Download Lecture 2 - University of Delaware

Introduction to Data Networking
Introduction to this class
•
•
•
•
Me: Stephan Bohacek
[email protected], 302-831-4274, skype: Stephan.Bohacek
http://www.eecis.udel.edu/~bohacek
Syllabus (online)
Syllabus (also online)
• Textbook: Kurose and Rose. Computer Networking, 2007 (the 4th
edition). This book is required
• Prerequisites: Introduction to probability, C/C++ programming
• Grading: homework=1/3, projects=1/3, final=1/3. Homework and
projects turned in late will be marked off 2.5% per day (including
weekends). Grades of online discussion are based on the number and
qualtiy of postings. These postings may be in the form of questions and
answers. A good question will count toward your discussion grade.
• There will be programming assignments. This can be done on Linux or
on Windows with Visual Studio. Evans 132 has linux machines and
remote access is possible. Also, with your EECIS user name and
password, MS visual studio can be downloaded for free from here
Syllabus (also online)
Today – networking basics
•
•
•
•
•
•
Movie on the history of the Internet
Core components of the Internet – the protocol stack
Multiplexing, circuit switching, and packet switching
Loss and delays
The structure of the Internet
This lecture covers much of chapter 1 in the textbook.
Today – networking basics
•
•
•
•
•
•
Movie on the history of the Internet
Core components of the Internet – the protocol stack
Multiplexing, circuit switching, and packet switching
Loss and delays
The structure of the Internet
This lecture covers much of chapter 1 in the textbook.
Today – networking basics
•
•
•
•
•
•
Movie on the history of the Internet
Core components of the Internet – the protocol stack
Multiplexing, circuit switching, and packet switching
Loss and delays
The structure of the Internet
This lecture covers much of chapter 1 in the textbook.
Core components
•
End-hosts
•
Applications
–
•
Packets
–
•
•
?
Routers and gateways
and groups of routers
(ISPs)
Links
–
•
?
?
Protocols
Core components
•
•
End-hosts
Applications
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–
–
–
•
Packets
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–
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•
TCP
UDP
Routers and gateways
and groups of routers
(ISPs)
Links
–
–
–
–
•
Web
Email
File transfer
File sharing
Fiber
Coaxial
Twisted pair
Wireless
Protocols
Application Layer – where the applications live
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•
End-hosts
Applications
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–
–
–
•
•
–
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TCP
UDP
Fiber
Coaxial
Twisted pair
Wireless
Protocols
Rules/protocols for how an end-host gets mail from the mail server
Web:
–
Routers and gateways
•
and groups of routers
(ISPs)
Links
–
–
–
–
•
Email:
Rules/protocols for how the end-hosts gets a web page from the web servers
Question:
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Packets
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–
•
Web
Email
File transfer
File sharing
•
–
How is a networking application different from a non-networking application
(e.g., MS Word). That is, why do we say that an application is a bunch of rules?
MS-Word is not a bunch of rules?
Answer: The networking applications must communicate, and rules are required
to define the communication.
Roles that end-hosts play:
–
–
Client, server, and peer
The client asks the server for a service.
•
•
•
–
E.g., The client asks the server to send a mail for it.
The client asks the server for a web page
The client asks the server to translate a web address to an IP address.
Peer: A host can act as both a client and a server. But usually in one transaction,
the host takes only one role
Layers 1-4
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•
End-hosts
Applications
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–
–
–
•
•

Routers and gateways 
and groups of routers

(ISPs)

Links


Protocols
–
–
–
–
•
Web
Email
File transfer
File sharing
Packets
–
–
•
Which are the end-host?
TCP
UDP
Fiber
Coaxial
Twisted pair
Wireless
client
server
Routers
Layers 1-4
•
•
End-hosts
Applications
–
–
–
–
•
•

Routers and gateways
and groups of routers

(ISPs)

