Introduction to Computer
Networks
Connection Services
This lesson examines connection service
options. Many network connection choices
are available, and each has advantages and
disadvantages. We begin with simple
telephone lines and move on to cover highspeed digital services.
Carriers
• All modem communication takes place over some
kind of communication line or cable. Which type of
cable it is, as well as who provides it and its
related services, makes a difference in network
performance and cost.
• Telephone Lines
• Dial-Up Lines: Dial-up lines are common
telephone lines. Some digital lines support data
transmission speeds of up to 56 Kbps.
• Dedicated Lines: from 56 Kbps to 45 Mbps or
more. (DSL)
Remote Access Service (RAS)
• Most server-based network OS provide a service,
called Remote Access Service (RAS).
• To establish a remote connection requires two
services: RAS and a client service known as dial-up
networking (DUN).
RAS Connections
• The physical connection to a RAS server can be
made using several different media. These include
the following:
• Public Switched Telephone Network (PSTN)
• X.25: This packet-switched network service can
be used to make dial-up or direct connections.
• Integrated Services Digital Network (ISDN)
• Digital Subscriber’s Line (DSL)
RAS Protocols
• RAS supports three connection protocols.
• Serial Line Interface protocol (SLIP): does not
support dynamic IP addressing or the NetBEUI or
IPX protocols, it cannot encrypt logon information,
and it is supported only by RAS clients.
• Point-to-Point Protocol (PPP): supports TCP/IP,
IPX, NetBEUI, compression and encrypted
passwords.
• Point-to-Point Tunneling Protocol (PPTP) is an
essential part of VPN technology. PPTP provides
secure transmission over TCP/IP networks because
its connections are encrypted.
RAS and Security
• The actual methods by which RAS ensures security
can vary with the operating system. RAS security
functions include:
• Auditing
• Callback
• Security host
• PPTP filtering
Limitations of RAS
• Do not use RAS if you need a higher bandwidth
than that provided by an asynchronous modem, or
if you need a dedicated full-time connection.
• Use RAS if you determine that your bandwidth
requirements are not greater than 128 Kbps.
• if you do not require a full-time connection,
• or if you must keep system costs down.
WAN Overview
• Most WANs are combinations of LANs and other
types of communications components connected
by communication links called "WAN links." WAN
links can include:
• Packet-switching networks.
Fiber-optic cable.
Microwave transmitters.
Satellite links.
Cable television coaxial systems.
WAN Transmission Technologies:
• Communication between LANs will involve one of
the following transmission technologies:
• Analog
• Digital
• Packet switching
Analog Connectivity
• The same network that your telephone uses is
available to computers.
• One name for this worldwide network is the Public
Switched Telephone Network (PSTN).
• In the context of computing, the PSTN, offering
voice-grade dial-up telephone lines, can be thought
of as one large WAN link.
Digital Connectivity
• Organizations requiring a faster, more secure
transmission environment than that which analog
lines provide can turn to digital data service (DDS)
lines.
• DDS provides point-to-point synchronous
communication
• Point-to-point digital circuits are dedicated circuits
that are provided by several telecommunications
carriers.
• Digital lines are available in several forms,
including DDS, T1, T3, T4, and switched 56.
Digital data service (DDS)
• DDS sends data
from a bridge
or router
through a
device called a
Channel
Service
Unit/Data
Service Unit
(CSU/DSU).
T1 Service
• T1 service is perhaps the most widely used type of
digital line.
• It is a point-to-point transmission technology that
uses two-wire pairs to transmit a full-duplex signal
at a rate of 1.544 Mbps.
• Subscribers who do not need or cannot afford the
bandwidth of an entire T1 line can subscribe to
one or more T1 channels in 64 Kbps increments,
known as Fractional T-1 (FT-1).
Digital Transmission Rates
• Signal Carrier T1-Chs Voice-Chs Data-Rate
• DS-0
DS-1
DS-2
DS-3
DS-4
N/A
T1
T2
T3
T4
N/A
1
4
28
168
1
24
96
672
4032
64 K
1.544 MB
6.312 MB
44.736 MB
274.760 MB
• T3 and Fractional T-3 leased line service provides
voice and data-grade service from 6 Mbps to 45
Mbps. These offer the highest-capacity leased-line
service commonly available.
