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ECE 4450:427/527 - Computer Networks
Spring 2017
Dr. Nghi Tran
Department of Electrical & Computer Engineering
Lecture 5.1: Link Layer
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Some Discussions
• Previously, we saw a network consisting of LINKS
interconnecting NODES
• Simplest network:
• A link connects two nodes
• Even though it looks trivial, link is a
fundamental aspect of network: one of the
first fundamental problems we face
• Next few lectures focus on LINK LAYER
• Recall: Our approach is bottom up.
• How about Physical Layer?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Link Layer: Five Common Problems
• Basic problem: you can’t just send datagrams over the link!
• We first consider how to encode bits into the signal at the
source and recover bits at the receiving node
• Once it is possible to transmit bits, we need to figure out
how to package these bits into FRAME
• Assume each node is able to recognize the collections of
bits making up a frame, the third problem is to determine if
those bits are in error: Error Detection and Correction
• If frames arriving at destination contain errors, how to
recover from such losses: ARQ
• Final problem related to multiple-access link: how mediate
access to a shared link so that all nodes have a chance to
transmit: We focus on Ethernet
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Link Layer: Outline
• Physical Medium and FIVE additional functions/services
in Link Layer:
• How to 1) encoding bits onto medium so that they
can be understood by receiving node
• With packed-switch, need to consider block of data:
2) Framing
• Transmission of Frames: corrupted errors: 3) Error
detection/correction
• Reliable delivery: 4) Automatic Repeat reQuest
(ARQ).
• A link shared by multiple nodes: Media access
control problem
• Introduction of Carrier Sense Multiple Access (CSMA)
network, e.g., Ethernet
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Link Layer
• Examples of Link Layer Protocols: Ethernet,
WiFi, token ring, LTE, 3G etc
• Where is Link Layer Implemented?
• In each node
• Most parts in a Network Adaptor or a
Network Interface Card (NIC): Ethernet
card, PCMCI card, 802.11 card
• Combination of hardware, software,
firmware
• Note: NIC also implements Physical Layer
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Link Layer
datagram
datagram
controller
controller
receiving host
sending host
datagram
frame
• receiving side
• sending side:
– encapsulates datagram in
frame
– adds error checking bits, flow
control, etc.
Dr. Nghi Tran (ECE-University of Akron)
– looks for errors, flow control,
etc
– extracts datagram, passes to
upper layer at receiving side
ECE 4450:427/527
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What is Communications?
• Two nodes connected by a link and the
ultimate objective is to send information from
one to the other
“The fundamental problem of communication is that
of reproducing at one point either exactly or
approximately a message selected at another point.”
(Claude Shannon 1948)
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Communications System
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Physical Medium & Link Capacity
• Signals travel through the medium/channel
carrying binary bits
• Maximum bits carried reliably – Link capacity?
• Shannon theorem: the upper bound to the
capacity of a link in terms of bits per second
(bps) as a function of signal-to-noise ratio
(S/N) of the link measured in decibels (dB):
C = W log2(1+S/N) (bits/s)
• W: Available ``bandwidth” of link (Hz)
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Link Capacity: Example
•
•
•
•
•
•
• C = W log2(1+S/N)
What is W for telephone line?
What is typical S/N
Can signal be weaker than noise?
How to get higher C?
Higher W, higher C?
Higher S/N, higher C?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Link Capacity as Function of W
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Link
• All practical links rely on some sort of
electromagnetic radiation propagating through a
medium or, in some cases, through free space
• One way to characterize links, then, is by the medium
they use
– Typically copper wire in some form (as in Digital Subscriber
Line (DSL) and coaxial cable),
– Optical fiber (as in both commercial fiber-to-the home
services and many long-distance links in the Internet’s
backbone), or
– Air/free space (for wireless links)
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Link
• Another important link characteristic is the frequency
– Measured in hertz, with which the electromagnetic waves
oscillate
• Distance between the adjacent pair of maxima or
minima of a wave measured in meters is called
wavelength
– Speed of light divided by frequency gives the wavelength.
– Frequency on a copper cable range from 300Hz to 3300Hz;
Wavelength for 300Hz wave through copper is speed of
light on a copper / frequency
– 2/3 x 3 x 108 /300 = 667 x 103 meters.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Link
Electromagnetic spectrum
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
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Links
Common services available to connect your home
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
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1) Encoding
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
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Encoding
• Now we start with the first service: Encoding –
How to convert binary bits to signal
• Not focus on the entire modulation. Instead,
we assume the simplest situation: working
with two discrete signals: high and low
• In practice, it corresponds to two different
voltages or two different power levels
• What is an obvious way to convert?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Encoding
Signals travel between signaling components; bits flow between adaptors
NRZ (Non-Return to Zero) encoding of a bit stream:
Low signal represents a 0, high signal represents a 1
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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NRZ: Problems
• Problem: Long periods of silence (0s) or high
signals (1s) are possible
• Baseline wander
• Clock recovery
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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NRZ Problem: Baseline Wander
• Baseline wander
• The receiver keeps an average of the signals it has
seen so far
• Uses the average to distinguish between low and
high signal
• When a signal is significantly low than the
