Transcript 3rd Edition, Chapter 5

Chapter 5
Link Layer and LANs
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Computer Networking:
A Top Down Approach
4th edition.
Jim Kurose, Keith Ross
Addison-Wesley, July
2007.
Thanks and enjoy! JFK/KWR
All material copyright 1996-2007
J.F Kurose and K.W. Ross, All Rights Reserved
5: DataLink Layer
5-1
Chapter 5: The Data Link Layer
Our goals:
 understand principles behind data link layer
services:




error detection, correction
sharing a broadcast channel: multiple access
link layer addressing
reliable data transfer, flow control: done!
 instantiation and implementation of various link
layer technologies
5: DataLink Layer
5-2
Link Layer
 5.1 Introduction and




services
5.2 Error detection
and correction
5.3 Multiple access
protocols
5.4 Link-layer
Addressing
5.5 Ethernet
 5.6 Link-layer switches
 5.7 PPP
 5.8 Link virtualization:
ATM, MPLS
5: DataLink Layer
5-3
Link Layer: Introduction
Some terminology:
 hosts and routers are nodes
 communication channels that connect adjacent nodes
along communication path are links
 wired links
 wireless links
 LANs
 layer-2 (LINK) packet is a frame, encapsulates
datagram
5: DataLink Layer
5-4
Link Layer
“link”
 Link-layer protocol
has the responsibility
of transferring
datagram from one
node to adjacent
node over a link.
5: DataLink Layer
5-5
Review
Internet protocol stack
 application: Process-Process data
communication

FTP, SMTP, HTTP
application
 transport: end-end data transfer
 TCP, UDP
transport
 network: host-host data transfer
 IP, routing protocols
network
 link: Point-Point data transfer
 PPP, Ethernet
 physical: bits “on the wire”
link
physical
5: DataLink Layer
5-6
Everything over IP
Application Layer
Transport Layer
…
HTTP
DNS
TCP
…
RTP
UDP
IP
Network Layer
Datalink Layer
SMTP
Link 1
Link 2
…
Link 3
5: DataLink Layer
5-7
Layer Traversing
Host A
P2
P1
P2
Packet 1
Packet 2
P2
Data
C
P2
Packet N
P1
P1
D
P1
P2
Data
P1
P1
P2
Host B
5: DataLink Layer
5-8
Link layer Protocols
 Datagram transferred by different link
protocols over different links:

e.g., Ethernet on first link, frame relay (帧中继) on
intermediate links, IEEE 802.11 on last link
 Each link protocol provides different services

e.g., may or may not provide reliable data transfer
over link
5: DataLink Layer
5-9
Link Layer Services
NOS Implementation
High Levels
Network
Logical Link Control
Data Link
Media Access Control
Phy
Physical Layer
The architecture specification of data link Layer
5: DataLink Layer
5-10
Sublayer
 MAC (Media Access Control，介质访问控制)
 Coordinate the frame transmissions of many
nodes if multiple nodes share a medium
 LLC (Logical Link Control，逻辑链路控制)
 Reliable delivery： between adjacent nodes
 Used on wireless links (seldom used on low bit
error link (fiber, some twisted pair), Correct an
error locally at link level.
5: DataLink Layer
5-11
Link Layer Services
 Framing:
encapsulate datagram into frame
 adding header, trailer, physical addresses used in
frame headers to identify source, destination

 Flow Control:

pacing between adjacent sending and receiving
nodes
5: DataLink Layer
5-12
Link Layer Services and Functions
 Access Control
 Media Access Control (Link access) ：Coordinate
the frame transmissions of many nodes if multiple
nodes share a medium
 Half-duplex and full-duplex：with half duplex,
nodes at both ends of link can transmit, but not at
same time
 “MAC” addresses used in frame headers to identify
source and destination.
• MAC Address is different from IP address!
5: DataLink Layer
5-13
Link Layer Services and Functions
 Reliability Control

Error Detection: errors caused by signal
attenuation, noise；receiver detects presence
of errors: signals sender for retransmission
or drops frame
Error Correction: receiver identifies and
corrects bit error(s) without resorting to
retransmission
 RDT (reliability data transfer): sequence #, retransmission, …

5: DataLink Layer
5-14
Link Layer Services
 Reliable delivery between adjacent nodes
 We learned how to do this already (chapter 3)!
 Seldom used on low bit-error link (fiber, some
twisted pair)
 Wireless links: high error rates
• Q: why both link-level and end-end reliability?
5: DataLink Layer
5-15
Link layer: context
Transportation analogy
 trip from Princeton to Lausanne



limo: Princeton to New York
plane: New York to Geneva
train: Geneva to Lausanne
 tourist = datagram
 transport segment = communication link
 transportation mode = link layer protocol
 travel agent = routing algorithm
5: DataLink Layer
5-16
Where is the link layer implemented?
 in each and every host
 link layer implemented in
“adaptor” (aka network
interface card NIC)


