Transcript 例外(exception）とは何か

Lecture 6
Long Distance Connection and WAN
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Digital Telephone, PCM and Nyquist Sampling Theorem
DSU/CSU, T Line Series and OC line Series
Local Loop
DSL Technologies
- ADSL, HDSL, SDSL, VDSL
Cable Modem
WAN and Packet Switch
Switch Addressing and Routing
Typical WANs
- ARPANET, X.25, Frame Rely, SMDS, ATM
Wireless WANs, Cellular Networks
Lecture 6
Long Distance Telephone Network and Others
- A single LAN can’t handle an arbitrary number of computers
- A LAN is usually owned by a company, organization, university, …, which
can’t build their own long distance links to connect computers in arbitrary sites
- Telephone and other companies offer long distance connections which
can be leased to build large computer networks covered many distance sites
Telephone Company
AD/DA
Comp
Modem
Circuit Switch Analogy Network
Modem
AD/DA
Circuit Switch Digital Network
Convert
LAN
Packet Switch Digital Network
Convert
Comp
LAN
Other Company
Modem
CATV/Power Network
Modem
Modem
Satellite Network
Modem
...
Lecture 6
Digital Telephone Network and PCM
Analogy telephone network: transfers analogy voice signal in the frequency range [0, 3.5KHz]
Digital telephone network: transfers digital voice signal
PCM (Pulse Code Modulation): a standard of conversion from analogy voice to digital voice
- Analogy signal is first sampled in sampling interval T or with sampling frequency F=1/T
- Nyquist sampling theorem: Original analogy signal can be reconstructed
from a sequence of sampled values if F>2B where B is the maximum signal frequency
- Sampling frequency of digital voice = 8 KHz
- Each sampled value is quantized to an integer in [-127, 127]
- Encoding the integer to an 8-bit binary value, PCM hardware is a analogy-to-digital (AD) device
- Bit rate of one digital voice channel: 8KHz x 8 bits = 64 Kbps
Nyquist Rate
00000111
00000110
F<2B
00000101
00000100
F=2B
00000011
00000010
00000001
00000000
F>2B
T
T, sampling interval
F=1/T, sampling frequency
Lecture 6
DSU/CSU
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Computer industry and telephone industry use own standards in different encoding
DSU/CSU perform the conversion between two standards
DSU (Data Service Unit): Translations between two encoding
CSU (Channel Service Unit): management of line termination and diagnose
DSU/CSU  Gateway
http://en.wikipedia.org/wiki/Gateway_(telecommunications)
Lecture 6
Digital Telephone Line Standard
Japan
Japan
Japan
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Specified by the telephone industry in each country, different from the world
Fractional T1= 64, 9.6 or 4.8Kbps, use multiple fractional T1 with multiplxier
Can be leased in two points
Three campus networks in Hosei University are connected by T1 (before 2001)
To outside
T2
Ichigaya
T1
Koganei
T1
T1
Tama
Lecture 6
High Capacity Digital Lines
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Use both electrical signal and optical signal
Electrical standard called Synchronous Transport Signal (STS)
Optical standard called Optical Carrier (OC)
Engineers usually use OC- terminology for everything
OC-3 is popular
OC-3: three OC-1 lines, OC-3C: a single line operating at 155.520 Mbps
Synchronous Optical Networking
Lecture 6
Local Loop: Analogy Line, ISDN and DSL
Analogy line
ISDN line
Subscribers
DSL line
Local
Central
Office
( CO )
Telephone
network
Local Loop
Analogy line:
- Analogy signal (300~3300Hz), modem needed, low speed (34/56Kbps)
ISDN (Integrated Services Digital Network):
- BRI (Basic Rate Interface) = 2B+D, B=64Kbps for data, D=16Kbps for control
- PRI (Primary Rate Interface)=23B+64D=1.544Mbps=T1
- called Narrowband ISDN (N-ISDN), relatively slow
- B-ISDN (Broadband ISDN), 155 or 622 Mbps to each subscriber
xDSL (Digital Subscriber Line)
- use existing telephone line to provide high speed transmission, current technology
Lecture 6
xDSL Technology
xDSL is a family of technologies to use existing analogy telephone line (copper pair)
for delivery of high-speed data between a subscriber and telephone central office.
