Lecture 4
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Project 1 posted, due September 15
(Thursday).
Reminder: Wireshark Project 1 is due today,
Homework 1 due on Thursday.
Questions?
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Outline
Chapter 1 - Getting Connected
2.1 Perspectives on Connecting
2.2 Encoding
2.3 Framing
2.4 Error Detection
2.5 Reliable Transmission
2.6 Ethernet
2.8 Wireless
2.9 Summary
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Introduction
After connecting two hosts there are other issues
to resolve before the two hosts can
communicate:
1) How are bits encoded onto the medium?
2) How do we separate bits into frames?
3) How are errors detected?
4) How do we ensure reliability?
5) How is access controlled on a shared link?
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Perspectives on Connecting
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Recall that networks
are constructed from
nodes and links.
Network needs to
connect all types of
links together to
provide reliable and
useful service to the
higher layers of the
protocol stack.
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Figure 2.1 End-user's view of the Internet
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Classes of Links
1) Speed of Propagation and frequency depends
on medium (e.g. copper, optic fiber, air)
WaveLength = SpeedOfPropagation/Frequency
2) Different encoding methods can be used to
translate bits to “high” and “low” signals.
3) Different modulation methods can be used to
translate the “high” and “low” signals to electronic
waveforms.
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Classes of Links
4) How the links are
used
"Last mile" links to
users.
Long-distance
backbone links,
mostly optical fiber
Building-wide LAN
links, mostly Ethernet
but also wireless
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Service
Dial-up
Bandwidth (typical)
28 – 56 kbps
ISDN
DSL
CATV
FTTH
64 – 128 kbps
128 kbps – 100 Mbps
1 – 40 Mbps
50 Mbps - 1 Gbps
Table 2.1 Common services
for home connections
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Encoding
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Most encoding/decoding functions are
performed by a network adapter.
Figure 2.3 Signals/bits flow between nodes
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NRZ Encoding
There are several methods used to map (encode)
bits to “high” and “low” signals.
Nonreturn to zero (NRZ) is the simplest.
Figure 2.4 NRZ encoding of a bitstream.
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NRZ Encoding
There are two problems with NRZ:
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The receiver averages the signal to determine a
detection threshold. A long stream of lows or
highs can cause the threshold to drift from the
desired value.
Frequent transitions are needed for the receiver
clock to maintain synchronization with the
transmitter clock. A long stream of lows or
highs may again cause problems.
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NRZI Encoding
Nonreturn to Zero Inverted (NRZI) encoding uses
a transition to represent a 1. No transition
represents a 0.
Figure 2.5 Different encodings
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Manchester Encoding
NRZI fixes problems due to a stream of 1s, but
does nothing to address the problems due to a
stream of 0s.
In Manchester encoding the NRZ signal is
exclusive-or'ed with the clock signal. This results
in a transition each clock period.
With Manchester encoding the bit transmit rate is
one-half the signal change rate (the baud rate).
The encoding is considered to be 50% efficient.
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4B/5B Encoding
In 4B/5B encoding, four bits are encoded as five
bits in such a way that there are never more than
three consecutive 0s. The bits are then further
encoded using NRZI so consecutive 1s do not
present a problem. 4B/5B coding is 80% efficient.
Only 16 of the 32 five bit codes are needed for
encoding, some of the remaining 16 codes are
used for signaling (11111 – line idle) and control.
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In-class Exercise, Part 1
Problem 2.2 (page 153)
The 4B/5B encoding table is on page 82.
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Framing
In packet-switched networks packets (called
frames at this level) are exchanged between
hosts, not continuous bit streams.
The central challenge is determining what set of
bits make up a frame.
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Byte-Oriented Protocols
In byte oriented framing protocols a frame is
viewed as a collection of bytes.
Two approaches (sentinel and byte-count) are
used to indicate the message size.
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BISYNC - Binary Synchronous
Communication
The BISYNC format uses sentinel characters to
mark the start of the frame (SYN) and the start
and end of the message (STX & ETX).
An ETX character in the body is “escaped”
(preceded by) a DLE character. A DLE character
is escaped by another DLE. This technique is
known as character stuffing.
Fig. 2.7 BISYNC Frame Format
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PPP - Point-to-Point Protocol
PPP is often used over modem links to carry IP
packets. It is also used on some fiber optic links.
The protocol field is used to demultiplex high-level
protocols (IP/IPX/LPC).
PPP uses the Link Control Protocol (LPC) to
negotiate the frame format, datagram size and
escaped characters.
Fig. 2.8 PPP Frame
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DDCMP - Digital Data Comm.
Message Protocol
Instead of using a sentinel character, DDCMP
includes a Count field in the frame header that
indicates the number of bytes in the body.
Fig. 2.9 DDCMP Frame
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HDLC - High Level Data Link
Control
HDLC is a bit-oriented protocol that uses the bit
sequence 01111110 to mark the start and end of
the frame.
Bit stuffing is used if this sequence appears
anywhere else in the frame. After five 1s the
transmitter inserts a 0. The receiver discards any
0 that appears after five 1s.
Fig. 2.10 HDLC Frame
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Clock-Based Framing
SONET - Synch. Optical Network
A SONET STS-1 frame consists of 9 rows of 90
bytes. The first two bytes in the frame contain a
special pattern to indicate the start of the frame.
Fig. 2.11 A SONET STS-1 Frame
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SONET - Synch. Optical Network
Each row contains three overhead bytes. The
overhead bytes are encoded using NRZ. The
payload bytes are scrambled by XORing them
with a well-known bit pattern. The pattern is
chosen to ensure enough transitions in the
payload to maintain clock synchronization.
Refer to the text for a more complete discussion
of SONET.
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In-class Exercise, Part 2
Problem 2.7 (page 154)
Problem 2.8 (page 154)
Turn in the in-class exercises before you leave.
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