Ethernet card, PCMCI
card, 802.11 card
implements link, physical
layer
 attaches into host’s
system buses
 combination of
hardware, software,
firmware
host schematic
application
transport
network
link
cpu
memory
controller
link
physical
host
bus
(e.g., PCI)
physical
transmission
network adapter
card
5: DataLink Layer
5-17
Adaptors Communicating
datagram
datagram
controller
controller
receiving host
sending host
datagram
frame
 sending side:
 encapsulates datagram in
frame
 adds error checking bits,
rdt, flow control, etc.
 receiving side
 looks for errors, rdt, flow
control, etc
 extracts datagram, passes
to upper layer at receiving
side
5: DataLink Layer
5-18
Adapters Communicating
datagram
sending
node
rcving
node
link layer protocol
frame
Adapter card
frame
Adapter card
 link layer implemented in  receiving side
“adapter” (NIC)
 looks for errors, rdt, flow
control, etc
 Ethernet card, PCMCI
 extracts datagram, passes to
card, IEEE 802.11 card
receiving node
 sending side:
 adapter is semi-autonomous,
 encapsulates datagram in
includes link & physical
a frame
layers
 adds error checking bits,
rdt, flow control, etc.
5: DataLink Layer
5-19
Link Layer
 5.1 Introduction and




services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-layer
Addressing
5.5 Ethernet
 5.6 Link-layer switches
 5.7 PPP
 5.8 Link Virtualization:
ATM. MPLS
5: DataLink Layer
5-20
Error Detection
EDC= Error Detection and Correction bits (redundancy)
D = Data protected by error checking, may include header fields
• Error detection not 100% reliable!
• protocol may miss some errors, but rarely
• larger EDC field yields better detection and correction
otherwise
5: DataLink Layer
5-21
Parity Checking
Single Bit Parity:
Detect single bit errors
Two Dimensional Bit Parity:
Detect and correct single bit errors
0
0
5: DataLink Layer
5-22
Internet checksum (review)
Goal: detect “errors” (e.g., flipped bits) in transmitted
packet (note: used at transport layer only)
Sender:
 treat segment contents
as sequence of 16-bit
integers
 checksum: addition (1’s
complement sum) of
segment contents
 sender puts checksum
value into UDP checksum
field
Receiver:
 compute checksum of
received segment
 check if computed checksum
equals checksum field value:
 NO - error detected
 YES - no error detected.
But maybe errors
nonetheless?
5: DataLink Layer
5-23
Checksumming: Cyclic Redundancy Check
 view data bits, D, as a binary number
 choose r+1 bit pattern (generator), G
 goal: choose r CRC bits, R, such that



<D,R> exactly divisible by G (modulo 2)
receiver knows G, divides <D,R> by G. If non-zero remainder:
error detected!
can detect all burst errors less than r+1 bits
 widely used in practice (802.11 WiFi, ATM)
5: DataLink Layer
5-24
CRC Example
Want:
D.2r XOR R = nG
equivalently:
D.2r = nG XOR R
equivalently:
if we divide D.2r by
G, want remainder R
R = remainder[
D.2r
G
]
5: DataLink Layer
5-25
Link Layer
 5.1 Introduction and




services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-layer
Addressing
5.5 Ethernet
 5.6 Link-layer switches
 5.7 PPP
 5.8 Link Virtualization:
ATM, MPLS
5: DataLink Layer
5-26
Multiple Access Links and Protocols
Two types of “links”:
 point-to-point
 PPP for dial-up access
 point-to-point link between Ethernet switch and host
 broadcast (shared wire or medium)
 old-fashioned Ethernet
 upstream HFC
 IEEE 802.11 wireless LAN
shared wire (e.g.,
cabled Ethernet)
shared RF
(e.g., 802.11 WiFi)
shared RF
(satellite)
humans at a
cocktail party
(shared air, acoustical)
5: DataLink Layer
5-27
Multiple Access protocols
 Single shared broadcast channel
 Two or more simultaneous transmissions by nodes:
interference

Collision if node receives two or more signals at the same time
multiple access protocol
 Distributed algorithm that determines how nodes
share channel, i.e., determine when node can transmit
 Communication about channel sharing must use channel
itself!

no out-of-band channel for coordination
5: DataLink Layer
5-28
Ideal Multiple Access Protocol
Broadcast channel of rate R bps
1. When one node wants to transmit, it can send at
rate R.
2. When M nodes want to transmit, each can send at
average rate R/M
3. Fully decentralized:


no special node to coordinate transmissions
no synchronization of clocks, slots
4. Simple
5: DataLink Layer
5-29
MAC Protocols: a taxonomy
Three broad classes:
 Channel Partitioning