Links


Protocols
–
–
–
–
•
Why is this a good
approach?
1.Small problems are easier
to understand/solve.
2.Different solutions can be
mixed and matched
Packets
–
–
•
Web
Email
File transfer
File sharing
Goal: move messages from server to the client
Approach: break the problem into little pieces.
Each piece is a layer in the “protocol stack”
TCP
UDP
Fiber
Coaxial
Twisted pair
Wireless
client
server
Layers 1-4
•
•
End-hosts
Applications
–
–
–
–
•
•
•
•
Web
Email
File transfer
File sharing
Packets

Routers and gateways
and groups of routers

(ISPs)

Links


Protocols
–
–
TCP
UDP
–
–
–
–
Fiber
Coaxial
Twisted pair
Wireless
client
server
Top down approach of breaking problems into small pieces
1. Transport layer
1.Reliability: The server must make sure that the client gets the data
Congestion control (or lack there of)
2.Congestion Control: The server should send data as fast as possible, but not too fast
3.TCP provides these features (services), while UDP does not
2. Network layer (could be called the routing layer, but it isn’t)
1.The packets must find their way through the network.
2.Each packet has the IP address of the destination
3.By examining the IP address, routers decide where to send the packet next
3. Link Layer or MAC layer
1.Links connect the routers/gateways and end-hosts
2.This layer provides logical and control for communicating across links.
3.Services that this layer might provide include
1. congestion control, media access, error detection/correction
Layers 1-4
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•
End-hosts
Applications
–
–
–
–
•
•
•
•
Web
Email
File transfer
File sharing
Top down approach of breaking problems into small pieces
…..
3.
Link Layer or MAC layer
1.
Links connect the routers/gateways and end-hosts
2.
This layer provides logical and control for communicating across links.
3.
Services that this layer might provide include
1.
congestion control, media access, error detection/correction
Packets

Routers and gateways
and groups of routers

(ISPs)

Links


Protocols
–
–
TCP
UDP
–
–
–
–
Fiber
Coaxial
Twisted pair
Wireless
1. Media access. The “air” is a shared medium. If two nodes transmit at the same time,
there will be a collision. Thus, a scheme must be developed to determine which node
transmits when.
2. Error detection/correction. If interference does occur, then errors might occur. If an
error is detected, then
1. the error could be corrected with forward error correction, or
2. the receiving link could request a retransmission
Layers 1-4
•
•
End-hosts
Applications
–
–
–
–
•
•
•
•
Web
Email
File transfer
File sharing
Packets

Routers and gateways
and groups of routers

(ISPs)

