Transcript Large Networks I: Transmission

Wide Area Networking
Chapter 9
Copyright 2001 Prentice Hall
Revision 2: July 2001
2
Orientation
 Chapters 6 & 7
Simple Ethernet
LAN
 Chapter 8
Site Networks
 Chapter 9
Wide Area
Networks
Link sites together
Carriers and
regulation
Leased Line Networks
Public Switched Data
Networks (PSDNs)
Virtual Private
Networks
3
Wide Area Networks
 WANs Link Sites (Locations)
Usually sites of the same organization
Sometimes, sites of different
organizations
Site B
Site A
Site C
WAN
4
Carriers
You can only install wires on your own property
Called your customer premises
To send signals between sites or to customers,
you must use a carrier
Customer
Premises
Carrier
5
Carriers
 Carriers transport data and voice traffic between
customer premises, charging a price for their
services
Receive rights of way from the government to
lay wires and radio links
Carrier
Carrier Regulation
6
Traditionally, Carriers Have been Regulated
Given rights of way
Given monopoly protection from competition
In return, services normally must be tariffed
Tariff specifies exact terms of the service to be
provided, and
Tariff specifies price to be charged
Prevents special deals, which would be
inappropriate for a regulated monopoly
Regulators must approve price for reasonableness
Carrier Regulation
There is a Strong Trend Toward Deregulation
Gradual removal of monopoly protections
Allows competition, so lower prices and more
service options
Fewer services need to be tariffed, allowing
price negotiation
7
Carrier Regulation
 Service Level Agreements (SLAs)
Even under competition, carriers may
guarantee specific levels of service for certain
service parameters in an SLA
Throughput
Latency
Availability
Error Rates, etc.
Penalties are paid to customers if carrier fails
to meet agreed-upon service levels
8
High Cost of Long-Distance
LAN Communication is Inexpensive per Bit
Transmitted
So most LANs operate at 10 Mbps to a few
gigabits per second
Long-Distance Communication is Very Expensive
per Bit Transmitted
So Most WANs use low speeds
Most WAN demand is 56 kbps to a few Mbps
9
Types of U.S. Carriers
10
Module D
The United States is divided into about 200
regions called local access and transport areas
(LATAs)
Within a LATA, services are provided by two
types of local exchange carriers (LECs)
The incumbent local area carrier (ILEC),
which is the traditional telephone carrier
within the LATA
Competitive local area carriers (CLECs),
which compete with the ILEC
Types of U.S. Carriers
11
Module D
Between LATAs, services are provided by interexchange carriers (IXCs)
Some carriers provide service both within and
between LATAs
However, their services may be regulated
differently when they act as ILECs/CLECs or
IXCs
12
Leased Lines
Leased Lines are Circuits (From Chapter 1)
