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Link Layer&
Physical Layer
CPE 400 / 600Computer Communication Networks
Lecture 24
2
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
DataLink Layer 3
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 4
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 5
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 6
PPP Data Control ProtocolBefore exchanging network-
layer 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 7
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 8
The Internet: virtualizing networks
ARPAnet satellite net
gateway
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
DataLink Layer 9
Cerf & Kahn’s Internetwork ArchitectureWhat 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 10
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 11
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 12
ATM architecture
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
physical
ATM
AAL
physical
ATM
AAL
physical
ATM
physical
ATM
end system end systemswitch switch
DataLink Layer 13
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)
AAL PDU
ATM cell
User data
small payload -> short cell-creation delay for digitized
voice
DataLink Layer 14
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 15
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 16
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 17
IP-Over-ATM
AALATMphyphy
Eth
IP
ATMphy
ATMphy
apptransport
IPAALATMphy
apptransport
IPEthphy
IP datagrams into ATM AAL5
PDUs
IP addresses to ATM addresses
DataLink Layer 18
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
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
DataLink Layer 19
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 20
R1R2
D
R3R4
R50
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
1
in out outlabel label dest interface 8 6 A 0
0
in out outlabel label dest interface 10 A 0
12 D 0 8 A 1
MPLS forwarding tables
DataLink Layer 21
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
Physical Layer
Slides are modified from Behrouz A. Forouzan22
TCP/IP and OSI model
23
Source-to-destination delivery
24
Physical layer
25Physical Layer
To be transmitted, data must be transformed to electromagnetic signals.
Physical Layer
Chapter 3: Data and Signals
Chapter 4: Digital Transmission
Chapter 5: Analog Transmission
26
3-1 ANALOG AND DIGITAL3-1 ANALOG AND DIGITAL
Data can be Data can be analoganalog or or digitaldigital
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 useperiodic analog signals and nonperiodic digital signals.
27Physical Layer
Comparison of analog and digital signals
28Physical Layer
3-2 PERIODIC ANALOG SIGNALS3-2 PERIODIC ANALOG SIGNALS
Periodic analog signals can be classified as Periodic analog signals can be classified as simplesimple or or compositecomposite..
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.
29Physical Layer
Signal amplitude
30Physical Layer
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.
Frequency
31Physical Layer
Frequency and period are the inverse of each other.
Units of period and frequency
Frequency and Period
32Physical Layer
Two signals with the same amplitude,but different frequencies
33Physical Layer
The power we use at home has a frequency of 60 Hz. What is the period of this sine wave ?
Examples
The period of a signal is 100 ms. What is its frequency in kilohertz?
34Physical Layer
Phase describes the position of the waveformrelative to time 0
Phase
Three sine waves with the same amplitude and frequency,but different phases
35Physical Layer
A sine wave is offset 1/6 cycle with respect to time 0. What is its phase in degrees and radians?
Example
SolutionWe know that 1 complete cycle is 360°. Therefore, 1/6 cycle is
36Physical Layer
Wavelength and period
37Physical Layer
Wavelength = Propagation speed x Period = Propagation speed / Frequency
Time-domain and frequency-domain plots of a sine wave
38Physical Layer
A complete sine wave in the time domain can be represented by one single spike in the frequency domain.
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. 39Physical Layer
According to Fourier analysis,any composite signal is a combination of simple sine
waves with different frequencies, amplitudes, and phases.
Fourier analysis
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.
40Physical Layer
A composite periodic signal
Decomposition of the composite periodic signal in the time and frequency domains
41Physical Layer
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.
42Physical Layer
The bandwidth of a composite signal is the difference between the highest and the lowest
frequencies contained in that signal.
Bandwidth
43Physical Layer
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.
SolutionThe lowest frequency must be at 40 kHz and the highest at 240 kHz.
Example
44Physical Layer