Links


Protocols
–
–
TCP
UDP
–
–
–
–
Fiber
Coaxial
Twisted pair
Wireless
client
server
Top down approach of breaking problems into small pieces
1.
Transport layer
1.
Reliability: The server must make sure that the client gets the data
Congestion control (or lack there of)
2.
Congestion Control: The server should send data as fast as possible, but not too fast
3.
TCP provides these features (services), while UDP does not
2.
Network layer (could be called the routing layer, but it isn’t)
1.
The packets must find their way through the network.
2.
Each packet has the IP address of the destination
3.
By examining the IP address, routers decide where to send the packet next
3.
Link Layer or MAC layer
1.
Links connect the routers/gateways and end-hosts
2.
This layer provides logical and control for communicating across links.
3.
Services that this layer might provide include
1.
congestion control, media access, error detection/correction
4.
Physical layer
1.
Logical bits are encoded as physical quantities, e.g., as voltage levels, as shifts in phase, …
2.
This course does not cover the physical layer
Protocols
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End-hosts
Applications
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–
–
–
•
Packets
–
–
•
•
TCP
UDP
Routers and gateways
and groups of routers
(ISPs)
Links
–
–
–
–
•
Web
Email
File transfer
File sharing
Fiber
Coaxial
Twisted pair
Wireless
Protocols
protocols define format, order of msgs sent and received
among network entities, and actions taken on msg
transmission, receipt
Hi
TCP connection
request
Hi
TCP connection
response
Got the
time?
Get http://www.awl.com/kurose-ross
2:00
<file>
time
Internet protocol stack
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application: supporting network applications
– FTP, SMTP, HTTP
•
transport: process-process data transfer
application
– TCP, UDP
•
network: routing of datagrams from source to
destination
– IP, routing protocols
•
link: data transfer between neighboring network
elements
– PPP, Ethernet
•
physical: bits “on the wire”
transport
network
link
physical
ISO/OSI reference model
•
•
•
presentation: allow applications to interpret meaning of
data, e.g., encryption, compression, machine-specific
conventions
session: synchronization, checkpointing, recovery of
data exchange
Internet stack “missing” these layers!
– these services, if needed, must be implemented in
application
– needed?
application
presentation
session
transport
network
link
physical
Today – networking basics
•
•
•
•
•
•
Movie on the history of the Internet
Core components of the Internet – the protocol stack
Multiplexing, circuit switching, and packet switching
Loss and delays
The structure of the Internet
This lecture covers much of chapter 1 in the textbook.
Circuit switching versus Packet switching
• Packet switching brought the networking revolution
• Circuit switching
• Virtual circuit networking
– A half-way point between packet switched and circuit switched
networking
Circuit switching
•
Circuit switching
– Old style phone system
– Each connection gets its own wire or
bandwidth
– Note: calls must be set-up.
– E.g.,
• Me: operator, get my the president.
• Operator: one moment please.
• Then she plugs a cable into a socket so
now I have a physical wired between me
and the president.
– Instead of each connection getting a whole
wire, connections can share a wire via
multiplexing
– The first automatic circuit switching was
developed by Almon Strowger – an
undertaker. There were two undertakers in
a small town and the switch board
operator was the wife of the other
undertaker. So Strowger invented an
automatic circuit switch to rid both
husband and wife of employment.
Frequency division multiplexing
On each hop, the connection gets its own bandwidth