divide channel into smaller “pieces” (time slots, frequency,
code)
allocate piece to node for exclusive use
 Random Access
 channel not divided, allow collisions
 “recover” from collisions
 “Taking turns”
 nodes take turns, but nodes with more to send can take
longer turns
5: DataLink Layer
5-30
Channel Partitioning MAC protocols: TDMA
TDMA: time division multiple access
 access to channel in "rounds"
 each station gets fixed length slot (length = pkt
trans time) in each round
 unused slots go idle
 example: 6-station LAN, 1, 3, 4 have packet, slots
2, 5, 6 idle
6-slot
frame
1
3
4
1
3
4
5: DataLink Layer
5-31
Channel Partitioning MAC protocols: FDMA
FDMA: frequency division multiple access
 channel spectrum divided into frequency bands
 each station assigned fixed frequency band
 unused transmission time in frequency bands go idle
 example: 6-station LAN, 1, 3, 4 have packet,
FDM cable
frequency bands
frequency bands 2, 5, 6 idle
5: DataLink Layer
5-32
Channel Partitioning MAC protocols: CDMA
 CDMA-Code Division Multiple Access
 employs spread-spectrum technology and a special
coding scheme (where each transmitter is assigned a
code) to allow multiple users to be multiplexed over
the same physical channel
5: DataLink Layer
5-33
Channel Partitioning MAC protocols: CDMA
5: DataLink Layer
5-34
Channel Partitioning MAC protocols: SDMA
SDMA (Space-Division Multiple Access)
is a channel access method based on
creating parallel spatial pipes next to
higher capacity pipes through spatial
multiplexing and/or diversity, by
which it is able to offer superior
performance in radio multiple access
communication systems.
By using smart antenna technology and
differing spatial locations of mobile units
within the cell, SDMA offers attractive
performance enhancements.
5: DataLink Layer
5-35
The Synthetic Use of Multiple Protocols
GSM
Link bandwidth: 25MHz, upper link: 890～915MHz，
downlink:935～960MHz.
each 200kHz treated as a channel，which means upper/down
link combined by 124 frequency channels, and each channel
contains multiple TDMA frame, each frame（4.615ms) with 8
slots, each slot is 148bit（576.9us), the total transmission
speed is 2.8kbps.
TD-SCDMA
5: DataLink Layer
5-36
The Structure of a TDMA frame
5: DataLink Layer
5-37
Random Access Protocols
 When node has packet to send
 transmit at full channel data rate R.
 no a priori coordination among nodes
 two or more transmitting nodes ➜ “collision”,
 random access MAC protocol specifies:
 how to detect collisions
 how to recover from collisions (e.g., via delayed
retransmissions)
 Examples of random access MAC protocols:
 slotted ALOHA
 ALOHA
 CSMA, CSMA/CD, CSMA/CA
5: DataLink Layer
5-38
Signal on Broadcast channel
A
B
C
D
E
F
C
Bus
 Advantage: 1  N
 Disadvantage: contention, collision domain
5: DataLink Layer
5-39
Random Access Protocols
 Examples of random access MAC protocols:

ALOHA: Pure / slotted
 CSMA:
CSMA/CD, CSMA/CA
5: DataLink Layer
5-40
Slotted ALOHA
Assumptions:
 all frames same size
 time divided into equal size slots (time to transmit 1
frame)
 nodes start to transmit only slot beginning
 nodes are synchronized
 if 2 or more nodes transmit in slot, all nodes detect
collision
5: DataLink Layer
5-41
Slotted ALOHA
Operation:
 when node obtains fresh frame, transmits in next
slot
 if no collision: node can send new frame in next
slot
 if collision: node retransmits frame in each
subsequent slot with probability p until success
5: DataLink Layer
5-42
Slotted ALOHA
Pros
 single active node can
continuously transmit
at full rate of channel
 highly decentralized:
only slots in nodes
need to be in sync
 simple
Cons
 collisions, wasting slots
 idle slots
 nodes may be able to
detect collision in less
than time to transmit
packet
 clock synchronization
5: DataLink Layer
5-43
Slotted Aloha efficiency
Efficiency : long-run
fraction of successful slots
(many nodes, all with many frames to send)
 suppose: N nodes with many frames to send, each
transmits in slot with probability p
 probability that given node has success in a slot =
p(1-p)N-1
 probability that any node has a success = Np(1-p)N-1
5: DataLink Layer
5-44
Slotted Aloha efficiency
 max efficiency: find p* that maximizes Np(1-p)N-1
 for many nodes, take limit of Np*(1-p*)N-1 as N goes to
infinity, gives:
Max efficiency = 1/e = 0.37
At best: channel used for useful transmissions 37%
of time!
5: DataLink Layer
!
5-45
Pure (unslotted) ALOHA
 unslotted Aloha: simpler, no synchronization
 when frame first arrives
 transmit immediately
 collision probability increases:
 frame sent at t0 collides with other frames sent in [t0-1,t0+1]
5: DataLink Layer
5-46
Pure Aloha efficiency
P(success by given node) = P(node transmits) .
P(no other node transmits in [t0-1,t0] .
P(no other node transmits in [t0, t0+1]
= p . (1-p)N-1 . (1-p)N-1
= p . (1-p)2(N-1)
… choosing optimum p and then letting n -> infty ...
= 1/(2e) = 0.184
even worse than slotted Aloha!
5: DataLink Layer
5-47
CSMA (Carrier Sense Multiple Access)
CSMA: listen before transmit:
If channel sensed idle: transmit entire frame
 If channel sensed busy, defer transmission
 human analogy: don’t interrupt others!
5: DataLink Layer
5-48
CSMA (Carrier Sense Multiple Access)
P-CSMA:
1. If the medium is idle, transmit with probability p, and delay
for one time unit with probability (1 - p)
2. If the medium is busy, continue to listen until medium becomes
idle, then go to Step 1
3. If transmission is delayed by 1 time unit, continue with Step 1
A good trade-off between Non-persistent CSMA
(wait random time) and 1-persistent CSMA.
5: DataLink Layer
5-49
CSMA collisions
collisions can still occur:
propagation delay means two nodes may not hear each other’s
transmission
collision
B
A
Collision is inevitable, and need detect and respond
5: DataLink Layer
5-50
CSMA collisions
spatial layout of nodes
collisions can still occur:
propagation delay means
two nodes may not hear
each other’s transmission
collision:
entire packet transmission
time wasted
note:
role of distance & propagation
delay in determining collision
probability
5: DataLink Layer
5-51
CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA
collisions detected within short time
 colliding transmissions aborted, reducing channel
wastage

 collision detection:
 easy in wired LANs: measure signal strengths,
compare transmitted, received signals
 difficult in wireless LANs: received signal strength
overwhelmed by local transmission strength
 human analogy: the polite conversationalist
5: DataLink Layer
5-52
CSMA/CD collision detection
5: DataLink Layer
5-53
Collision window

