Module 2: WAN Technologies

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Oct 24, 2013 (3 years and 11 months ago)

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Module 2: WAN Technologies

CCNA IV

Clark College Business & Industry
Spring/Summer 2004

Steve Raymond

Module 2 Overview

1.
LANs spanning geographic distances are connected together by WANs.

2.
WANs are typically much slower in terms of link speed and can be of high cost. A
variety of technologies and equipment can be used to mitigate the cost or tradeoff
performance for expense.


Students completing this module should be able to:

*

Differentiate between a LAN and WAN

*

Identify the devices used in a WAN

*

List WAN standards

*

Describe WAN encapsulation

*

Classify the various WAN link options

*

Differentiate between packet
-
switched and circuit
-
switched WAN technologies

*

Compare and contrast current WAN technologies

*

Describe equipment involved in the implementation of various WAN services

*

Recommend a WAN service to an organization based on its needs

*

Describe DSL and cable modem connectivity basics

*

Describe a methodical procedure for designing WANs

*

Compare and contrast WAN topologies

*

Compare and contrast WAN design models

*

Recommend a WAN design to an organization based on its needs



2.1 WAN Technologies Overview

2.1.1 WAN Technology

WANs are data networks which span geographic distances beyond the reach of LANs. WAN
technology usually requires the service of a carrier to haul circuits between locations.

Customer WAN equipment is called CPE or customer premises equipment. The carrier
connects customer CPE at different sites together through a variety of technologies.

The device which interfaces with the telco is called data circuit
-
terminating equipment, or
DCE. This is usually a modem or a CSU/DSU. The device served by DCE is data
terminal equipment, or DTE; typically a router or even a dumb terminal.


2.1 WAN Technologies Overview

2.1.2 WAN devices

LAN communication protocols must be converted to a signaling format and protocol
compatible with WAN interfaces. Various types of equipment are used in WAN
communications. Digital circuits use CSU/DSUs, whereas analog circuits use a
modem. ISDN uses a terminal adapter.


2.1 WAN Technologies Overview

2.1.3 WAN standards

WANs typically operate at layers 1 and 2 of the OSI model. WAN standards are specified by
the following organizations:











Physical layer standards specify electrical, mechanical, operational, and functional
connectors.

2.1 WAN Technologies Overview

2.1.3 WAN standards continued

Physical standards:


2.1 WAN Technologies Overview

2.1.3 WAN standards continued

Physical connectors:


2.1 WAN Technologies Overview

2.1.3 WAN standards continued

Data link layer WAN protocols:


2.1 WAN Technologies Overview

2.1.4 WAN encapsulation

Packets are passed down from layer 3 to layer 2 and encapsulated with the data link protocol
appropriate for the WAN technology in use. Most serial encapsulations are some form
of the HDLC protocol.


2.1 WAN Technologies Overview

2.1.4 WAN encapsulation continued

HDLC frames have signaling fields which vary depending upon the vendor implementation.
The address field is not used for HDLC because it is for point
-
to
-
point links. The
control field indicates the frame type:

*

Unnumbered frames carry line setup messages.

*

Information frames carry network layer data.

*

Supervisory frames control the flow of information frames and request data
retransmission in the event of an error.

PPP and Cisco HDLC have an extra field to identify the layer 3 protocol contained in the
data portion of the frame.


2.1 WAN Technologies Overview

2.1.5 Packet and circuit switching

Packet switched networks send individual datagrams or segments serially with share trunks
carrying traffic for all users in the network, perhaps even public networks.

Circuit switched networks provide dedicated bandwidth at least temporarily, with exclusive
access to the timeslot. Circuit switched service is usually more expensive due to the
exclusive nature and lower efficiency of bandwidth utilization.


Circuit Switched




Packet Switched

2.1 WAN Technologies Overview

2.1.5 Packet and circuit switching continued

In time division multiplexed networks, circuits are provisioned permanently and stay until
disconnected by the carrier.

Packet switched networks rely on complex switching devices in the network core to
determine the layer 2 path throughout the WAN network. Frame relay networks use
Data Link Connection Identifiers to forward frames to the destination.