Often goes through multiple switches and trunk lines
Looks to user like a simple direct link
Switch
Trunk
Line
Leased
Line
Leased Lines
 Leased lines
Limited to point-to-point communication
Limits who you can talk to
Carriers offer leased lines at an attractive
price per bit sent to keep high-volume
customers
Leased Line
13
Leased Line Meshes
If you have several sites, you need a mesh of
leased lines among sites
Mesh
Leased Line
14
Leased Line Speeds
 Largest Demand is 56 kbps to a few Mbps
 56 kbps (sometimes 64 kbps) digital leased lines
DS0 signaling
 T1 (1.544 Mbps) digital leased lines
24 times effective capacity of 56 kbps
Only about 3-5 times cost of 56 kbps
DS1 signaling
 Fractional T1
Fraction of T1’s speed and price
Often 128, 256, 384 kbps
15
Leased Line Speeds
T3: is the next step
44.7 Mbps in U.S.
Europe has E Series
E1: 2.048 Mbps
E3: 34 Mbps
SONET/SDH lines offer very high speeds
156 Mbps, 622 Mbps, 2.5 Gbps, 10 Gbps
16
17
SONET/SDH
Created as Trunk Lines for Internal Carrier
Traffic
As were other leased lines
The Trunk Line Breakage Problem
Problem: unrelated construction products often break
carrier trunk lines, producing service disruptions
The most common cause of disruptions
X
SONET/SDH Uses a Dual Ring
18
Normally, Traffic Travels in One Direction on One
Ring
If Trunk Line Breakage, Ring is Wrapped; Still a
Ring, So Service Continues
Switch
Normal Operation
Wrapped
Digital Subscriber Lines (DSLs)
19
Can Use DSLs Instead of Traditional Leased
Lines
Less expensive
HDSL (High-Speed DSL)
Symmetrical: Same speed in each direction
HDSL: 768 kbps (Half a T1) on a single twisted pair
HDSL2: 1.544 Mbps (T1) on a single twisted pair
SHDSL (Super-High-Speed DSL)
New
Can run at multiple rates up to 4.6 megabits/second
Symmetrical
Digital Subscriber Line
Normal Leased Lines Used Data Grade Wires
High-quality, high-cost
Two pairs (one in each direction)
DSLs Normally Use Voice Grade Copper
Not designed for high-speed data
So sometimes works poorly
Usually one pair (ADSL, HDSL)
Sometimes two pairs (HDSL2)
20
Problems of Leased Lines
21
With many sites, meshes are expensive and
difficult to manage
There are many leased lines between the sites
Each site is likely to have several leased lines
connected to it
These leased lines tend to span long distances
between sites
New
Problems of Leased Lines
22
User firm must handle switching and ongoing
management
Expensive because this requires planning and
the hiring, training, and retention of a WAN
staff
23
T1 Leased Lines
Mod B
Voice Requirements
Analog voice signal is encoded as a 64 kbps
data stream (see Chapter 5)
8 bits per sample
8,000 samples per second
T1 Leased Lines
24
Mod B
T1 lines are designed to multiplex 24 voice
channels of 64 kbps each
T1 lines use time division multiplexing (TDM)
Time is divided into 8,000 frames per second
One frame for each sampling period
Each frame is divided into 24 8-bit slots
One for each channel’s sample in that time period
(24 x 8) 192 bits
Plus one framing bit for 193 bits per frame
25
T1 Leased Lines
Mod B
Speed Calculation
193 bits per frame
8,000 frames per second
1.544 Mbps
Framing Bit
One per frame
8,000 per second
Used to carry supervisory information (in
groups of 12 or 24 framing bits)
26
PSDNs
 Public Switched Data Networks
Designed for data rather than voice
Site-to-site switching is handled for you
You merely connect each site to the PSDN
“cloud” (No need to know internal details)
PSDN
27
PSDNs
Connect each site to the PSDN using one leased
line
Only one leased line per site
Line only runs a short distance—to the New
nearest PSDN access point
1 Leased
Line
PSDN
28
PSDNs
 Access Device Needed at Each Site
Connects each site to access line
Often a router
Sometimes a device specific to a particular
PSDN Technology
Access
Device
PSDN
29
PSDNs
 Point of Presence (POP)
Place where you connect to the cloud
May be several in a city
May not have any POP close
Need leased line to POP
