Midterm Review

1
Midterm Review

• CS460

2
Delay

• Speed of light
• 3.0 x 108 meters/second in a vacuum
• 2.3 x 108 meters/second in a cable
• 2.0 x 108 meters/second in a fiber

• Delay x Bandwidth Product

Delay
Bandwidth
3
Sequence Number

• Suppose you are designing a sliding window
  protocol for a 1-Mbps point-to-point link to the
  moon, which has a one-way latency of 1.25
  second.  How many bits do you need for the
  sequence number?
• 2.5 seconds 1Mbps 2.5Mbits 312Kbytes. We
  need a sequence range that will cover double this
  value or 624K. So 20 bits are needed.

4
Seven Layer Model
Application Presentation Session Transport
Application Presentation Session Transport
Email, FTP, www integer size, big
endian Synchronization(video/audio), name
space reliability, congestion control (TCP)
Routing Address framing Errors electrical signa
ls
Network Data Link Physical
Network Data Link Physical
IP Ether
Network Data Link Physical
Network Data Link Physical
5
Process Model

• Process per protocol
• Context switch at each protocol boundary
• No Semaphores
• Incoming and outgoing queue
• Slow

• Process per message
• Procedure call at each protocol boundary
• Semaphores needed
• Faster
• Error prone because of multiple threads running
  in a single protocol

6
Avoiding Copies

• Assume Data rate of 600Mbps73MBps
• Assume 16MHz memory bus that is 16 bits wide,
  this results in 32MBps
• For a MB message, one copy takes 1/32 sec
• The resulting maximum data rate is 32MBps
• Two copies take 1/32 sec 1/32 sec 1/16 sec
• The resulting maximum data rate is 16MBps
• Copies are necessary between user and system
  space if no special interface

7
Links
Sometimes you install your own Sometimes
leased from the phone company
STSSynchronous Transport SignalOCOptical
carrier
8
Non-Return to Zero (NRZ)
0 0 1 0 1 1 1 1 0 1 0 0
0 0 1 0
Bits
NRZ

• Problem Consecutive 1s or 0s
• Low signal (0) may be interpreted as no signal
• High signal (1) leads to baseline wander
• Unable to recover clock
• NRZI eliminates consecutive 1s

9
4B/5B

• Problem consecutive zeros
• Idea Every 4 bits of data is encoded in a 5-bit
  code, with the 5-bit codes selected to have no
  more than one leading 0 and no more than two
  trailing 0 (i.e., never get more than three
  consecutive 0s).
• Resulting 5-bit codes are then transmitted using
  the NRZI encoding. Achieves 80 efficiency.
• We already dealt with consecutive 1s with NRZI

10
Bit stuffing Example

• Original Data
• 0011111011000011111100
• Bit Stuffed
• 001111100110000111110100
• Receiver
• 01111110 00111110110000111110100 01111110

11

• Sender
• multiply M(x) by xk for our example, we get
• x10 x7 x6 x4 (10011010000)
• divide result by C(x) (1101)
• Send 10011010000 - 101 10011010101, since this
  must be exactly divisible by C(x)

11111001 10011010 000 Message 1101 1001
1101 1000 1101 1011 1101
1100 1101 1 000
1 101 101 Remainder
Generator 1101
12
IP Addresses
 