2.1 WAN Technologies Overview

2.1.6 WAN link options


2.2 WAN Technologies

2.2.1 Analog dialup

Primary benefit of analog dialup service is the ubiquitous and pervasive availability.
Modems can convert a digital signal to analog and send over the voice network, which
provides a circuit
-
switched path for the duration of the call. Cost and throughput are
low.


2.2 WAN Technologies

2.2.2 ISDN

Integrated Services Digital Network delivers a digital connection to locations where analog
service can reach. ISDN uses several channels in a single line: B for bearer or user
data, and D for delta or signaling.

Basic Rate Interface or BRI is a two 64kbps B channel plus one 16kbps D channel often
delivered to a home or small office.

Primary Rate Interface or PRI is a business line consisting of 23 64kbps B channels and 1
64kbps D channel, usually delivered on a T1 line.










BRI B channels can be used for for any combination of simultaneous voice or data calls. B
channels can be bonded together for 128kbps of data throughput. PRI channels are
sometimes bonded together for video conferencing.


2.2 WAN Technologies

2.2.3 Leased line

Leased line service provides dedicated exclusive bandwidth on a TDM system with speeds
ranging at least up to 10Gbps. Leased lines are point
-
to
-
point and provisioned
permanently by a service provider.

Since leased lines are dedicated, they must be paid for even when not being used. Therefore
leased lines are typically the most expensive type of data WAN networking service.


2.2 WAN Technologies

2.2.4 X.25

X.25 is a packet
-
switching technology intended to be more cost effective than leased lines
for low bandwidth applications.

X.25 is a network
-
layer protocol and each subscriber is provided with a network address.
Virtual circuits are established by call request packets to the destination system. The
resulting switched virtual circuit (SVC) is identified by a channel number and packets
are labeled with the channel number for delivery to the destination.

Subscribers connect to the X.25 cloud through leased lines of dialup connections. X.25 can
be cost effective as usage can be metered and billed. X.25 typically has a maximum
speed of 48kbps.

Frame relay service has superseded X.25, especially in the US. Some common users of
X.25 are point
-
of
-
sale equipment and other low
-
bandwidth applications.



2.2 WAN Technologies

2.2.5 Frame Relay

Frame relay is a packet
-
switched network that appears somewhat like X.25, data rates can go
up to 45Mbps.

Frame relay is a data
-
link layer protocol (layer 2) and does not implement error correction
nor application flow control. Frame relay offers low latency and jitter.

Frame relay uses mostly PVCs configured by the service provider once ordered, but can also
used SVCs which are dynamically set up and torn down by subscribers to the same
network.

Frame relay provides permanent point
-
to
-
point service with a quality that comes close to
that of leased line service, with the added benefit of offering point
-
to
-
multipoint
connections where several PVCs can terminate on one physical circuit, avoiding the
requirement of an additional leased line for every WAN connection.




2.2 WAN Technologies

2.2.6 ATM

A WAN technology was needed for assured quality of service and even higher available
bandwidth options than frame relay. Asynchronous Transfer Mode (ATM) can go to at
least 622Mbps, and higher with some vendor applications.

ATM is designed to carry voice, video, and data all on the same network and assign the
appropriate QOS to the packets to ensure reliable delivery of multiple services.

ATM PDUs are fixed 53
-
byte cells of which 5 bytes is overhead and 48 bytes payload.
Small cells are chosen due to their lending well to video and voice applications.

The 5 byte overhead in every cell is nearly 10% of the bandwidth and thus relatively costly
compared to other variable
-
length technologies like frame relay. ATM takes
approximately 20% more bandwidth on average than frame relay to achieve the same
throughput.

ATM can carry PVCs and SVCs with PVCs being more common. ATM provides for
multiple virtual circuits on on physical connection to the ATM cloud.