average, it is 0, else it is 1
• Too many consecutive 0’s and 1’s cause this
average to change, making it difficult to detect
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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NRZ Problem: Clock Recovery
• Clock recovery
• Frequent transition from high to low or vice versa
are necessary to enable clock recovery
• Both the sending and decoding process is driven
by a clock
• Every clock cycle, the sender transmits a bit and
the receiver recovers a bit
• The sender and receiver have to be precisely
synchronized
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Other Coding Scheme: NRZI
– Non Return to Zero Inverted
– Sender makes a transition from the current signal
to encode 1 and stay at the current signal to
encode 0: transition at a clock boundary
– Solves for consecutive 1’s: Used in USB with bit
stuffing
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Other Coding Scheme: Manchester
– Merging the clock with signal by transmitting ExOR of the NRZ encoded data and the clock
– In Manchester encoding (used widely in 802.3)
• 0: low high transition
• 1: high low transition
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Issues with Manchester
– Doubles the rate at which the signal transitions are
made on the link
• Which means the receiver has half of the time to detect
each pulse of the signal
– The rate at which the signal changes is called the
link’s baud rate
– In Manchester the bit rate is half the baud rate
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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4B/5B Encoding
• Encode 4-bit symbols into 5-bit codes
• 24 symbols must be mapped to 24 codewords out of the possible 25
• Each codeword has no more than one starting zero, and no more
than two trailing zeros
• Already solve consecutive 0s problem?
• Then use NRZI to solve the consecutive 1s problem
• 80% efficiency (1 bit is overhead)
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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4B/5B Encoding
4-bit
data
symbol
5-bit
code
4-bit
data
symbol
5-bit
code
0000
11110
1000
10010
0001
01001
1001
10011
0010
10100
1010
10110
0011
10101
1011
10111
0100
01010
1100
11010
0101
01011
1101
11011
0110
01110
1110
11100
0111
01111
1111
11101
Dr. Nghi Tran (ECE-University of Akron)
16 left
11111 – when the line is idle
00000 – when the line is dead
.. 00100 – to mean halt
13 left : 7 invalid, 6 for various
control signals
ECE 4450:427/527
Computer Networks
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4B/5B Encoding
4-bit data symbol
5-bit code
4-bit data symbol
5-bit code
0000
11110
1000
10010
0001
01001
1001
10011
0010
10100
1010
10110
0011
10101
1011
10111
0100
01010
1100
11010
0101
01011
1101
11011
0110
01110
1110
11100
0111
01111
1111
11101
4B/5B: popularized by Fiber Distributed Data Interface (FDDI); also
adopted by 802.3, Multichannel Audio Digital Interface
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Other Schemes
• RZ, RZ-L
• Bi – Phase, Bi-Phase-L
• Miller Code, Miller L etc.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Example
• Show 4B/5B encoding and the resulting NRZI
signal for the following bit sequences
1101 1110 1010
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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2) Framing
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
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Why Framing?
• We are focusing on packet-switched networks, which
means that blocks of data (called frames at this
level), not bit streams, are exchanged between
nodes.
• It is the network adaptor that enables the nodes to
exchange frames.
Bits flow between adaptors, frames between hosts
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Why Framing?
• When node A wishes to transmit a frame to node B, it
tells its adaptor to transmit a frame from the node’s
memory. This results in a sequence of bits being sent
over the link.
• The adaptor on node B then collects together the
sequence of bits arriving on the link and deposits the
corresponding frame in B’s memory.
• Recognizing exactly what set of bits constitute a
frame—that is, determining where the frame begins
and ends—is the central challenge faced by the
adaptor
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Framing
• Byte-Oriented Protocols
• To view each frame as a collection of bytes (characters)
rather than bits
• BISYNC (Binary Synchronous Communication) Protocol –
Developed by IBM (around 60s)
• DDCMP (Digital Data Communication Protocol) – Used in
DECnet (Digital Equipment Corporation)
• Widely used PPP (Point-to-Point Protocol) over Internet
• Bit-Oriented Protocols
• HDLC (High-Level Data Link Control)
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Frame/Packet Format
• Frame/packet: Sequence of labeled fields
• Above each field: A number indicating the
length of that field in bits
• Frame/packet: transmission beginning with the
leftmost field
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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BISYNC
BISYNC: Binary synchronous communication
Frame is a collection of bytes
Need to indicate the beginning and end of a frame
Sentinel characters are used
SYN: Synchronization character
SOH: Start of header
STX, ETX: Start of text, End of text
CRC: Cyclic redundancy check
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Problem with BISYNC
• ETX may occur in the payload
• Precede it with a DLE (data-link-escape)
character
• Problem propagates, precede DLE with another
DLE (extra overhead). Cost is overhead
• This approach is called character stuffing: extra
characters are inserted in data portion
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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DDCMP: Byte-Counting Framing
Include the # of bytes in the frame as a field in the header
Digital Data Communications Protocol (DDCMP)
Count: Specifies # of bytes in the body
What happen if Count corrupted?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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PPP Framing
• Recent PPP which is commonly run over
Internet links uses sentinel approach
– Special start of text character denoted as Flag
• 01111110
– Address, control : default numbers
– Protocol for demux : IP / IPX
– Payload : negotiated (1500 bytes)
– Checksum : for error detection 2-4 bytes
Overhead: 8/1508 =0.5%
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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HDLC –Bit-oriented Framing