Packet-Switching Networks
• Because packet technology is fast, convenient, and
reliable, it is used to transmit data over extensive
areas such as across cities, states, or countries.
Virtual Circuits
• Many packet-switching networks use virtual circuits.
• These are circuits composed of a series of logical
connections between the sending computer and the
receiving computer.
• The circuit is bandwidth allocated on demand, not
an actual cable or permanent, physical link between
two stations.
• Virtual circuits can last either as long as the
conversation lasts (Switched Virtual Circuits
(SVCs)), or as long as the two communicating
computers are up and running (Permanent Virtual
Circuits (PVCs))
Sending Data Across a WAN
• WAN environments are becoming more popular to
deliver the speed or bandwidth an organization
needs.
• These WAN technologies include:
• X.25.
Frame relay.
Asynchronous Transfer Mode (ATM).
Integrated
Services Digital Network (ISDN).
Fiber
Distributed Data Interface (FDDI).
Synchronous
Optical Network (SONET).
Switched
Multimegabit Data Service (SMDS). Internet.
X.25
• X.25 is a set of protocols incorporated in a packetswitching network.
• The early X.25 networks used telephone lines to
transmit data.
Frame Relay
• Frame relay is an advanced fast-packet variablelength, digital, packet-switching technology.
• This technology stripped away many X.25
accounting and checking functions that are not
necessary in a reliable, secure, fiber-optic circuit
environment.
• Frame relay
uses a point-topoint system
Asynchronous Transfer Mode (ATM)
• ATM is an advanced implementation of packet
switching that provides high-speed data
transmission rates to send fixed-size packets (cells)
over broadband and baseband LANs or WANs.
ATM Switches
• ATM switches act as multiplexers allowing multiple
data input.
• ATM can offer throughput rates from 155MB up to
1.2 gigabits per second.
Integrated Services Digital Network
(ISDN)
• ISDN is an inter-LAN digital connectivity
specification that accommodates voice, data.
• The 64 Kbps channels, B channels, can carry
voice, data, or images. The slower 16 Kbps
channel, the D channel, carries signaling and link
management data. ISDN Basic Rate desktop
service is called 2B+D.
• Primary Rate ISDN uses the entire bandwidth of a
T1 link by providing 23 B channels at 64 Kbps and
one D channel at 64 Kbps.
Fiber Distributed Data Interface
(FDDI)
• Fiber Distributed Data Interface (FDDI) is a
specification that describes a high-speed (100
Mbps) token-ring network that uses fiber-optic
media.
• FDDI can be used for metropolitan area networks
(MANs) to connect networks in the same city with
a high-speed fiber-optic cable connection.
• It is limited to a maximum ring length of 100
kilometers (62 miles), so FDDI is not really
designed to be used as a WAN technology.
FDDI
• FDDI uses the token-passing system in a dual-ring
setting. Traffic usually flows on the primary ring.
• If the primary
ring fails, FDDI
automatically
reconfigures the
network so that
the data flows
onto the
secondary ring in
the opposite
direction.
Synchronous Optical Network
(SONET)
• SONET is a standard for optical transport that was
formulated by the Exchange Carriers Standards
Association (ECSA) for ANSI.
• SONET has also been incorporated into the
Synchronous Digital Hierarchy recommendations of
the CCITT (ITU).
• SONET defines optical-carrier (OC) levels and
electrical-equivalent synchronous transport signals
(STSs) for the fiber optic-based transmission
hierarchy.
Synchronous Optical Network
(SONET)
• SONET defines optical-carrier (OC) levels and
electrical-equivalent synchronous transport signals
(STSs) for the fiber optic-based transmission
hierarchy.
Switched Multimegabit Data Service
(SMDS)
• SMDS) is a switching service provided by some local
exchange carrier services.
• Transmission speeds range from 1 Mbps to 34
Mbps with many-to-many connectivity. SMDS does
not perform error checking or flow control; that is
left up to the sites being connected.
• SMDS uses the same fixed-length cell relay
technology as ATM. SMDS is a dual-bus topology
that forms a ring that is not closed.
Internet
• Internet (Web) has become the most popular WAN
connection for all LANs.
• The Internet is a network of networks, linking
computers to computers sharing the TCP/IP
protocols.
• A VPN solution provides today's companies the
ability to interconnect different corporate network
sites in a completely secure environment using the
Internet.