Time needed to detect the collision after transmitting?
τ= distance (m) / signal-speed (200m/μs)。
2 τ： the largest time to detect the collision.
Succeed: no collision during 2τ
5: DataLink Layer
5-54
Ethernet uses CSMA/CD Protocol
 adapter may begin to
transmit at anytime, i.e.,
no slots are used
 adapter doesn’t transmit
if it senses that some
other adapter is
transmitting, that is,
carrier sense
 transmitting adapter
aborts when it senses
that another adapter is
also transmitting, that is,
collision detection
 Before attempting a
retransmission,
adapter waits a
random time, that is,
random access
 Retransmit after a
random amount of time
5: DataLink Layer
5-55
Ethernet CSMA/CD algorithm
1. Adaptor receives
4. If adapter detects
datagram from network
another transmission while
layer and creates frame
transmitting, aborts and
sends jam signal
2. If adapter senses channel
idle, it starts to transmit 5. After aborting, adapter
frame.
enters exponential
backoff (指数回退):
If it senses channel busy,
waits until channel idle and
Adapter waits
then transmits
Ti = r x
3. If adapter transmits
entire frame without
slot
detecting another
transmission, the adapter
and returns to Step 2
is done with frame !
5: DataLink Layer
5-56
Exponential Backoff
the ith collision,
Ti = r x slot
r = random（0 ~ 2i-1），i最大为10
Slot= 512bit time
Bit time: 0.1 msfor 10 Mbps Ethernet ; Slot=51.2 ms
 first collision: choose r from {0, 1}; delay is K x 512
bit transmission times
 after second collision: choose r from {0, 1, 2, 3}…
 after ten collisions, choose r from {0, 1, 2, 3, 4,…,
1023}，for K=1023, wait time is about 50 ms
5: DataLink Layer
5-57
CSMA/CA (Collision Avoidance )
 Collision detection:
easy in wired LANs: measure signal strengths,
compare transmitted and received signals
 difficult in wireless LANs: receiver shut off
while transmitting; i.e., can’t transmit and
receive at the same time

 CSMA/CA
 RTS (RTS: Request to Send)
 CLS (CTS: Clear to Send)
5: DataLink Layer
5-58
CSMA/CA (Collision Avoidance )
CSMA/CA
Collision Avoidance
5: DataLink Layer
5-59
“Taking Turns” MAC protocols
Channel partitioning MAC protocols:
 share channel efficiently and fairly at high load
 inefficient at low load: delay in channel access,
1/N bandwidth allocated even if only 1 active
node!
Random access MAC protocols
 efficient at low load: single node can fully
utilize channel
 high load: collision overhead
“Taking turns” protocols
look for best of both worlds!
5: DataLink Layer
5-60
“Taking Turns” MAC protocols
Polling:
 master node “invites”
slave nodes to transmit in
turn
 typically used with “dumb”
slave devices
 concerns:
polling overhead
 latency
 single point of failure
(especially master failure)

data
poll
master
data
slaves
5: DataLink Layer
5-61
“Taking Turns” MAC protocols
Token passing:
 control token passed
from one node to next
sequentially.
 token message
 concerns:



token overhead
latency
single point of failure
(token)
T
(nothing
to send)
T
data
5: DataLink Layer
5-62
Summary of MAC protocols
 channel partitioning, by time, frequency, code or space
 Time Division, Frequency Division, Code Division
 random access (dynamic),
 ALOHA, S-ALOHA, CSMA, CSMA/CD
 carrier sensing: easy in some technologies (wire), hard in
others (wireless)
 CSMA/CD used in Ethernet
 CSMA/CA used in IEEE 802.11
 taking turns
 polling from central site, token passing
 Bluetooth, FDDI, IBM Token Ring
5: DataLink Layer
5-63
Homework
 Reviews: 1, 2，6
5: DataLink Layer
5-64
LAN Technologies
Data link layer so far:

services, error detection/correction, multiple
access
Next: LAN technologies
addressing
 Ethernet
 switches
 PPP

5: DataLink Layer
5-65
Link Layer
 5.1 Introduction and




services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet
 5.6 Link-layer switches
 5.7 PPP
 5.8 Link Virtualization:
ATM, MPLS
5: DataLink Layer
5-66
Vocabulary and Term
 PAN/LAN/MAN/WAN
 Personal/Local/Metropolitan/Wide