phone
End office
300 3400
toll office
100300 103400
End office
200300 203400
TV is frequency division multiplexing
phone
300 3400
Time division multiplexing
There are standard bit-rates that support
multiplexing different numbers of calls
64kbits
1 2 3
bytes
1/8000 sec per byte
111 2
Multiplex 24 channels
= 24*64kbps + overhead
= 1.544Mbps DS1 (T1)
Overhead is 1 bit per 8*24 bits
= 8000bps
Multiplex 28 DS1
= 28*24*64kbps + overhead = 44.736Mbps DS-3
Multiplexing 810 channels + overhead = 51.84 = STS-1/OC-1
STS is electrical and oc is optical
OC3 = 155.52Mbps (150.336 payload)
OC12 = 633.08 Mbps (601.344 payload)
OC48 = 2.488Gbps (2.405Gbps)
OC192 = 9.953Gbps (9.6Gbps payload)
7 bits of data and
one bit for control
(data or not), so it
really 56kbps of
data
Note all the control overhead: if the bit is 1, then payload is control. Lots of control is
needed to setup a circuit. How is it possible to get channels at each hop?
Also, if there is not data, then nothing is sent. This wastes data.
But the circuit is yours, guaranteed!
Packet switching - Statistical multiplexing
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•
•
•
Data is in packets, not streams.
Must be digital
Each packet has an address
A switch/router reads the whole packet, then reads the address and forwards the packet –
store and forward
packet. Note format
specification specifies
where the address is
data 1
client
Server: address = 1
Packet switching - Statistical multiplexing
•
•
•
•
Data is in packets, not streams.
Must be digital
Each packet has an address
A switch/router reads the whole packet, then reads the address and forwards the packet –
store and forward
If destination
If destination
is 1, then next
hop is B
A
data 1
is 1, then next
hop is C
B
If destination
is 1, then next
data 1
hop is
data 1
C
data 1
D
client
Server: address = 1
F
E
Packet switching - Statistical multiplexing
•
•
•
•
•
•
Data is in packets, not streams.
Must be digital
Each packet has an address
A switch/router reads the whole packet, then reads the address and forwards the packet –
store and forward
No reservations are needed. First come first serve.
Major benefit:
–
•
If you need more bandwidth, then you can get it, it you don’t need it, then maybe someone else
can use it.
Major drawback:
–
What happens if two packets arrive at a switch and both need to go to the same output interface.
Picture. One packet is either dropped, or is placed in a buffer. Either way, something bad has
happened, the packet is gone, or is delayed. This would never happen on a circuit switched
network.  queuing delay and packet loss 
Packet vs. Circuit Switching
If usage is random (e.g., web surfing) statistical multiplexing is better.
Suppose that
1. A 5Mbps link
2. Each user needs 50kbps
3. And each user is active 20% of the time. (note that this condition does not matter for circuit switching. Why?)
How many users can be accommodated under packet switching and how many can be accommodated under packet
switching?
Circuit switching case
The total number of users that can be accommodated with circuit switching is 5e6/50e3 =
100 users
Packet Switching Case
Now if there are 200 users, what is the probability that there are 150 or more active users?
In this case, there would be a problem, since the network cannot support more than 100 active users.
Simpler questions: What is the probability of 150 particular users being active and 50 other being inactive?
Packet Switching Case
What is the probability of more than 100 users being active?
The probability of 101 users being active plus, 102 users being active, plus, …., 200 users being active,
which is
Packet Switching vs. Circuit Switching
A couple of things:
What does this probability really mean?
 k 
  300 0.2 1  0.2
300
k
k 101


