Chapter 11: WAN Technologies
1. On the Test
2.11: Identify the basic characteristics (e.g., speed, capacity, media) of the
following WAN technologies: Packet switching vs. circuit switching; ISDN;
FDDI; ATM; Frame Relay; SONet/SDH; T1/E1; T3/E3; OCx.
2. Wide-Area Networking (WAN) Technologies
a. A wide area network (WAN) is a data communications network that spans
a large geographic area and usually makes use of transmission facilities of
third-party vendors.
b. Third-party vendors are usually referred to as common carriers.
c. WANs are used to interconnect remote locations of a business operation.
d. The functions of WAN technologies are clustered at the three lowest
layers of the OSI Model.
3. Switching Technologies
Two types of switching technologies are used with WAN connections: circuit
switching and packet switching.
4. Circuit Switching
a. A circuit is a path between two or more communicating entities and
usually refers to the physical path the data takes.
b. Circuits may be pre-reserved, as with a dedicated connection between two
end points.
5. Virtual Circuits
a. Logical circuits represent a path between two points and appear to indicate
a physical path, but usually the data will travel over many different paths
between the source and destination. This type of logical circuit is called a
virtual circuit.
b. One type of virtual circuit is called a permanent virtual circuit (PVC)
because it guarantees that a certain amount of bandwidth will always be
available when needed.
c. The PVC eliminates the need to pre-reserve a specific path in advance.
d. The switched virtual circuit (SVC) is dynamically created when a request
for transmission is made. When the transmission ends, the circuit is
deleted. This type of circuit is good for those sites that transmit traffic in
sporadic or inconsistent patterns.
e. Circuit switching is used by the public telephone network (POTS or
PSTN) to carry telephone calls from source to destination.
f. Circuit switching does involve some setup time to establish the circuit, but
the time delay is usually not apparent to the user.
6. Packet Switching
a. Packet switching is commonly used for WAN connections.
b. A message will be broken into smaller units called packets.
c. Each packet has a header that contains the source address, destination
address, and a sequence number. The sequence number is used to
reassemble the message once it reaches the destination.
d. With packet switching, the pieces of the message (the packets) often take
different paths from the source to the destination. Once the packets reach
the destination, the message is reassembled using the sequence numbers in
the headers of the packets so the message can be read by the destination.
e. Packet switching involves no circuit setup time. The transmission is
connectionless, so there is no guarantee of delivery.
f. Routers make the decisions about the path a series of packets will take.
7. Which Switching Method Is Better?
a. Every business must evaluate the types of traffic that will be transmitted in
order to determine which type of switching method is most suitable.
b. Benefits of circuit switching:
i. There is a guaranteed pathway for the data.
ii. A guaranteed portion of the bandwidth is available for
transmission.
iii. Packets are smaller, because there is no need to place source and
destination information in the header, only the circuit number so
intervening devices will recognize which message the packet
belongs to.
c. Benefits of packet switching:
i. The public infrastructure can be used for data transmission instead
of incurring the expense for dedicated links between locations.
ii. Resources, such as bandwidth, are used more fairly and efficiently
in packet switching.
iii. Pipelining, the ability to simultaneously use a communication link
for two or more transmissions, increases the efficiency of use for
the available bandwidth.
8. Integrated Services Digital Network (ISDN)
a. ISDN is a cost-effective WAN connection that uses digital transmission
over traditional telephone networks.
b. ISDN used two types of channels to carry data:
i. Bearer Channels (B channels) carry the payload, which may be
voice or data.
ii. Data Channels (D channels) carry the control information for
connection setup, timing, and disconnection.
c. ISDN is offered in two types of interfaces:
i. Basic Rate Interface (BRI) provides two B channels and one D
channel for a total transfer rate of 128Kbps.
ii. Primary Rate Interface (PRI) allocates 23 B channels for data and
one 64Kbps D channel for control information for a total transfer
rate of 1.544Mbps.
d. The benefits of ISDN include:
i. Data capacity to service many users at the same time
ii. Voice and data transmission over the same physical media at the
same time because one of the two channels of the BRI can be
switched to voice if a call comes in
iii. Video conferencing
iv. Widespread availability
v. Cost-effective solution for small businesses and small office/home
office (SOHO) operations
e. To implement ISDN on the network, contact the local telephone service
provider.
9. Fiber Distributed Data Interface (FDDI)
a. FDDI is a set of standards developed by the ISO and ANSI that provides
the guidelines for data transmission over fiber-optic cable.
b. FDDI uses a dual counter-rotating ring formation capable of 100Mbps
token passing.
c. FDDI internetworks have extremely high capacity, and so can service
thousands of users over great distances because the data is transmitted
over fiber-optic cable.
d. The two FDDI rings provide data flow in opposite directions.
e. When a portion of the FDDI network fails (a station or a cable segment),
the FDDI network can heal itself by closing the ring at the gap and
allowing data to be transmitted. This results in little or no network down
time.
f. FDDI and fiber optic networks are usually used for backbone deployments
because of the expense of constructing a fiber network. Professionals must
install fiber-optic cable to minimize damage to the cable and to the
installer.
10. Synchronous Optical Networks/Synchronous Digital Hierarchy (SONet/SDH) and
Optical Cable X (OCx)
These three technologies have as their common ground the fiber-optic cable
plant. Because of this commonality, the three cannot be easily separated when
discussing the technology.