2.2 WAN Technologies

2.2.7 DSL

Digital Subscriber Line attempts to deliver high
-
bandwidth data service to edge subscribers
over a copper line similar to phone lines. xDSL refers to related by different
technologies:

*

Asymmetric DSL (ADSL)

*

Symmetric DSL (SDSL)

*

High Bit Rate DSL (HDSL)

*

ISDN (like) DSL (IDSL)

*

Consumer DSL (CDSL), also called DSL
-
lite or G.lite







DSL works by splitting the available frequency bands of a copper pair into voice and data
channels, with different frequencies carrying traffic in the upstream and downstream
direction.

The varying bandwidths and flavors of DSL exist to provided different performance based
upon varying distances and line qualities on the copper pair to the central office. DSL
service availability is dependent upon a maximum distance from the CO.

2.2 WAN Technologies

2.2.8 Cable modem

Significant cable plant has already been deployed by cable TV companies to most homes in
the US. Cable modems can use this circuit for high
-
speed 2
-
way data communication.
Cable modems are always on and usually easy to install. Cable modem equipment can
theoretically deliver 30 to 40Mbps on a 6Mhz cable channel. Cable TV signal can
coexist with the data signal on the same wire.


2.3 WAN Design

2.3.1 WAN communication

WAN links connect LANs together with one of the technologies discussed prior. WAN links
almost always must be leased from a service provider or carrier. WAN links are both
more expensive than LAN circuits but also much slower.

WANs carry voice, video, and data. WANs are simply interconnection points, and services
are not hosted in the WAN themselves but provide access to LANs where services
reside. WANs operate upon the first three levels of the OSI model.


2.3 WAN Design

2.3.2 Steps in WAN design

To design a WAN, the type of traffic, the source and destination must be identified along
with the bandwidth, latency, and jitter requirements of the application:











Input from the users and a networking budget play important roles in designing a WAN.

Installation and operational costs must be determined and budgeted for when designing and
implementing WANs.

2.3 WAN Design

2.3.2 Steps in WAN design continued


2.3 WAN Design

2.3.3 How to identify and select networking capabilities

The following must be determined during WAN design:

*

Selecting an interconnection pattern or layout for the links between the various
locations

*

Selecting the technologies for those links to meet the enterprise requirements at an
acceptable cost

A star or hub
-
and
-
spoke topology is the most common. While a full
-
mesh topology may
provider higher performance, it usually comes with higher cost than star.


Star




Full Mesh

2.3 WAN Design

2.3.3 How to identify and select networking capabilities continued

Different technologies offer various performance and cost tradeoffs. Leased lines offer
dedicated bandwidth, low latency and jitter. Packet
-
switched networks share
bandwidth, are lower cost, but may bring higher latency and or jitter. ATM offers QOS
guarantees and traffic prioritization, but that comes at additional cost.

Packet
-
switched technologies allow multiple virtual connections to interface on a single
physical circuit. Leased lines are point
-
to
-
point and require a 1:1 circuit ratio to
achiever a full mesh topology.

WANs can incorporate several technologies to meet a variety of goals, for example lease
lines between headquarters, frame relay to smaller branch offices, and ISDN for as
-
need dial backup lines.



2.3 WAN Design

2.3.4 Three
-
layer design model

A hierarchical design can increase the scalability and reliability of a complex WAN design
especially when a full
-
mesh topology would be either impossible or too costly.

Access links connect LANs to the WAN edge. Distribution links interconnect regions to the
WAN core. The Core layer then provides reliable high
-
speed transport of data through
the network:


2.3 WAN Design

2.3.5 Other layered design models

A two
-
layer design may be sufficient in simpler or smaller networks. In the following the
heaviest traffic load is on the local networks with only a small need for traffic
exchange between sites across the WAN core:




2.3 WAN Design

2.3.6 Other WAN design considerations

One design decision is whether or not each branch will have it’s own separate Internet
access, or if a single connection to the Internet will be serviced in the core network.
The more accesses to the Internet are installed, the greater a chance for network
intrusion and security problems. The benefit of each site having its own Internet is the
bandwidth savings on the WAN link due to offloading of the Internet traffic locally.

Further possibilities include using the Internet itself for WAN connectivity between remote
sites while implementing some sort of VPN security technology.