xDSL family
ADSL: Asymmetric Digital Subscriber Line
HDSL: High-rate Digital Subscriber Line
SDSL: Symmetric Digital Subscriber Line
VDSL: Very high bite rate Digital Subscriber Line
Telephone Network
Local Central Office (CO)
Analogy phone
DSL
f (KHz)
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20 25
Phone
Fa Fb
Upstream
Fc
Downstream
HDSL
SDSL
VDSL
1.5M
1.5M
1.5~2.3M Up
1.5M
1.5M
13~52M Down
Symmetric Symm
Asym
2 pairs
1 pair
1 pair
3.7km
3.0km
1.4km
More and latest at Digital Subscriber Line (DSL):
Computer
http://en.wikipedia.org/wiki/Digital_subscriber_line
Spectrum of copper pair
0
xDSL
ADSL
Bit rate 32K~3M Up
32K~40M Down
Mode
Asymmetric
wire
1 pair
Length 3.7~5.5km
f (KHz)
FDM – Frequency Division Multiplexing
Frequency range (<20KHz) for analogy phone
Frequency range (>25Kz) for data transmission
Local line quality
Low frequency is better than high frequency
Characteristic of each line is different from others
Lecture 6
ADSL
DMT (Discrete MultiTone) by ANSI
- FDM and QAM modulation
- 31 channels (carriers) for upstream
- 255 channels (carriers) for downstream
Analogy
phone
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0
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f (KHz)
20 25
200 250
1M
Upstream Downstream
Spectrum of copper pair
ADSL2/2+, VDSL2, etc.
http://en.wikipedia.org/wiki/Asymmetric_Digital_Subscriber_Line_2
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f (KHz)
- Concurrent and independent uses of analogy phone and data transmission
- Upstream ~few Mbps, Downstream ~tenths M, suited for accessing Internet
Lecture 6
Cable Modem Technology
Cable TV already brings high bandwidth coax into houses
Conventional Cable TV is one direction
- Signal broadcast at central location
- Amplifiers boost signal through network
- Amplifiers are unidirectional
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M
CATV
Center
Amplifier
Solutions:
- Alternate upstream path - e.g., dialup
- Replaced by bi-directional amplifiers
M - Cable Modem
Cable modems encode and decode data from cable TV coax
- One in cable TV center connects to network
- One in home connects to computer
Users share the cable
- Each subscriber is assigned an address
- TDM is used like multiple computers in LAN
TV
Upstream Downstream
(shared by multiple users)
TV
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f (MHz)
Lecture 6
Cable Modem Connections
Hybrid Fibre-Coaxial (HFC)
Lecture 6
Cable Modem
Connections
Lecture 6
FTTX - Fiber To The X
Fiber to the x
Lecture 6
WAN and Switch
WAN – Wide Area Network: A network that can span a large geographic area,
e.g., multiple cities, countries or continents.
WAN building blocks:
(1) Long-distance connection (e.g., T1/T3 lines, OC1/OC3 lines, etc.)
(2) Packet switch
- A hardware device connected to other switches and computers
- Has CPU, memory, I/O interface, etc.