Separate from PSDN charges
POP
Leased
Line
PSDN
PSDNs in Perspective
 Simpler than Networks of Leased Lines
Less staffing
Fewer leased lines and shorter distances
 Less Expensive than Networks of Leased Lines
Less staffing
PSDN prices are very low
PSDN is less expensive overall
PSDNs are replacing many leased line mesh
networks
30
Circuit-Switched PSDNs
End-to-End Capacity is Guaranteed
If you need it, it is always there
When you don’t need it, you still pay for it
Expensive for data traffic, which usually has
short bursts and long silences
A
bcd
PSDN
efg
31
Packet-Switched PSDNs
32
Messages are divided into small units called
packets
Short packets load switches more effectively
than fewer long messages
Packet Switched PSDNs
Packets are multiplexed on trunk lines
Cost of trunk lines is shared
Packet switching lowers transmission costs
Dominates PSDN service today
Multiplexed
Trunk Line
33
Packet Switched PSDNs: Virtual Circuits
34
All commercial packet switched PSDNs use
virtual circuits
Eliminates forwarding decisions for individual packets
Reduces switching load, so reduces switching costs
Virtual
Circuit
Unreliable PSDNs
Most commercial PSDNs are Unreliable
(Only obsolete X.25 PSDN technology was
reliable)
No error correction at each hop between
switches
Reduces costs of switching
Note that both virtual circuits and unreliable
service reduce switching costs
35
PSDN Cost Savings
Packet Switching
Reduces costs of transmission lines through
multiplexing
Virtual Circuits
Reduces costs of switches because they do
not have to make decisions for each frame
Unreliability
Reduces costs of switches because they do
not have to do error correction
36
WAN Products
ISDN
X.25
Frame Relay
ATM
Virtual Private Networks (VPNs)
38
ISDN
Integrated Services Digital Network
2B+D Basic Rate Interface (BRI) to the desktop
Two 64-kbps B channels
Can be bonded for 128 kbps service
One 16-kbps D channel, usually for
supervisory signals
64
kbps
64
kbps
BRI
2B+D
ISDN Modem
39
ISDN
 Primary Rate Interface (PRI)
Connection between firm and ISDN carrier
23B+D (on a T1 line)
30B+D (on an E1 line)
One 64 kbps D channel for supervision
2B+D
BRI
23B+D
PRI
ISDN
ISDN
40
Circuit-Switched
Dedicated capacity
Expensive for data
 Dial-Up Connection
Must connect each time you wish to communicate
Other PSDNs are dedicated (always on)
Good for backup since only pay when use
New
Unreliable
 Only Popular PSDN that is either circuit-switched
or dial-up
ISDN
Never achieved strong market use
Being overtaken by PSDNs that are both faster
and less expensive
Often, ISDN is spelled out as “It still does
nothing”
However, there is enough ISDN in use that you
must know it
Also, if connectivity is only needed a short time
each day, ISDN is still a good choice for lowspeed transmission
41
X.25
42
First Packet-Switched PSDN Standard
Developed in the 1970s
Now obsolete
But still used, especially in third-world
countries and Europe
Slow: Usually 64 kbps or slower
Some faster X.25 services are available
Reliable, so costs of switches are high
So cost of service is high
But works even if transmission lines are poor
Frame Relay
 Most Popular PSDN Today
Offers speeds of 64 kbps to about 40 Mbps;
This covers the range of greatest corporate
demand
Most demand is at
the low end of the range
Priced aggressively
Both reasons
are critical
43
Frame Relay
Low-Cost Service
Packet-Switched
Uses virtual circuits to cut costs
Unreliable
Relatively low speeds
Dedicated Connections
Always ready to send
and receive
44
ATM
 Like Frame Relay:
Packet switched
Virtual circuits
Dedicated (Always On) Connections
 Unlike Frame Relay
Much faster top speed
1 Mbps, 25 Mbps, 45 Mbps, 156 kbps, 622 kbps, several
Gbps
Offers quality of service (QoS) guarantees
Maximum latency for time-critical applications
Exact cell-by-cell timing
45
46
Frame Relay and ATM
Most Vendors Offer Both
To cover speeds from
56 kbps to a few
gigabits per second
Price