Area Network
个域网/局域网/城域网/广域网
 MAC（Media Access Control）
 介质访问控制
 LLC（Logical Link Control）
 逻辑链路控制
 ARP（Address Resolution Protocol）
 地址解析协议
5: DataLink Layer
5-67
MAC Addresses and ARP
 32-bit IP address:

network-layer address

used to get datagram to destination IP subnet
 MAC address:


function: get frame from one interface to another
physically-connected interface (same network)
48 bit MAC address (for most LANs)
• burned in NIC ROM, also sometimes software settable
5: DataLink Layer
5-68
LAN Address (more)
 MAC address allocation administered by IEEE
 Manufacturer buys portion (24bit) of MAC
address space (to assure uniqueness)
 Analogy:
(a) MAC address: like Social Security Number
(b) IP address: like postal address
 MAC flat address ➜ portability

can move LAN card from one LAN to another
 IP hierarchical address NOT portable
 address depends on IP subnet to which node is attached
5: DataLink Layer
5-69
LAN Addresses and ARP
Each adapter on LAN has unique LAN/MAC address
1A-2F-BB-76-09-AD
71-65-F7-2B-08-53
Broadcast IP address :
FF-FF-FF-FF-FF-FF
LAN
(wired or
wireless)
: adapter/NIC
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
5: DataLink Layer
5-70
ARP: Address Resolution Protocol
Question: how to determine
MAC address of B if we
know B’s IP address before?
137.196.7.78
1A-2F-BB-76-09-AD
137.196.7.23
137.196.7.14
 Each IP node ( host,
router) on LAN has ARP
table
 ARP table: IP/MAC
address mappings for
some LAN nodes
<IP address; MAC address; TTL>

LAN
71-65-F7-2B-08-53
137.196.7.88
58-23-D7-FA-20-B0
TTL (Time To Live): time
after which address
mapping will be forgotten
(typically 20 min)
0C-C4-11-6F-E3-98
5: DataLink Layer
5-71
Recall earlier routing discussion
Starting at A, given IP
datagram addressed to B:
A
223.1.1.1
223.1.2.1
 look up network address of B,
find B on same network as A
 link layer send datagram to B
inside link-layer frame
frame dest
address
223.1.1.2
223.1.1.4 223.1.2.9
B
223.1.1.3
frame source datagram source,
address
dest address
B’s MAC A’s MAC
addr
addr
A’s IP
addr
B’s IP
addr
223.1.3.27
223.1.3.1
223.1.2.2
E
223.1.3.2
IP payload
datagram
frame
5: DataLink Layer
5-72
Hi – listen, everyone! 223.1.1.3 Tell me your MAC!
My IP is 223.1.1.1, my MAC is 87-A2-15-35-02-CC
A: 223.1.1.1
B: 223.1.1.3
not me
I am 223.1.1.3, My MAC is 87-A2-15-35-02-C3
5: DataLink Layer
5-73
ARP datagram in Ethernet Frame
Dest. MAC
ARP Request Frame
111…111
Src. MAC
MAC
My
10101001 0806H
1
A1
Protocol Type：ARP= 0806H
IP
My
My A2
MAC
Your
IP
Your
?
Your B2
Operation: request
Hardware Type=1
Protocol type=0800H
Hw Addr len=6
Protocol Addr len=4
ARP Reply Frame
10101001
A1
11100011 0806H
2
B1
B2
A1
A2
B1
Hardware Type=1
Protocol type=0800H
Hw Addr len=6
Protocol Addr len=4
5: DataLink Layer
5-74
ARP protocol: Same LAN (network)
 A wants to send datagram
to B, and B’s MAC address
not in A’s ARP table.
 A broadcasts ARP query
packet, containing B's IP
address
 Dest. MAC address =
FF-FF-FF-FF-FF-FF
 all machines on LAN
receive ARP query
 B receives ARP packet,
replies to A with its (B's)
MAC address

frame sent to A’s MAC
address (unicast)
 A caches (saves) IP-to-
MAC address pair in its
ARP table until information
becomes old (times out)
 soft state: information
that times out (goes
away) unless refreshed
 ARP is “plug & play”:
 nodes create their ARP
tables without
intervention from net
administrator
5: DataLink Layer
5-75
Routing to another LAN
Walkthrough: send datagram from A to B via router R
assume A has already known B’s IP address
A
R
B
 Two ARP tables in router R, one for each IP network
5: DataLink Layer
5-76
 A creates IP datagram with source A, destination B
 A uses ARP to get Router’s MAC address for 111.111.111.110, E6





E9-00-17-BB-4B
A creates link-layer frame with R's MAC address as destination,
frame contains A-to-B IP datagram
A’s NIC sends the frame
R’s NIC receives frame
R removes IP datagram from Ethernet frame, sees its destined
to B
R uses ARP to get B’s MAC address, i.e., 49-BD-D2-C7-56-2A
R creates frame containing A-to-B IP datagram sends to B
88-B2-2F-54-1A-0F
74-29-9C-E8-FF-55
A
E6-E9-00-17-BB-4B
111.111.111.111
222.222.222.220
111.111.111.110
111.111.111.112
222.222.222.221
1A-23-F9-CD-06-9B
R
222.222.222.222
B
49-BD-D2-C7-56-2A
CC-49-DE-D0-AB-7D
5: DataLink Layer
5-77
ARP Commands：
C:\> arp -a
---- 显示目前的ARP表
C:\> arp -s 192.168.25.198 00-AA-BB-F2-33-C4
---- 增加一个ARP 项
C:\> arp – d 192.168.25.198
---- 删除一个ARP 项
5: DataLink Layer
5-78
Link Layer
 5.1 Introduction and