300 k
 10 8
This means that
• when you walk into the switching center, the probability of finding overload is 10^-8.
• Or, if you random access the link, the probability of finding it in overload.
• Once you find it in overload, or not, the probability that is will be in overload in the next second is more complicated
and requires queuing theory. This analysis might reveal worst performance.
In this example, we assumed 20% user utilization (they were active 20% of the time)
Is this large or small?
If it the user utilization is smaller, then the difference between packet switching and circuit switching is
even larger. But it is smaller, then there is less of a difference.
What is your user utilization?
•For web surfing
•For cell phone usage
•For music streaming
Packet Switching vs. Circuit Switching
• If loss and delay are permissible and usage is random, then packet
switching is better than circuit switching.
• If usage is very regular (e.g. TV!), circuit switching is best.
• If losses and delay are not permissible, then circuit switching is best
(e.g., remote controlled surgery).
• With packet switching, congestion control is required. Also, there is
more overhead for each packet.
• For circuit switching, once the circuit is setup, it can be very efficient.
But circuits must be set-up.
• So, for short file transfer, packet switching is good but for long file
transfers, circuit switching might be better.
There is a subtle difference between packet switching and statistical multiplexing.
Statistical multiplexing means to use the resource as needed. This leads to the
performance improvements mentioned but also the complications (delay and loss).
The phone network uses circuit switching, but the circuits are statistically
multiplexed between users. In packet switching, links are statistically multiplexed.
Packet Switching: Statistical Multiplexing
100 Mb/s
Ethernet
A
B
statistical multiplexing
C
1.5 Mb/s
queue of packets
waiting for output
link
D
E
Sequence of A & B packets does not have fixed pattern, bandwidth shared on demand 
statistical multiplexing.
TDM: each host gets same slot in revolving TDM frame.
Packet-switching: store-and-forward
L
R
R
R
Example:
• L = 7.5 Mbits
• R = 1.5 Mbps
• transmission delay = 15 sec
• takes L/R seconds to
transmit (push out) packet
of L bits on to link at R bps
• store and forward: entire
packet must arrive at
router before it can be
transmitted on next link
• delay = 3L/R (assuming more on delay shortly …
zero propagation delay)
Today – networking basics
•
•
•
•
•
•
Movie on the history of the Internet
Core components of the Internet – the protocol stack
Multiplexing, circuit switching, and packet switching
Loss and delays
The structure of the Internet
This lecture covers much of chapter 1 in the textbook.
Losses and delay in packet switched networks
• Losses
– Transmission losses
• In fiber links, bit-error is 10^-12 or better (i.e., less).
– What is the probability of packet error when there are 1400 bytes in a packet?
• In wireless links, the bit-error rate can be very high
– Congestion losses.
• If too many packets arrive at the same time, then the buffers will fill up and
packets are lost.
• Increasing the link speeds or reducing the number of users can reduce the
probability of loss.
• Increasing the size of the buffer reduces losses, but also increases delay.
• Delay
–
–
–
–
Queuing delay
Transmission delay
Propagation delay
Processing delay
packet being transmitted (delay)
A
B
packets queueing (delay)
free (available) buffers: arriving packets
dropped (loss) if no free buffers
Queuing delay
packet being transmitted (delay)
A
B
•
•
•
packets queueing (delay)
free (available) buffers: arriving packets
dropped (loss) if no free buffers
Queuing delay occurs for the same reason as congestion losses.
The more the network is utilized, the high the queueing delay (and losses)
Utilization =  := actual use / maximum possible use
Suppose that
• the link bit-rate is Z,
• there are X users
• Each users uses data rate Y, with probability P, and use no bandwidth with probability 1-p.
 = X*P/Z
Queuing delay
Is it possible to have a network run at full utilization?
No! The average delay would be infinite!
From queuing theory
Delay = /(1- )

