CPE 400 / 600
Computer Communication Networks
Lecture 24
Link Layer
&
Physical Layer
Lecture 24: Outline
 5.1 Introduction and Services
 5.2 Error-detection and Error-correction
 5.3 Multiple Access Protocols
 5.4 Link-layer Addressing
 5.5 Ethernet
 5.6 Link-layer Switches
 5.7 Point to Point Protocol
 5.8 Link Virtualization

ATM , MPLS
 Physical Layer

Data and Signals
2
Point to Point Data Link Control
 one sender, one receiver, one link: easier than
broadcast link:
 no Media Access Control
 no need for explicit MAC addressing
 e.g., dialup link, ISDN line
 popular point-to-point DLC protocols:
PPP (point-to-point protocol)
 HDLC: High level data link control (Data link used
to be considered “high layer” in protocol stack!)

DataLink Layer
3
PPP Design Requirements [RFC 1557]
 packet framing: encapsulation of network-layer
datagram in data link frame
 ability to demultiplex upwards
 bit transparency: must carry any bit pattern in the
data field
 error detection (no correction)
 connection liveness: detect, signal link failure to
network layer
 network layer address negotiation: endpoint can
learn/configure each other’s network address
Error recovery, flow control, data re-ordering
all relegated to higher layers!
DataLink Layer
4
PPP Data Frame
 Flag: delimiter (framing)
 Address: does nothing (only one option)
 Control: does nothing; in the future possible multiple
control fields
 Protocol: upper layer protocol to which frame delivered
(eg, IP, PPP-LCP, IPCP, etc)
 info: upper layer data being carried
 check: cyclic redundancy check for error detection
DataLink Layer
5
PPP Data Control Protocol
Before exchanging networklayer data, data link peers
must
 configure PPP link (max.
frame length, authentication)
 learn/configure network
layer information
 for IP: carry IP Control
Protocol (IPCP) msgs
(protocol field: 8021) to
configure/learn IP address
DataLink Layer
6
Virtualization of networks
Virtualization of resources: powerful abstraction in
systems engineering:
 computing examples: virtual memory, virtual devices
Virtual machines: e.g., java
 IBM VM os from 1960’s/70’s

 layering of abstractions: don’t sweat the details of
the lower layer, only deal with lower layers abstractly
DataLink Layer
7
The Internet: virtualizing networks
Internetwork layer (IP):
 addressing: internetwork
appears as single, uniform
entity, despite underlying
local network heterogeneity
 network of networks
Gateway:
 “embed internetwork packets
in local packet format or
extract them”
 route (at internetwork level)
to next gateway
gateway
ARPAnet
satellite net
DataLink Layer
8
Cerf & Kahn’s Internetwork Architecture
What is virtualized?
 two layers of addressing: internetwork and local
network
 new layer (IP) makes everything homogeneous at
internetwork layer
 underlying local network technology
 cable
 satellite
 telephone modem
 today: ATM, MPLS
… “invisible” at internetwork layer. Looks like a link
layer technology to IP!
DataLink Layer
9
ATM and MPLS
 ATM, MPLS separate networks in their own
right

different service models, addressing, routing
from Internet
 viewed by Internet as logical link connecting
IP routers

just like dialup link is really part of separate
network (telephone network)
DataLink Layer
10
Asynchronous Transfer Mode: ATM
 1990’s/00 standard for high-speed (155Mbps to
622 Mbps and higher) Broadband Integrated Service
Digital Network architecture
 Goal: integrated, end-end transport of carry voice,
video, data
 meeting timing/QoS requirements of voice, video
(versus Internet best-effort model)
 “next generation” telephony: technical roots in
telephone world
 packet-switching (fixed length packets, called
“cells”) using virtual circuits
DataLink Layer
11
ATM architecture
AAL
AAL
ATM
ATM
ATM
ATM
physical
physical
physical
physical
end system
switch
switch
end system
 adaptation layer: only at edge of ATM network
data segmentation/reassembly
 roughly analagous to Internet transport layer
 ATM layer: “network” layer
 cell switching, routing
 physical layer

DataLink Layer
12
ATM Adaptation Layer (AAL)
 Different versions of AAL layers, depending on ATM
service class:

AAL1: for CBR (Constant Bit Rate) services, e.g. circuit emulation

AAL2: for VBR (Variable Bit Rate) services, e.g., MPEG video

AAL5: for data (eg, IP datagrams)
User data
small payload -> short
cell-creation delay for
digitized voice
AAL PDU
ATM cell
DataLink Layer
13
ATM Layer: Virtual Circuits
 VC transport: cells carried on VC from source to dest
 call setup, teardown for each call before data can flow
 each packet carries VC identifier (not destination ID)
 every switch on source-dest path maintain “state” for each
passing connection
 link,switch resources (bandwidth, buffers) may be allocated
to VC: to get circuit-like perf.
 Permanent VCs (PVCs)
 long lasting connections
 typically: “permanent” route between to IP routers
 Switched VCs (SVC):
 dynamically set up on per-call basis
DataLink Layer
14
ATM VCs
 Advantages of ATM VC approach:

QoS performance guarantee for connection
mapped to VC (bandwidth, delay, delay jitter)
 Drawbacks of ATM VC approach:
 Inefficient


support of datagram traffic
one PVC between each source/dest pair) does not
scale (N*2 connections needed)
SVC introduces call setup latency, processing
overhead for short lived connections
DataLink Layer
15
ATM cell header
 5-byte ATM cell header
 VCI: virtual channel ID
 will change from link to link thru net
 PT: Payload type (e.g. RM cell versus data cell)
 CLP: Cell Loss Priority bit
 CLP = 1 implies low priority cell, can be discarded if congestion
 HEC: Header Error Checksum
 cyclic redundancy check
DataLink Layer
16
IP-Over-ATM
app
transport
IP
Eth
phy
IP datagrams into
ATM AAL5 PDUs
IP
AAL
Eth
ATM
phy phy
ATM
phy
ATM
phy
app
transport
IP
AAL
ATM
phy
IP addresses to
ATM addresses
DataLink Layer
17
Multiprotocol label switching (MPLS)
 initial goal: speed up IP forwarding by using fixed
length label (instead of IP address) to do forwarding

borrowing ideas from Virtual Circuit (VC) approach

but IP datagram still keeps IP address!
PPP or Ethernet
header
MPLS header
label
20
IP header
remainder of link-layer frame
Exp S TTL
3
1
5
DataLink Layer
18
MPLS capable routers
 a.k.a. label-switched router
 forwards packets to outgoing interface based only on
label value (don’t inspect IP address)

MPLS forwarding table distinct from IP forwarding tables
 signaling protocol needed to set up forwarding



RSVP-TE
use MPLS for traffic engineering
forwarding possible along paths that IP alone would not allow
(e.g., source-specific routing) !!
 must co-exist with IP-only routers
DataLink Layer
19
MPLS forwarding tables
in
label
out
label dest
10
12
8
out
interface
A
D
A
0
0
1
R4
R6
in
label
out
label dest
out
interface
10
6
A
1
12
9
D
0
R3
0
0
D
1
1
R5
0
0
R2
in
label
8
out
label dest
6
A
out
interface
0
A
R1
in
label
6
out
label dest
-
A
out
interface
0
DataLink Layer
20
Chapter 5: Summary
 principles behind data link layer services:
 error detection, correction
 sharing a broadcast channel: multiple access
 link layer addressing
 instantiation and implementation of various link layer
technologies




Ethernet
switched LANs
PPP
virtualized networks as a link layer: ATM, MPLS
DataLink Layer
21
Physical Layer
Slides are modified from Behrouz A. Forouzan
22
TCP/IP and OSI model
23
Source-to-destination delivery
24
Physical layer
To be transmitted,
data must be transformed to electromagnetic signals.
Physical Layer
25
Physical Layer
Chapter 3: Data and Signals
Chapter 4: Digital Transmission
Chapter 5: Analog Transmission
26
3-1 ANALOG AND DIGITAL
Data can be analog or digital
 Analog data refers to information that is continuous
 Analog data take on continuous values
 Analog signals can have an infinite number of values in a range
 Digital data refers to information that has discrete states
 Digital data take on discrete values
 Digital signals can have only a limited number of values
In data communications, we commonly use
periodic analog signals and nonperiodic digital signals.
Physical Layer
27
Comparison of analog and digital signals
Physical Layer
28
3-2 PERIODIC ANALOG SIGNALS
Periodic analog signals can be classified as simple or composite.
 A simple periodic analog signal, a sine wave, cannot be
decomposed into simpler signals.
 A composite periodic analog signal is composed of multiple
sine waves.
Physical Layer
29
Signal amplitude
Physical Layer
30
Frequency
Frequency is the rate of change with respect to time.
 Change in a short span of time means high frequency.
 Change over a long span of time means low frequency.
 If a signal does not change at all, its frequency is zero
 If a signal changes instantaneously, its frequency is infinite.
Physical Layer
31
Frequency and Period
Frequency and period are the inverse of each other.
Units of period and frequency
Physical Layer
32
Two signals with the same amplitude,
but different frequencies
Physical Layer
33
Examples
The power we use at home has a frequency of 60 Hz. What is the period of
this sine wave ?
The period of a signal is 100 ms. What is its frequency in kilohertz?
Physical Layer
34
Phase
Phase describes the position of the waveform
relative to time 0
Three sine waves with the same amplitude and frequency,
but different phases
Physical Layer
35
Example
A sine wave is offset 1/6 cycle with respect to time 0. What is its phase in
degrees and radians?
Solution
We know that 1 complete cycle is 360°. Therefore, 1/6 cycle is
Physical Layer
36
Wavelength and period
Wavelength = Propagation speed x Period
= Propagation speed / Frequency
Physical Layer
37
Time-domain and frequency-domain plots of a sine wave
A complete sine wave in the time domain can be
represented by one single spike in the frequency domain.
Physical Layer
38
Frequency Domain
 The frequency domain is more compact and useful when we are
dealing with more than one sine wave.
 A single-frequency sine wave is not useful in data communication
o We need to send a composite signal, a signal made of many simple
sine waves.
Physical Layer
39
Fourier analysis
According to Fourier analysis,
any composite signal is a combination of simple sine
waves with different frequencies, amplitudes, and phases.
 If the composite signal is periodic, the decomposition
gives a series of signals with discrete frequencies;
 If the composite signal is nonperiodic, the decomposition
gives a combination of sine waves with continuous frequencies.
Physical Layer
40
A composite periodic signal
Decomposition of the
composite periodic
signal in the time and
frequency domains
Physical Layer
41
Time and frequency domains of a nonperiodic signal
 A nonperiodic composite signal
o It can be a signal created by a microphone or a telephone set
when a word or two is pronounced.
o In this case, the composite signal cannot be periodic
because that implies that we are repeating the same word or words
with exactly the same tone.
Physical Layer
42
Bandwidth
The bandwidth of a composite signal is
the difference between the highest and the lowest
frequencies contained in that signal.
Physical Layer
43
Example
A nonperiodic composite signal has a bandwidth of 200 kHz,
with a middle frequency of 140 kHz and peak amplitude of 20 V.
The two extreme frequencies have an amplitude of 0. Draw the
frequency domain of the signal.
Solution
The lowest frequency must be at 40 kHz and the highest at
240 kHz.
Physical Layer
44