11. SONet
a. SONet defines the standards for optical carrier levels and synchronous
transport signals (STSs) for the infrastructure.
b. SONet uses multiplexing to use all bandwidth efficiently for the
transmission of data.
c. This permits SONet to use low-level digital signals and a synchronous
structure.
d. Synchronous structure refers to the transitions of the digital signals so that
they occur at exactly the same rate. This tells the media and the receiving
station where the 1s and 0s are in the signal.
e. A clocking signal provides the constant and even pulse to keep traffic in
line.
f. Asynchronous Transfer Mode (ATM) uses SONet because of its capacity
and its ability to use multiplexing and synchronization.
12. Synchronous Digital Hierarchy (SDH)
SDH was the result of several standards organizations defining a global
synchronization standard for transmission. SDH unified the various existing
standards for international communication.
13. The Optical Carrier (OC)
a. The Optical Carrier standards are based on the Synchronous Transport
Signals (STSs) used by SONet. The STS standards have an equivalent OC
(Optical Carrier) standard that is expressed by a numeric indicator.
b. The basic building block of OC is based on the STS-1, which has a
capacity of 51.84Mbps transmission speed.
c. Other examples of OC levels are:
i. OC-3: Also called STS-3, this signal transmits at a line rate of
155.52Mbps.
ii. OC-12: Also referred to as STS-12, transmission rates of
622.08Mbps can be achieved
iii. OC-48: This is STS-48, with a 2488.32Mbps rate
iv. OC-192: At the top of the line, this STS-192 rate is 9953.28Mbps
14. Asynchronous Transfer Mode (ATM)
a. Asynchronous Transfer Mode (ATM) is a WAN transmission technology
that is capable of speeds ranging from 1.54Mbps to 622Mbps.
b. ATM uses a switching technology to move high volumes of data, voice,
video, and audio transmissions between end points.
c. ATM is expensive, so it is seldom used for LAN transmission.
d. ATM uses cells instead of packets. Each cell is 53 bytes in length. The
fixed-sized cell reduces the overhead for processing the package, reduces
the number of bits needed for error control, and functions much more
efficiently because of the reduced overhead.
e. ATM uses virtual circuits between defined end points and routes, but does
not allocate bandwidth ahead of time (does not pre-reserve bandwidth).
This allows ATM to support several classes of service and to support
transmission of traffic like multimedia files.
f. ATM can be used over fiber-optic cable or some of the newer, highcapacity copper media.
15. Layered Technology of ATM.
a. ATM functions with several layers:
i. At the Physical layer, there are specifications for transmission
media, signal-encoding schemes, data rates, and compatibility.
ii. At the ATM layer, provisions are made to access services in the
upper layers, packet transfer capabilities, cell-size definition, and
logical connection specifications.
iii. The ATM Adaptation layer (AAL) changes depending on the
service being used. AAL maps higher layer information into the
cells and passes it down to the ATM layer, or it assembles
information from the ATM cells and passes it up to higher layer
technologies.
16. Virtual Channel Connections (VCCs) and Virtual Path Connections (VPCs)
a. A VCC is a virtual circuit that carries a sequenced, single flow of cells
from end to end.
b. VCCs can be statically configured permanent virtual circuits or
dynamically configured switched virtual circuits.
c. A set of VCCs can be bundled together to form a VPC for transmission.
All VCCs that are made part of a VPC will be transmitted from end to end
across the circuit as a single entity, resulting in reduced overhead and
easier recovery from a failure in the route.
d. A VPC acts like a virtual trunk between two sites.
17. Frame Relay
a. Frame relay is a packet-switching technology that supports data transport
at reasonable cost. Frame relay uses the public infrastructure of common
carriers to transmit at rates between 56Kbps and 1.544Kbps.
b. Frame relay uses the “cloud” of common carriers to span large distances.
c. To create a frame relay network, a connection to the provider must be
established. Circuits are purchased that allow the transmission between
each end of each circuit.
d. Frame relay offers both PVCs and SVCs.
e. Frame relay requires that a committed information rate be established.
This number is the minimum amount of bandwidth that will always be
available to the circuit.
f. Vendors also establish the committed burst rate (Bc), which identifies how
much excess bandwidth a circuit may use.
g. A third rate, called the burst excess rate (Be), indicates the maximum burst
transmission bandwidth available to the circuit.
h. If the network is heavily congested, the vendor has the option of dropping
packets if the transmission rate exceeds the CIR.
i. Frame relay networks are easily scalable because they require minimal
amounts of hardware.
18. The T-Carrier Connection
a. The T-carrier connection refers to the telecommunications links that
provide remote access for business using the public telephone
infrastructure as the physical media.
b. T-carrier solutions are leased line solutions, billed from the local
telephone company.
c. T-carrier connections are digital in nature, eliminating analog to digital
conversions and making them a good choice for data network
interconnection.
d. T-carrier connections are based on the same building block as ISDN. A T1
leased line uses 24 channels for a transmission rate of 1.544Mbps. T3 lines
transmit at 44.376Mbps.
e. The signal level determines the speed of the channel, which is the OSI
Model Physical layer characteristic. The unit of measurement is the Data
Signal X, or DSx. One data channel is a DS0; twenty-four channels are
called a DS1.
f. T-carrier connections are always on, making internetwork
communications simple for the users.
g. T-carrier lines can be purchased in increments. These increments are
called fractional T1 lines. Each channel transmits at 64Kbps.
h. The European equivalent of the T1 carrier is the E1, which has a capacity
of 2.048Mbps. The European equivalent of the T3 line is the E3 line,
transmitting at 34.368Mbps.
i. T-carrier connections require the following hardware:
i. CSU/DSU
ii. Multiplexer