Delay in packet switched networks
•
Delay
–
–
–
–
Queuing delay
Transmission delay
Propagation delay
Processing delay
How long does it take to transmit a packet?
How long does it take to get all the bits from node on to the wire/air/fiber?
Suppose
•Link bit rate is 10 Mbps
•Packet size is 1400 bytes
How long to transmit the packet?
1400 *8 bits / packet
10*10^6 bits / sec
= .0011 sec = 1.1 ms
Delay in packet switched networks
•
Delay
–
–
–
–
Queuing delay
Transmission delay
Propagation delay
Processing delay
–
Suppose
•
•
How long does it take for a bit to travel along a wire/fiber/through the air?
Speed of light in a vacuum 3e8 m/s while in a fiber it is 2e8m/s
How long does it take to transmit a bit from NY to LA = 3962km
–
20ms propagation delay
•
How about from NY to Jakarta, Indonesia = 16,179km
•
How about to a Geostationary satellite?
–
–
•
250-300 (up and back)
Medium orbit satellites (e.g., GPS)
–
•
80ms
120ms
Low-earth orbit satellites (low earth? What about middle-earth?)
–
–
Iridium at 10ms
» Note, Iridium paid 5 billion for the network and sold for 25million (1/2%->on sale 99.5% off, everything must go)
Teledesic. 10ms
Fun with Propagation Delay
How long is a bit?
Suppose that a links transmits at 10mbps. How long is a bit?
How long does it take to a bit?
1/10*10^6 = 10^-7
How far does the electric signal go in 10^7 sec?
10^-7 * 2e8 = 20 meters.
How long many bits fit in a fiber at 10Mbps from NY to Jakarta?
16,179km*10^3/20 = 0.1 MB
How long many bits fit in a fiber at 10 Gbps from NY to Jakarta?
16,179km*10^3/20 = 100 MB
Satellite transmissions are subject to transmission loss (e.g., rain can cause interference),
The satellite sending station could wait for and ACK from the other side and resend the data if no
ACK appeared (a link layer solution)
But this would cause out-of-order delivery
So the satellite could hold the packets until the lost one is retransmitted.
How large would the buffer need to be if the bit rate was 3Gbps?
Answer .5*3e9/8=187MB (assuming no processor delay)
Delay in packet switched networks
•
Delay
–
–
–
–
Queuing delay
Transmission delay
Propagation delay
Processing delay
Routers take a bit of time to process packets.
• moving packets inside the router
• Finding which is the next hop
• Applying security or QoS
How to measure delay?
•
•
•
•
Ping: > ping 216.109.124.73
Ping gives help
(linux) Ping –I 10 216.109.124.73 > file.txt
Then read it in excel and plot delay
•
•
Traceroute (linux), tracert (windows)
Traceroute 216.109.124.73 gives the routers and an estimate of the delay to
each router.
Today – networking basics
•
•
•
•
•
•
Movie on the history of the Internet
Core components of the Internet – the protocol stack
Multiplexing, circuit switching, and packet switching
Loss and delays
The structure of the Internet
This lecture covers much of chapter 1 in the textbook.
Internet structure: network of networks
•
•
roughly hierarchical
at center: “tier-1” ISPs (e.g., Verizon, Sprint, AT&T, Cable and Wireless),
national/international coverage
– treat each other as equals
Tier-1
providers
interconnect
(peer)
privately
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Tier-1 ISP: e.g., Sprint
POP: point-of-presence
to/from backbone
peering
…
…
.
…
…
…
to/from customers
Internet structure: network of networks
•
“Tier-2” ISPs: smaller (often regional) ISPs
– Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs
Tier-2 ISP pays
tier-1 ISP for
connectivity to
rest of Internet
 tier-2 ISP is
customer of
tier-1 provider
Tier-2 ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
Tier 1 ISP
Tier-2 ISP
Tier-2 ISPs
also peer
privately with
each other.
Tier-2 ISP
Internet structure: network of networks
•
“Tier-3” ISPs and local ISPs
– last hop (“access”) network (closest to end systems)
local
ISP
Local and tier3 ISPs are
customers of
higher tier
ISPs
connecting
them to rest
of Internet
Tier 3
ISP
Tier-2 ISP
local
ISP
local
ISP
local
ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local
local
ISP
ISP
Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP
Internet structure: network of networks
•
a packet passes through many networks!
local
ISP
Tier 3
ISP
Tier-2 ISP
local
ISP
local
ISP
local
ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local
local
ISP
ISP
Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP
ISPs and the structure of the Internet
•
Video of a Network Access Point (NAP) in Los Angeles
MEET ME ROOM
Said to be the most interconnected space in the world and the most expensive real
estate in North America, the “Meet Me Room” (a telco industry term) is the heart of
One Wilshire. Here the primary fiber optic cables are routed, split, and shared.
Because of the presence of so many telcos in this room and the ability to freely
interconnect between them, rackspace here becomes extremely valuable. For
comparison, the average price for office space in downtown Los Angeles is $1.75 per
square foot per month. At the Meet Me Room, $250 per square foot would be a
bargain.
CABLE RISERS
Some 1,800 known conduits contain the fiber optic cables that
flow through the building’s stairwells and vertical utility
corridors, called “risers.” Cable connects the commercial telco
tenants on floors 5 through 29 to the 4th floor Meet Me Room,
and to a new, “wireless” Meet Me Room constructed on the
30th floor.
SURFACE CABLE MAP
Whenever a permit is pulled by a city contractor for any
underground repairs outside One Wilshire, the various telco
companies with cable in the area come out and paint the cable
routes on the asphalt, creating a visible graphic of the complexity
of what lies just under the surface.
HVAC
Computers generate a lot of heat, and maintaining a stable, cool temperature and
a low humidity is essential in telco hotels, so tenants sometimes demand to install
their own cooling systems to safeguard their equipment. At One Wilshire, these
units are installed primarily on the third floor roof. A new closed loop cooling