services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet
 5.6 Link-layer switches
 5.7 PPP
 5.8 Link Virtualization:
ATM and MPLS
5: DataLink Layer
5-79
Ethernet Inventor
 3Com Inc. Founder
 Popular IT writer
 InfoWorld (In 1993 he became
vice-president of technology for
the International Data Group,
parent company of InfoWorld
Magazine)
Bob Metcalfe
(1946-)
 Metcalfe Law
 The usefulness, or utility, of a
network equals square of the
numbers of users.
5: DataLink Layer
5-80
Bob Metcalfe
 Born in Brooklyn, New York in 1946

So fascinated was he by technology and gadgets as a child, that by the
age of ten he already knew he wanted to become an electrical engineer
and that he wanted to attend the Massachusetts Institute of
Technology (MIT).
 University Education

Metcalfe did attend MIT eventually, and he graduated in 1969 with a
bachelor's degrees in electrical engineering and business management.
In 1970, he received a master's degree in applied mathematics from
Harvard University, and he completed his Ph.D. in computer science at
Harvard in 1973.
 Work on ARPAnet and ALOHA

B. Metcalfe began working at Xerox PARC. The first job he did:
connected Xerox ALTO computer to ARPAnet. Metcalfe
invented what has come to be known as Ethernet, the local area
networking (LAN) technology that turns PCs into communication
tools by linking them together
5: DataLink Layer
5-81
Ethernet History-1
 1973-05-22

每台Xerox计算机通过光纤电缆相连，并接上打印机。速度：
2.94Mbps

该速度值有点零碎，究其原因是第一个以太网的接口定时器采用
ALTO系统时钟，意味着每340毫微秒就发送一次脉冲。
 What is Ethernet？

灵感来自于“电磁辐射是可以通过发光的以太来传播的这一想法”

天体要素：在古代和中古时代被认为是充满于月球空间并且是构
成恒星和行星的元素。

以太【物理学】：一种在以前被假定为电磁波的传播媒质并具有
绝对连续性、高度弹性的极其稀薄的媒体。
5: DataLink Layer
5-82
Ethernet History-2
 1976年6月

Metcalfe和 Boggs发表："以太网：局域网的分布型信息包交换"的
著名论文。
 1977年底

Metcalfe和三位合作者获得了“具有冲突检测的多点数据通信系统”
专利，多点传输被称为 CSMA/CD。从此，以太网正式诞生。
 技术、市场与产业标准


1979：技术推向市场
1970年代末，数十种局域网技术涌现出来，以太网只是其中之一。
当时著名的网络还有：DEC公司 MCA、NS公司 Hyperchannel、
Data Point公司ARCnet和 Corvus公司 Omninet。最终以太网一统
天下，不是其技术优势和速度，而是以太网变成产业标准。
5: DataLink Layer
5-83
Ethernet History-3
 Standardization

1980年：DEC、Intel和Xerox联合发表了10Mbps Ethernet标准
DIX 80。
• 物理介质粗同轴电缆
• Xerox提供技术， DEC有雄厚的技术力量，而且是以太网硬件的
强有力的供应商，Intel提供以太网芯片构件。


1982年：DIX Version 2规范，增加网管功能。
IEEE成立了802计划组，目标是为LAN技术标准化提供广泛的工业
框架。IEEE802.3研究以太网技术标准。1983年，通过IEEE 802.3
CSMA/CD规范。
5: DataLink Layer
5-84
Ethernet History-4
 以太网的发展

1982~1990：10Mbps以太网成熟，可依托多种物理介质
• 3COM公司为Apple机配置的第一批以太网产品投放市场
• IBM正忙于发明令牌环网, 放弃与3Com 合作
• EtherLink成为 IBM PC的第一个以太网ISA总线适配器
• EtherLink设计师 Ron Crane发明细同轴电缆
• 在无屏蔽双 绞线(UTP)电话电缆上运行以太网 [1Mbps]

