (see
Figure 12-4)
– The term value-added network (VAN) is often used in conjunction with
PDNs because network proprietors offer additional services beyond mere
data transmission including virtual circuits, error recovery, network
management, message priorities, and store-and-forward capabilities
• X.25 PDNs are more widely available outside the U.S.
– In the U.S., frame relay services are more common than X.25

19. Figure 12-4

20. PDN Error Correction Processes
• PDNs employ node-to-node (aka hop-to-hop or point-to-
point) error detection and correction (see Figure 12-8)
• Each packet is checked for errors at each packet switch
before being forwarded to the next hop on its path
• If no errors are detected, an ACK is sent to the previous
hop
• If errors are detected, a NAK is sent to the previous hop
which triggers retransmission of the packet
• This process means that PDNs are store-and-forward
networks; packets are stored at switching nodes until
positive acknowledgements are received

21. Figure 12-8

22. Frame Relay Technologies
• Key technologies in frame relay networks are illustrated in
Figure 12-9. These include:
– Frame assembler/dissembler devices (FRADs) which like X.25
PADs are responsible for building outgoing frames and unpacking
incoming frames
– Frame relay switches which are responsible for accepting frames,
checking them for errors, and transmitting them to their next hops
in the network
• Both switched and permanent virtual circuits are supported in frame
relay networks
– Frame relay circuits. Frame relay switches are typcially connected
by DS-1 (T-1) or DS-3 (T-3) circuits. The Frame Relay Form
(FRF) has addressed connections up to 622 mbps (OC-12)

23. Figure 12-9

24. Frame Formats
• Frames are formatted by FRAD devices or software
• Variable length frames may be supported; some may
include up to 8,000 characters
• Figure 12-11 shows a LAPD (Link access Procedure—D
channel) frame relay
• The address field carries the recipient’s network address as
well as a data link connection identifier (DLCI) that serves
the same purpose as a virtual circuit identifier in X.25 (see
Figure 12-12)
• The BCEN, FCEN, and DE fields are used to address
network congestion during peak usage periods

25. Figure 12-11
Figure 12-12

26. Asynchronous Transfer Mode (ATM)
• ATM is a high-bandwidth, low-delay, packet-switching and multiplexing
technology that can handle many types of network traffic and WAN services
• ATM represents a step in the evolution of frame relay by using frames (called
cells) that do not vary in size
– The use of small fixed-size packets translates into easier switching and
faster transmission rates.
– By 2002, ATM transfer rates of 38.813 gbps had been achieved over OC-
768 circuits
• Virtual channels are used in ATM to establish logical connections between
senders and receivers (see Figure 12-13)
– Once setup up, full-duplex variable-rate transmission is possible over the
connections
• Virtual paths are also supported. These are bundles of virtual channels with
the same end-points that are switched as a set. Each channel can carry a
different type of data

27. Figure 12-16

28. ATM Cell Formats
• Two cell formats have been specified for
ATM (see Figure 12-14):
– User-network interface (UNI): UNI cells carry
data between the user and the ATM network
– Network-network interface (NNI): NNI cells
carry network control information between
ATM switches
• NNI also enables network control information to be
exchanged between different ATM networks

29. Figure 12-14

30. T- Carrier
T-carrier Bandwidth No. Of T1 Channels used
• T1 1.544Mbps 1
• T2 6.312Mbps 4
• T3 44.736Mbps 28
• T4 274.176Mbps 168

31. T-1 Service Access Technologies
• Businesses use a variety of services to
access T-1 services (see Figure 12-18).
These include:
– T-1 CSU/DSUs
– T-1 multiplexers
– T-1 channel banks
– T-1 switches

32. Figure 12-18

33. SONET Services
• Synchronous optical network (SONET) is an optical transmission
interface/specification for high-speed digital transmission over optical
fiber
• SONET specifications define a hierarchy of standardized data transfer
rates over optical media. An abbreviated set is provided in Table 12-2
– Each level is capable of carrying multiple lower-speed signals. An STS-1
channel, for example, is capable of carrying multiple DS-1 (T-1) signals
• STS-1 frames are the fundamental data transmission format in SONET
(see Figure 12-19)
– Each consists of 810 octets that can logically be depicted as a matrix of 9
rows with 90 octets in each row
– 87 octets in each row carry data and can be flexibly allocated to lower
bandwidth channels such as DS-0, DS-1, and DS-2
• SONET service access technologies include add-drop multiplexors,
cross-connect switches, and broadband bandwidth managers

34. Table 12-2
Figure 12-19

35. ISDN
• Integrated Services Digital Network (ISDN) is widely used by
business to provide digital WAN services among geographically
dispersed operating locations
• ISDN switches are the core of the ISDN network (see Figure 12-20)
• Two major categories of ISDN are:
– Narrowband ISDN. This is essentially a circuit-switched digital network
service that allows temporary connections to be dynamically created and
terminated among ISDN subscribers. Two narrowband service levels
exist:
• Basic rate interface (BRI) that supports two 64 kbps bearer channels and one
16 kbps data channel (2B+D)
• Primary rate interface (PRI) with 23 64 bps bearer channels and a 64 bps data
channel (23B+D)
– Broadband ISDN (B-ISDN) which may be described as ATM over
SONET