system has been installed on the 30th floor roof.
CABLE MINING
As tenants’ needs change, cables can go unused. Cable mining is performed to
thin out the obsolete cables and future congestion is alleviated through the
installation of dedicated new ducts.
ELECTRICITY
Power is supplied by DWP, but in the event of a blackout, the building’s
five generators will kick in. It takes the generators three seconds to start
up and stabilize. During this brief period, the entire building runs on
batteries. There are 11,000 gallons of diesel stored on site, enough to run
the generators for 24 hours before being refueled.
MICROWAVE
On the roof, microwave antennas link up One Wilshire to transmission towers located around
the city. Though fiber’s higher capacity has given it dominance over microwave at One Wilshire,
microwave’s relatively low cost over long distances continues to make it economical for some
applications. The roof’s clear line of sight to the south, west, and to other high-rises, along
with the ability to interface with the fiber inside, continues to make One Wilshire an attractive
location for microwave-based transmission.
READING A ROOF
Much can be learned about a building’s function by examining its roof. The
existence of telco hotels in the region around One Wilshire is indicated by the
presence of new and extensive cooling units on the roofs of adjacent buildings,
many of which were nearly vacant until the telco companies moved in.
POINT OF ENTRY
The main fiber optic cables connecting One Wilshire to the world enter the building
from under the street through closets in the walls of the building’s parking garage.
Given the importance of the building to the global communications network, access
to the parking garage is controlled, and the building is said to be monitored
continuously by federal security officials.
MEET ME ROOM
Said to be the most interconnected space in the world and the most expensive real
estate in North America, the “Meet Me Room” (a telco industry term) is the heart of
One Wilshire. Here the primary fiber optic cables are routed, split, and shared.
Because of the presence of so many telcos in this room and the ability to freely
interconnect between them, rackspace here becomes extremely valuable. For
comparison, the average price for office space in downtown Los Angeles is $1.75 per
square foot per month. At the Meet Me Room, $250 per square foot would be a
bargain.
CABLE RISERS
Some 1,800 known conduits contain the fiber optic cables that
flow through the building’s stairwells and vertical utility
corridors, called “risers.” Cable connects the commercial telco
tenants on floors 5 through 29 to the 4th floor Meet Me Room,
and to a new, “wireless” Meet Me Room constructed on the
30th floor.
SURFACE CABLE MAP
Whenever a permit is pulled by a city contractor for any
underground repairs outside One Wilshire, the various telco
companies with cable in the area come out and paint the cable
routes on the asphalt, creating a visible graphic of the complexity
of what lies just under the surface.
HVAC
Computers generate a lot of heat, and maintaining a stable, cool temperature and
a low humidity is essential in telco hotels, so tenants sometimes demand to install
their own cooling systems to safeguard their equipment. At One Wilshire, these
units are installed primarily on the third floor roof. A new closed loop cooling
system has been installed on the 30th floor roof.
CABLE MINING
As tenants’ needs change, cables can go unused. Cable mining is performed to
thin out the obsolete cables and future congestion is alleviated through the
installation of dedicated new ducts.
ELECTRICITY
Power is supplied by DWP, but in the event of a blackout, the building’s
five generators will kick in. It takes the generators three seconds to start
up and stabilize. During this brief period, the entire building runs on
batteries. There are 11,000 gallons of diesel stored on site, enough to run
the generators for 24 hours before being refueled.
MICROWAVE
On the roof, microwave antennas link up One Wilshire to transmission towers located around
the city. Though fiber’s higher capacity has given it dominance over microwave at One Wilshire,
microwave’s relatively low cost over long distances continues to make it economical for some
applications. The roof’s clear line of sight to the south, west, and to other high-rises, along
with the ability to interface with the fiber inside, continues to make One Wilshire an attractive
location for microwave-based transmission.
READING A ROOF
Much can be learned about a building’s function by examining its roof. The
existence of telco hotels in the region around One Wilshire is indicated by the
presence of new and extensive cooling units on the roofs of adjacent buildings,
many of which were nearly vacant until the telco companies moved in.
POINT OF ENTRY
The main fiber optic cables connecting One Wilshire to the world enter the building
from under the street through closets in the walls of the building’s parking garage.
Given the importance of the building to the global communications network, access
to the parking garage is controlled, and the building is said to be monitored
continuously by federal security officials.
Homework
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Page 61. Questions (3, 7), 8, (9), (10), 11, 13, 14, 19, (20), 21, (22), (23)
Page 63. Problems 2, (3), 6, 7, 8, (10), (11), (12)
Use trace route to determine the average number of hops between 10
destinations of your choice.
Use ping to determine the propagation delay. Specifically, send very small
packets (these will be 24 bytes).Then send ICMP packets with larger payload.
Compare the difference in the RTT and determine the transmission time.
Do links have time-varying delay? To answer this questions run trace route at
different times of the day (e.g., the middle of the night, morning, afternoon,
etc) and compare the delay times.