In general, a smooth price-speed
curve across the two services
At some speed, may offer both
If so, usually price them the same
FR
ATM
Speed
Frame Relay and ATM
47
Both are widely used
Frame Relay is more popular today because it
serves the range of greatest corporate need (56
kbps to a few megabits per second) at an
attractive price
As demand for higher-speed links grows, ATM
should become more widely used
Unless other alternatives to ATM appear,
such as 10 Gbps Ethernet for WANs
48
Frame Relay Pricing
 Frame Relay Access Device (FRAD)
Device at each site to connect site network
to Frame Relay WAN
Paid to PSDN carrier or an equipment
manufacturer
Access
Device
PSDN
49
Frame Relay Pricing
Leased Access Line
Usually paid to telephone company rather than
PSDN
PSDN often includes a bundled pricing
arrangement, but it must pay the telephone
company if this is the case
If POPs are not close, access line charges will be
high
Access
Line
PSDN
50
Frame Relay Pricing
Port Speed
At the POP there is a connection port
Ports come in different speeds
56 or 64 kbps, 1 Mbps are very common
Faster ports are more expensive
Monthly port speed usually is the most expensive
component of PSDN cost
Port at
POP
PSDN
51
Frame Relay Pricing
 Sometimes, Two Port Speeds are Quoted
Committed Information Rate (CIR)
Frames can be sent continuously at this speed
Available Bit Rate (ABR)
Higher speed
Frames sent above the CIR are on standby
If congestion appears, will be discarded first
Port at
POP
PSDN
52
Frame Relay Pricing
Port Speed and Access Line Speed
Port costs usually are higher than access line charges
Access line must be as fast as port speed or faster to
avoid wasting expensive port speed
For example, if the port speed is 1 Mbps, you should
get a T1 (1.544 Mbps) leased access line
Port at
POP
Access Line
PSDN
53
Frame Relay Pricing
 Permanent Virtual Circuits (PVCs)
Leased line meshes had a leased line to each site
PSDNs, usually have PVCs wherever there were
leased lines between sites in the previous leased line
network
Eases the transition to PSDNs from leased lines
PVC
Leased
Access Line
PVC
54
Frame Relay Pricing
 Permanent Virtual Circuits (PVCs)
PVCs to other sites are multiplexed over a
site’s single leased access line
Pay monthly charge per PVC, based on speed
PVC
Leased
Access Line
PVC
55
Frame Relay Pricing
Permanent Virtual Circuits (PVCs)
Leased access line must be fast enough to
handle all of the PVCs it is multiplexing
Example: if it multiplexes 15 64 kbps PVCs, the
access line must be 840 kbps (T1 line needed)
PVC
Leased
Access Line
PVC
Frame Relay Pricing
56
Other Aspects of Pricing
Sometimes only flat fees for access lines,
ports, etc, but sometimes also charges based
on traffic volume
Ongoing (monthly) fees for leased lines,
ports, PVCs, etc.
Usually also an initial setup or installation
charge
Frame Relay Pricing
57
Other Aspects of Pricing
If offer switched virtual circuits (which are
set up on demand), usually a fee for each set
up
Many vendors offer managed services that
have them manage the Frame Relay network
instead of the customer having to do it
Frame Relay Frame Structure
58
Variable Length Frames
Start flag (01111110) to signal start of frame
Address field has variable length (2-4 octets)
Information field to carry data (variable)
CRC (Cyclical Redundancy Check) field to
detect errors (2 octets)
If find errors, switch discards the frame
Stop flag (01111110) to signal end of frame
59
Frame Relay Frame Structure
Address Field of Frame Relay Frame
Variable Length: 2-4 octets
4-Octet form shown
Complex
Bits
0
DLCI (6 bits)
DLCI
FECN BECN
7
C/R
AE
DE
AE
DLCI
AE
DLCI
AE
Frame Relay Frame Structure
Address Extension Bit (AE)
0 unless last octet
1 if last octet
Bits
0
DLCI (6 bits)
DLCI
FECN BECN
7
C/R
0
DE
0
DLCI
0
DLCI
1
60
61
Frame Relay Frame Structure
Address Field of Frame Relay Frame
Data Link Control Indicator (DLCI)
Indicates virtual circuit for switching