Bit-oriented Protocol

HDLC : High Level Data Link Control

Beginning and Ending Sequences
01111110
HDLC Frame Format
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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HDLC –Bit-oriented Framing

HDLC Protocol

On the sending side, any time five consecutive
1’s have been transmitted from the body of
the message (i.e. excluding when the sender
is trying to send the distinguished 01111110
sequence)

The sender inserts 0 before transmitting the next
bit: Bit stuffing
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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HDLC –Bit-oriented Framing

HDLC Protocol

On the receiving side

5 consecutive 1’s

Next bit 0 : Stuffed, so discard it
1 : Either End of the frame marker
Or Error has been introduced in the bitstream
Look at the next bit
If 0 ( 01111110 )  End of the frame marker
If 1 ( 01111111 )  Error, discard the whole frame
The receiver needs to wait for next
01111110 before it can start
receiving again
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Example
Example
of Bit-stuffing
Sender
1111110111111111110111110
0
Receiver
x
0
0
0
x
x
x
11111010111110111110101111100
Length of frame
Variable, depends on the data
We can calculate and optimize the overhead of bit
stuffing
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Example
Frame
Size
Let each frame contain V overhead bits
Let a message of M bits be broken into frames of size Kmax
# of packets : M / K max 
The total # of bits for all frames: M  M / Kmax V
If Kmax ↓, # of frames ↑, and overhead also ↑
If # of frames ↑, then each frame must be processed then more
processing delay at each host
Increase frame length as much as possible
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Example
Frame
Size
• Why just make Kmax = M?
Pipelining delay: frame must be
received before forwarding
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Example
Framing
• What we have discussed so far
• Sentinel framing uses special “sentinel” characters
(STX, ETX) to indicate the start and end of each
frame.
• Length-based framing specifies the length of the
frame in the beginning of the header, so the
receiver knows exactly how much data to read
• Frames usually not same size
• Clock-based framing (SONET): all frames are the same
size - Sender and receiver must have synchronized
clocks. All frames fit into a specific time interval. Refer
to 2.3.3 textbook.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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