1983~1997：LAN桥接与交换技术日趋成熟

1995：IEEE 802.3u Fast Ethernet

1998：IEEE 802.3z Gigabit Ethernet

2002：IEEE 10G Ethernet

2010: 100G?
5: DataLink Layer
5-85
Ethernet History-5
 以太网已经成为局域网(LAN)中的主导技术。
 而且随着Gbps级 Ethernet出现，以太网开始向城域网(MAN)迈进。
 而且随着10 Gbps Ethernet出现，以太网已开始向广域网(WAN)渗透。
 虽然业界对于以太网下一步是40Gbps还是100Gbps还没有定论，但业界
的芯片领导厂家很快会推出单片支持20个10 Gbps Ethernet端口的以太
网芯片，现正在实验室试验100Gbps的以太网技术。
5: DataLink Layer
5-86
Ethernet
“dominant” wired LAN technology:
 cheap $20 for NIC
 first widely used LAN technology
 simpler, cheaper than token LANs and ATM
 kept up with speed race: 10 Mbps – 10 Gbps
Metcalfe’s Ethernet
Sketch
5: DataLink Layer
5-87
Star topology
 bus topology popular through mid-1990s
 all nodes in same collision domain (can collide with each
other)
 today: star topology prevails
 active switch in center
 each “spoke” runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus: coaxial cable
star
5: DataLink Layer
5-88
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble:
 7 bytes with pattern 10101010 followed by one
(8th) byte with pattern 10101011
 used to synchronize receiver, sender clock rates
5: DataLink Layer
5-89
Ethernet Frame Structure (more)
 MAC Addresses: 6 bytes
 if adapter receives frame with matching destination
address, or with broadcast address (e.g. ARP packet), it
passes data in frame to network layer protocol
 otherwise, adapter discards frame
 Type: indicates higher layer protocol (mostly IP
but others possible, e.g., Novell IPX, AppleTalk)
 CRC: checked at receiver, if error is detected,
frame is dropped
5: DataLink Layer
5-90
Ethernet: Unreliable, connectionless
 Connectionless: No handshaking between sending and
receiving NICs
 Unreliable: receiving NIC doesn’t send ACKs or
NACKs to sending NIC (source)



stream of datagrams passed to network layer can have gaps
(missing datagrams)
gaps will be filled if app is using TCP
otherwise, application will see gaps
 Ethernet’s MAC protocol: unslotted CSMA/CD
5: DataLink Layer
5-91
Ethernet CSMA/CD algorithm
1. NIC adapter receives
datagram from network
layer, creates frame
4. If NIC detects another
transmission while
transmitting, aborts and
sends jam signal
2. If NIC senses channel idle,
5. After aborting, NIC
starts frame transmission
enters exponential
otherwise, waits until
th
backoff:
after
m
channel idle, then transmits
collision, NIC chooses K at
3. If NIC transmits entire
random from
frame without detecting
{0, 1, 2,…, 2m-1}. NIC waits
another transmission, NIC
K·512 bit times, returns to
is done with frame !
Step 2
2Շ
5: DataLink Layer
5-92
Ethernet’s CSMA/CD (more)
Jam Signal:
make sure all other
transmitters are aware of
collision; 48 bits
Bit time:
0.1 μs for 1 bit on 10Mbps
Ethernet ; for K=1023,
wait time is about 52.38
ms
See/interact with Java
applet on AWL Web site:
highly recommended !
Exponential Backoff:
 Goal: adapt retransmission
attempts to estimated
current load
 heavy load: random wait
will be longer
 first collision: choose K from
{0, 1}; delay is 0 or 51.2μs
 after second collision: choose
K from {0, 1, 2, 3} …
 after 10 collisions, choose K
from {0, 1, 2, 3, 4,…, 1023}
5: DataLink Layer
5-93
CSMA/CD efficiency
 Tprop = max propagation delay between 2 nodes in LAN
 ttrans = time to transmit max-size frame
efficiency 
1
1  5t prop /ttrans
 efficiency always goes to 1
 as tprop goes to 0
 as ttrans goes to infinity
 better performance than ALOHA: and simple, cheap,
decentralized!
5: DataLink Layer
5-94
802.3 Ethernet Standards: Link & Physical Layers
 Many different Ethernet standards
common MAC protocol and frame format
 different speeds: 2 Mbps, 10 Mbps, 100 Mbps,
1Gbps, 10G bps
 different physical layer media: fiber, cable, etc.

application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T2
100BASE-FX
100BASE-T4
100BASE-SX
100BASE-BX
copper (twister
pair) physical layer
fiber physical layer
5: DataLink Layer
5-95
Ethernet : 10Base 2
 10: 10Mbps;
 2: under 200 meters max cable length
 thin coaxial cable in a bus topology
5: DataLink Layer
5-96
Ethernet Types
10 Base 5
10Base5
Thick Coax
10Base2
Thin Coax
10BaseT
Twisted
10BaseF
MMF
100BaseT
Twisted
100BaseF
MMF/SMF
1000BaseX
Shielded short Twisted/MMF/SMF
1000BaseT
Twisted
Mbps (data rate) Base（基带,未经过
频率变换）
Broad(宽带)
Max Length（100M）
Media type (T, F, X）
5: DataLink Layer
5-97
Ethernet PHY
以太网标准
IEEE规范
公布时间
速率
拓扑结构
网段长度
支持介质
10Base5
802.3
1983
10M
总线
500
50Ω粗缆
10Base2
802.3a
1988
10M
总线
185
50Ω细缆
1Base5
802.3c
1988
1M
星型
250
2对3类UTP（100Ω）
10Base-T
802.3i
1990
10M
星型
100
2对3类UTP（100Ω）
10Broad36
802.3b
1988
10M
总线
1800
75Ω同轴电缆
10Base-TF
802.3i
1992
10M
星型
2000
2股多模/单模光缆
100Base-TX
802.3u
1995
100M
星型
100
2对5类UTP（100Ω）
100Base-T4
802.3u
1995
100M
星型
100
4对3类UTP（100Ω）
100Base-TF
802.3u
1995
100M
星型
2000
2股多模/单模光缆
260
62.5μm芯多模光缆
525
50μm芯多模光缆
550
62.5μm芯多模光缆
525
50μm芯多模光缆
3000
单模光缆
1000Base-SX
1000Base-LX
802.3z
802.3z
1998
1998
1000M
1000M
星型
星型
1000Base-CX
802.3z
1998
1000M
星型
25
STP同轴电缆
1000Base-T
802.3ab
1999
1000M
星型
1000
5: DataLink Layer
5类UTP（100Ω
5-98
Manchester encoding
 used in 10BaseT
 each bit has a transition
 allows clocks in sending and receiving nodes to
synchronize to each other