13
IP Addresses
14
Example
A
(3,3,0)
B
B3
(5,5,0)
(7,7,0)
(3,3,0)
(5,5,0)
C
B5
(2,2,0)
(5,5,0)
D
B7
K
B2
(7,7,0)
(2,2,0)
(7,7,0)
E
F
(1,1,0)
(1,1,0)
(1,1,0)
B1
(1,1,0)
G
H
(6,6,0)
(4,4,0)
(4,4,0)
B6
B4
(6,6,0)
(4,4,0)
I
J
15
Next Phase
A
(3,2,1)
B
B3
(5,1,1)
(7,1,1)
(5,1,1)
C
B5
(2,1,1)
D
B7
K
B2
(7,1,1)
E
F
(1,1,0)
(1,1,0)
(1,1,0)
B1
(1,1,0)
G
H
(4,1,1)
B6
B4
(6,1,1)
(4,1,1)
I
J
16
Next Phase
A
B
B3
(5,1,1)
(5,1,1)
C
B5
(2,1,1)
(7,1,1)
D
B7
K
B2
E
F
(1,1,0)
(1,1,0)
(1,1,0)
B1
(1,1,0)
G
H
(4,1,1)
B6
B4
(4,1,1)
I
J
17
Routers? Hubs? Switches
D 128.187.174.10 D 44.fe.34.56.32.d5 S
128.187.171.2 S fe.34.56.32.d5.29
D 128.187.174.10 D 44.fe.34.56.32.d5 S
128.187.171.2 S fe.34.56.32.d5.29
173
Hub
171
H1
H2
H3
H7
H8
H9
172
174
H4
H5
H6
H10
H11
H12
D 128.187.174.10 D 44.fe.34.56.32.d5 S
128.187.171.2 S fe.34.56.32.d5.29
D 128.187.174.10 D 44.fe.34.56.32.d5 S
128.187.171.2 S fe.34.56.32.d5.29
H10 IP 128.187.174.10, Ethernet 44.fe.34.56.32.d5
18
Forwarding

• H1 LANE layer receives a packet for H2
• Since H2 is not in our cache, we send query to
  LES for H2 ATM address
• Since the query and VC setup may take seconds, we
  start sending data through the BUS
• We may wait until we receive several packets
  before setting up a separate virtual circuit
• The LES replies with the ATM address of H2
• H1 sets up a direct VC to H2 and adds an entry in
  the cache for H2

19
More Questions

• What are the advantages and disadvantages of
  statistical multiplexing?
• What are the three kinds of failures that network
  designers have to worry about?
• Why is the delay x bandwidth product important?
• What is jitter? Why would it be difficult to send
  video over a network with a high jitter value?

20
More Questions

• Given a source routing example, determine the
  destination that a message will go to. (Something
  like Figure 3.7)
• Given a virtual circuit routing example,
  determine the destination that a message will go
  to. (Something like Figure 3.5)
• Compare datagrams, virtual circuits and source
  routing. What are the advantages and
  disadvantages of each approach?

21
More Questions

• What is the spanning tree algorithm and which
  links would be eliminated in a problem similar to
  Figure 3.11? How do bridges learn host locations?
• Why does an ATM cell have 48 bytes of payload?
• What kind of routing does ATM use?
• Given a banyan network similar to Figure 3.33,
  decide if there will be a contention free route
  for two packets.

22
More Questions

• What is the hidden node problem (page 138) and
  how does 802.11 deal with it?
• Describe the process for selecting an AP with a
  mobile node (pages 140-141).
• What is an internetwork?
• What is the service model for IP?
• How is fragmentation performed with IPv4?
• Assume a host with an IP address of
  222.123.250.3. To what class does this IP address
  belong? To what network is it connected?

23
More Questions

• What is an ARP packet and how is it used? How
  would ARP be implemented in a network such as ATM
  that doesnt have broadcast capabilities?
• What does an ARP cache contain? How does the ARP
  protocol work?
• Is source routing practical? Explain.
• What type of IP address class would be adequate
  for a corporation that needs to establish up to
  100 sub-networks, each of which with up to 500
  hosts? Justify your choice.

24
More Questions

• Assume the following routing table
• What does the router do with the following
  packets?
• a) 128.96.39.10 b) 128.96.40.12 c) 128.96.40.151
  d) 192.4.153.17 e) 192.4.153.90
• What are the problems with stop and wait, and how
  are they solved?

25
Architecture
LES
Mac
Mac
Mac
Mac
Ethernet
BUS
Relay
ATM Switch
Mac
LEC
ATM
Plug and Play Allows ATM hosts to
connect Simplified Core Switch, complex edge
switch
26
Architecture
Mac
Mac
Mac
Mac
Ethernet
LEC
Relay
LES
BUS
ATM Switch
Mac
ATM
PVC static configuration Simplified Edge switch,
complex core switch
27
Architecture
POS
Mac
Mac
Mac
Mac
Ethernet
Relay
Relay
Ether Switch
Mac
POS
Simplified Edge switch, point to point link, not
scalable, time division multiplexing leased
line expense