WAN, Wiki
- Handles packet
Packet
Header
Dest Addr
Frame Data or Payload
Sour Addr
Trailer
Lecture 6
Illustration of A WAN
Animation
- Place one or more switches at each site
- Interconnect switches: LAN technology for local connections
Leased digital lines for long distance connections Leased Line
- Interconnections depend on traffic amount and reliability requirement
- Packet is sent from source, travels switch-to-switch, and delivered to destination
- Switch stores packet in memory, examines address, and forwards it toward destination
Lecture 6
Addressing in WAN
- Each computer has a unique address
- It is a two-part hierarchical address including 2 integers: [Integer1, Integer2]
Integer1 for switch number
Integer2 for computer number
Lecture 6
Next-Hop Forwarding and Routing Table
Routing table of switch 2
- Packet switch must choose outgoing connection for forwarding
If destination is local computer, packet switch delivers computer port
If destination is attached another switch, this packet switch
forwards to next hop through connection to another switch
- Packet switch doesn't keep complete information about all possible destination
- A routing table just keeps next hop
- For each packet, packet switch looks up destination in its routing table and
forwards through connection to next hop
- Next hop to destination does not depend on source of packet
- Called source independence
Lecture 6
Routing Table and Graph
Graph: Node models switch
Edge models connection
Routing tables
without default routes
Routing tables
with default routes
- Each switch or node has a routing table:
Left entry
right entry
destination switch
edge number pair
- Default route can remove duplicate routes, reduce memory and improve performance
Lecture 6
Routing Table Computation
- Static routing table:
fixed in switch booting and does not change
- Dynamic routing table: initialized in booting and alters as conditions in the network change
Shortest path computation:
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Label on edge represents “distance”
Possible distance metric: geographic distance, economic cost, capacity, etc
“Shortest” means the minimum sum of distances in all paths between two nodes
Darkened path is minimum from node 4 to node 5
Algorithms for computing shortest paths
- Dijkstra’s algorithm
- Distance Vector (DV) algorithm
Dijkstra's algorithm
Distance Vector Routing
Lecture 6
History of WAN Technologies
ARPANET
- The first WAN, began in 1960s, low speed: 56Kbps
- Funded by Advanced Research Projects Agency, an organization of US Defense Department
- Incubator for many of current ideas, algorithms and internet technologies
X.25
- Early standard for connection-oriented networking from ITU, which was originally CCITT
- Early commercial service, more popular in Europe
- Predates computer connections, used for terminal/timesharing connection
Frame Relay
- Telco service for delivering blocks of data
- Connection-based service;
- Typically 56Kbps or 1.5Mbps; can run to 100Mbps
-SMDS - Switched Multi-megabit Data Service
- Connectionless service; any SMDS station can send a frame to any other station
- Typically 1.5-100Mbps
ATM - Asynchronous Transfer Mode
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Designed as single technology for voice, video, data, ...
Low jitter (variance in delivery time) and high capacity
Uses fixed size, small cells - 48 bytes data, 5 bytes header
Can connect multiple ATM switches into a network
Potential in future B-ISDN
Can be worked as a LAN
Lecture 6
Wireless Networks
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Wireless LANs
Wireless WANs
Satellite Networks
Cellular Networks
Region
BS: Base Station
BS
BS
BS
TAXI
City
BS
BS
BS
laptops, PDAs
Campus
BS
BS
In-Building
F: Carrier Frequency
Lecture 6
Cellular Wireless Networks
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Geographic region divided into cells
Frequencies/timeslots/codes reused at spatially-separated locations.
Co-channel interference between same color cells.
Cllular Network: http://en.wikipedia.org/wiki/Cellular_network
Lecture 6
Cellular Phone Networks
Lecture 6
Handoff in Cellular Networks
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Base stations/main stations coordinate handoff and control functions
Shrinking cell size increases capacity, as well as networking burden
Lecture 6
4 Generations of Data Communications
Lecture 6
Data Transmission Speeds
4G WWAN: http://en.wikipedia.org/wiki/4G
Exercise 6
1. For an analogy TV signal, its frequency range is in [0, 6MHz]. To transmit the TV signal across
a digital network, it is necessary to convert the analogy signal to a digital TV signal. What is
the minimum sampling frequency in such conversion? Suppose that every sample will be
encoded into 16 bits binary value (this is called TV PCM coding). Calculate bit rate of
the digital TV signal after PCM coding.
2. Summarize the features and performance of typical long-distance connection technologies
including T and OC series services, conventional modem, ISDN, xDSL and Cable modem.
3. Explain why bit rates of the upstream and the downstream in ADSL are not fixed?
4. In a packet switch network, the address of each computer consists two parts: one identifies
a switch and other identifies a computer attached to that switch. Why?
5. Suppose that a packet switch network with a five nodes is given below. Give a routing table
for each of the five nodes.
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