CS475 – Networks
Lecture 4
Chapter 2: Direct Link Networks
Assignments
• Reading for Lecture 5: Sections 2.4 – 2.5
• Programming Project 1 out, due Thursday, September 15
Chapter 2: Introduction
After connecting two hosts there are other issues to resolve
3) How are errors detected?
before the two hosts can communicate:
4) How do we ensure reliability?
1) How are bits ________________ onto the medium?
5) How is access controlled on a shared link?
2) How do we separate bits into frames?
2.1 Perspectives on Connecting
Recall that networks are constructed from nodes and ________.
Network needs to connect all types of links together to provide
reliable and useful service to the higher layers of the protocol
stack.
Fig 2.1 End-user's view of the Internet
2.1.1 Classes of Links
1. Speed of Propagation and frequency depend on medium
(e.g. copper, optical fiber, air)
translate bits to “high” and “low” signals.
3.
WaveLength = SpeedOfPropagation / Frequency
Different modulation methods can be used to translate
the “high” and “low” signals to electronic waveforms.
2.
Different ______________ methods can be used to
4.
How the links are used
Service Bandwidth (typical)
Dial-up 28 – 56 kbps
"Last mile" links to users.
______ 64 – 128 kbps
Long-distance backbone links, mostly optical fiber
Building-wide LAN links, mostly Ethernet but also
DSL 128 kbps – 100 Mbps
CATV 1 – 40 Mbps
FTTH 50 Mbps - 1 Gbps
wireless
Table 2.1 Common services for home connections
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2.2 Encoding
Most encoding/decoding functions are performed by a
____________________________.
NRZ Encoding
There are several methods used to map (encode) bits to “high”
and “low” signals.
Figure 2.3 NRZ encoding of a bitstream.
________________________________ (NRZ) is the simplest.
There are two problems with NRZ:
2) Frequent transitions are needed for the receiver clock to
1) The receiver averages the signal to determine a detection
maintain ___________________________ with the transmitter
threshold. A long stream of lows or highs can cause the
clock. A long stream of lows or highs may again cause
threshold to drift from the desired value.
problems.
NRZI Encoding
Nonreturn to Zero _________________ (NRZI) encoding uses a
transition to represent a 1. No transition represents a 0.
NRZI fixes problems due to a stream of 1s, but does nothing to
address the problems due to a stream of 0s.
Figure 2.5 Different encodings
Manchester Encoding
In Manchester encoding the NRZ signal is exclusive-or'ed with
signal change rate (the _____________ rate). The encoding is
the clock signal. This results in a transition each clock period.
considered to be 50% efficient.
With Manchester encoding the bit transmit rate is one-half the
4B/5B Encoding
In 4B/5B encoding, four bits are encoded as five bits in such a
Only 16 of the 32 five bit codes are needed for encoding, some
way that there are never more than three consecutive 0s. The
of the remaining 16 codes are used for signaling (11111 – line
bits are then further encoded using NRZI so consecutive 1s do
idle) and _____________________.
not present a problem. 4B/5B coding is 80% efficient.
In Class Exercise:
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Problem 2.2 (page 153)
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2.3 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.
2.3.1 Byte Oriented Protocols
In byte oriented framing protocols a frame is viewed as a
Two approaches (__________________ and byte-count) are
collection of bytes.
used to indicate the message size.
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 “__________________” (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
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 __________________________ Protocol (LPC) to negotiate the frame format, datagram size and escaped characters.
Fig. 2.8 PPP Frame
DDCMP–Digital Data Communication Message Protocol
Instead of using a sentinel character, DDCMP includes a ___________________________ field in the frame header that indicates
the number of bytes in the body.
Fig. 2.9 DDCMP Frame
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2.3.2 Bit Oriented Protocols
HDLC–High Level Data Link Control
HDLC uses the bit sequence 01111110 to mark the start and end of the frame.
____________________ is used if this sequence appears anywhere else in the frame. After five 1s the transmitter inserts a 0 (bit
stuffing). The receiver discards any 0 that appears after five 1s.
Fig. 2.15 HDLC Frame
In-Class Exercises
Problem 2.7 (page 154)
Problem 2.8 (page 154)
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Turn in the in-class exercises before you leave.
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