36. Figure 12-20

37. Wireless WAN Services
• Increasingly, organizations are turning to wireless WAN
services to satisfy their data communication needs
including:
– Circuit-switched cellular systems (see Figure 12-21)
– Cellular digital packet data (CDPD)
– ARDIS (Advanced Radio Data Information Service)
– Mobitex
– Metricom (see Figure 12-22)
– Personal communication services (PCS)
– Broadband wireless services (such as wireless T-1 service)

38. Figure 12-21

39. Figure 12-22

40. Fiber Distributed Data Interface (FDDI)
• FDDI (standardized as ANSI X3T9.5) backbone
protocol was developed in the 1980s and popular
during the 80s and 90s.
• FDDI operates at 100 Mbps over a fiber optic
cable.
• Copper Distributed Data Interface (CDDI) is a
related protocol using cat 5 twisted wire pairs.
• FDDI’s future looks limited, as it is now losing
market share to Gigabit Ethernet and ATM.

41. FDDI Topology (Figure 7-15)
• FDDI uses both a physical and logical ring
topology capable of attaching a maximum
of 1000 stations over a maximum path of
200 km. A repeater is need every 2 km.
• FDDI uses dual counter-rotating rings
(called the primary and secondary). Data
normally travels on the primary ring.
• Stations can be attached to the primary ring
as single attachment stations (SAS) or both
rings as dual attachment stations (DAS).

42. Figure 7-15 FDDI

43. FDDI’s Self Healing Rings
• One important feature of FDDI is its ability
to handle a break in the ring to form a
temporary ring out of the pieces of the two
rings.
• Figure 7-16, show an example of a cable
break between two dual-attachment
stations.
• After the cable break is detected, a single
ring is formed out of the primary and
secondary rings until the cable break can be
repaired.

44. Figure 7-16 FDDI’s Self-healing Rings

45. FDDI Media Access Control
• FDDI uses a token passing system. Computers wanting to
send packets wait to receive a token before transmitting.
• Multiple packets can be attached to the token as it moves
around the network.
• When a station receives the token, it looks for attached
packets addressed to it and removes them from the incoming
packet.
• If the station wants to send a packet it attaches it to the token
and sends the token with its attached packets to the next
station.
• This controlled access technique provides a higher
performance level at high traffic levels compared to a
contention-based technique like Ethernet.

46. Advantages and Disadvantages of
FDDI: Advantages
• High bandwidth
• Security
• Physical durability
• Resistance to EMI
• Cable distance
• Weight
• Use of multiple token
• Ability to prioritize workstation
• System fault tolerance

47. Advantages and Disadvantages of
FDDI: Disadvantages
• Complex technology
• Installation and maintenance require a great
deal of expertise
• Cost

48. Asynchronous Transfer Mode (ATM)
• Asynchronous Transfer Mode (ATM) (also
called cell relay) is a technology originally
designed for use in wide area networks that
is now often used in backbone networks.
• ATM backbone switches typically provide
point-to-point full duplex circuits at 155
Mbps (total of 310 Mbps).

49. ATM vs. Switched Ethernet
• ATM is a switched network, but differs from
switched Ethernet in four ways:
1. ATM uses small, fixed-length packets of 53 bytes
(called cells). Ethernet frames are variable and can
be up to about 1 kilobyte in length.
2. ATM provides no error correction on the user
data. Switched Ethernet does error correction.
3. ATM uses virtual channels instead of the fixed
addresses used by traditional data link layer
protocols such as switched Ethernet (see Fig. 7-17).
4. ATM prioritizes transmissions based on Quality of
Service (QoS), while switched Ethernet does not.

50. Figure 7-17 Addressing & Forwarding
with ATM Virtual Circuits

51. ATM’s Virtual Circuits
• ATM is connection-oriented, meaning all
packets travel in order through the same
virtual circuit.
• There are two types of ATM virtual
circuits:
– Permanent Virtual Circuits (PVCs) - defined
when the network is established or modified.
– Switched Virtual Circuits (SVCs) - defined
temporarily for one transmission and deleted
when the transmission is completed.

52. LAN Encapsulation
• The first step in LAN Encapsulation is to create an
ATM virtual circuit identifier for the virtual circuit
that will connect the “gateway” ATM edge switch
to the ATM edge switch nearest the frame’s
destination (see Figure 7-18)
• Once the virtual circuit is ready, the Ethernet
frame is broken up into a series of ATM cells and
sent over the ATM backbone using the ATM
virtual circuit identifier.
• At the receiving edge switch the frame is
reassembled. Unfortunately LAN has very high
overhead and so network performance suffers as a
consequence.

53. Figure 7-18 ATM Encapsulation

54. ATM to the Desktop
• ATM-25 is a low-speed option that provides
point-to-point full duplex circuits at 25.6 Mbps in
each direction. It is an adaptation of token ring
that runs over cat 3 cable and can even use token
ring hardware if modified.
• ATM-51 is designed for the desktop allowing
51.84 Mbps from computers to the switch.
• Both these ATMs appear to be good choices for
desktop connections when ATM backbone
networks are used. However, industry has been
very slow to accept either and have instead moved
to Fast Ethernet which is both cheaper and faster.