Does not use destination addresses
Bits
0
DLCI (6 bits)
DLCI (4 bits)
FECN BECN
7
C/R
AE
DE
AE
DLCI (7 bits)
AE
DLCI (7 bits)
AE
62
Frame Relay Frame Structure
Address Field of Frame Relay Frame
If address field is 2 octets long, DLCI is 10 bits long
If address field is 3 octets long, DLCI is 17 bits long
If address field is 4 octets long, DLCI is 24 bits long
Bits
0
DLCI (6 bits)
DLCI (4 bits)
FECN BECN
7
C/R
AE
DE
AE
DLCI (7 bits)
AE
DLCI (7 bits)
AE
Frame Relay Frame Structure
63
Address Field of Frame Relay Frame
Discard Eligible Bit
If send faster than committed rate, DE bit is set to 1
indicating that it may be discarded first
Bits
0
DLCI (6 bits)
DLCI
FECN BECN
7
C/R
AE
DE
AE
DLCI
AE
DLCI
AE
Frame Relay Frame Structure
Congestion Notification
If a switch detects congestion, it sets the FECN or
BECN bit
Bits
0
DLCI
DLCI
FECN BECN
7
C/R
AE
DE
AE
DLCI
AE
DLCI
AE
64
Frame Relay Frame Structure
65
 Congestion Control
Traffic shaping (see Chapter 8)
 The Backward Explicit Congestion Notification (BECN) bit
is set to tell the station that receives the frame to slow
down; This is easy to implement
 The Forward Explicit Congestion Notification (FECN) bit
is more complex; If a station receives this notification in
an incoming frame, it should tell its communication
partner at the other end of the Frame Relay network to
slow down
66
Frame Relay Frame Structure
Address Field of Frame Relay Frame
Command/Response bit useful in applications that
have commands and responses
Not widely used
Bits
0
DLCI (6 bits)
DLCI
FECN BECN
7
C/R
AE
DE
AE
DLCI
AE
DLCI
AE
10 Gbps/40 Gbps Ethernet
67
New
Now Under Development
Potential alternative to PSDNs
Handle high data traffic loads
Cost should be attractive
Should be limited to metropolitan area networks
(MANs)—a city and its suburbs
Only optical fiber; No UTP versions
VPNs
Virtual Private Networks
Use the Internet for transmission instead of a PSDN
Sometimes called VPNs if use Frame Relay or ATM
with added security
Internet
68
VPNs
Why use the Internet?
Inexpensive
Business partners are already connected to
the same network (the Internet)
May use different PSDNs, but everybody is
connected to the Internet
69
VPNs
Problems with the Internet
Congestion: slows transmissions
Reliability: cannot always connect,
sometimes fails during transmissions
Lack of security
70
71
VPNs
IETF developing IPsec security standards
IP security
At the internet layer
Protects all messages at the transport and
application layers
E-Mail, WWW, Database, etc.
TCP
UDP
IPsec
72
VPNs
IPsec Transport Mode
End-to-end security for hosts
Local
Network
Secure Communication
Internet
Local
Network
73
VPNs
IPsec Tunnel Mode
IPsec server at each site
Secure communication between sites
Local
Network
Secure Communication
Internet
Local
Network
IPsec
Server
74
VPNs
IPsec Modes Can be Combined
End-to-end transport mode connection
Within site-to-site tunnel connection
Local
Network
Tunnel Mode
Internet
Local
Network
Transport Mode
75
VPNs
Module F
Another Security System for VPNs is the Pointto-Point Tunneling Protocol (PPTP)
For dial-up connections, based on PPP
Connects user with securely to a remote
access server at a site
Dial-Up
Connection
PPTP Connection
Internet
Local
Network
Remote Access Server
Virtual Private Networks
Other Problems Remain
Internet Congestion is Still a Problem
Internet throughput tends to be low
Internet Reliability is Low
Cannot get connections
Backbone fails occasionally
76
Virtual Private Networks
Alternative
Avoid the congested and unreliable
backbone!
Use one ISP that serves all sites
Should offer QoS service level agreement
(SLAs) for latency and reliability
Site 1
ISP
Site 2
77
WANs in Perspective
78
Both Leased Line Networks and PSDNs are
widely used and will be for several years to
come
Leased Line Networking is shrinking while PSDN
networking is growing rapidly
VPN technology and standards are still
immature and use will be very low for several
years to come