no need for a centralized, global clock among nodes!
 Hey, this is physical-layer stuff!
5: DataLink Layer
5-99
Device and Card
5: DataLink Layer 5-100
10Gb Ethernet Media Types
 10GBase-SR – up to 300m over dark fiber
 10GBase-SW – up to 300m over SONET
 Both 850nm, multimode fiber
 10GBase-LR – 2m-10km over dark fiber
 10GBase-LW – 2m-10km over SONET
 Both 1310nm, single mode fiber
 10GBase-ER – 2m – 40km over dark fiber
 10GBase-EW – 2m – 40km over SONET
 Both 1550nm, single mode fiber
 10GBase-LX4 – 4 parallel wavelengths over single
multi- or single-mode fiber pair at 1310nm
5: DataLink Layer 5-101
10 Gbps Applications: Internet Peering
ISP 1
10 GE
10 GE (Private)
ISP 4
10 GE
10 GE
10 GE
ISP 2
ISP 3
5: DataLink Layer 5-102
10 Gbps Applications: Server Connections
Customer Cages
Co-Location
Facility
Optical
Core
Layer 4-7
Switches, Servers
10 GE
Connections
IP Core
Direct
Attachment
5: DataLink Layer 5-103
10 Gbps Applications: MAN or WAN
SONET or
dark fiber
10 GbE
5: DataLink Layer 5-104
Link Layer
 5.1 Introduction and




services
5.2 Error detection
and correction
5.3 Multiple access
protocols
5.4 Link-layer
Addressing
5.5 Ethernet
 5.6 Link-layer switches
 5.7 PPP
 5.8 Link Virtualization:
ATM, MPLS
5: DataLink Layer 5-105
Hubs
… physical-layer (“dumb”) repeaters:
 bits coming in one link go out all other links at same rate
 all nodes connected to hub can collide with one another
 no frame buffering
 no CSMA/CD at hub: host NICs detect collisions
twisted pair
hub
5: DataLink Layer 5-106
Switch
 link-layer device: smarter than hubs, take active
role


store, forward Ethernet frames
examine incoming frame’s MAC address, selectively
forward frame to one-or-more outgoing links when frame
is to be forwarded on segment, uses CSMA/CD to access
segment
 transparent
 hosts are unaware of presence of switches
 plug-and-play, self-learning
 switches do not need to be configured
5: DataLink Layer 5-107
Switch: allows multiple simultaneous
transmissions
A
 hosts have dedicated,
direct connection to switch
 switches buffer packets
 Ethernet protocol used on
each incoming link, but no
collisions; full duplex

each link is its own collision
domain
 switching: A-to-A’ and B-
to-B’ simultaneously,
without collisions

not possible with dumb hub
C’
B
6
1
5
2
3
4
C
B’
A’
switch with six interfaces
(1,2,3,4,5,6)
5: DataLink Layer 5-108
Switch Table
 Q: how does switch know that A’
reachable via interface 4, B’
reachable via interface 5?
 A: each switch has a switch
table, each entry:

C’
B
6
 Q: how are entries created,
maintained in switch table?
something like a routing protocol?
1
5
(MAC address of host, interface to
reach host, time stamp)
 looks like a routing table!

A
2
3
4
C
B’
A’
switch with six interfaces
(1,2,3,4,5,6)
5: DataLink Layer 5-109
Switch: self-learning
 switch learns which hosts
can be reached through
which interfaces


Source: A
Destination: A’
A A A’
C’
when frame received,
switch “learns” location of
sender: incoming LAN
segment
records sender/location
pair in switch table
B
1
6
5
2
3
4
C
B’
A’
MAC addr. interface TTL
A
1
60
Switch table
(initially empty)
5: DataLink Layer
5-110
Switch: frame filtering/forwarding
When frame received:
1. record link associated with sending host
2. index switch table using MAC destination address
3. if entry found for destination
then {
if dest. on segment from which frame arrived
then drop the frame
else forward the frame on interface indicated
}
else flooding
forward on all but the interface
on which the frame arrived
5: DataLink Layer
5-111
Self-learning, forwarding:
example
Source: A
Destination: A’
A A A’
C’
 frame destination
unknown: flooding
B
A6A’
1
2
4
5
 destination A
location known:
selective send
C
A’ A
B’
3
A’
MAC addr. interface TTL
A
A’
1
4
60
60
Switch table
(initially empty)
5: DataLink Layer
5-112
Interconnecting switches
 switches can be connected together
S4
S1
S2
A
B
S3
C
F
D
E
I
G
H
 Q: sending from A to F - how does S1 know to
forward frame destined to F via S4 and S2?
 A: self learning! (works exactly the same as in
single-switch case!)
5: DataLink Layer
5-113
Institutional network
to external
network
mail server
router
web server
IP subnet
5: DataLink Layer
5-114
Switches vs. Routers
 both store-and-forward devices
 routers: network layer devices (examine network layer
headers)
 switches are link layer devices
 routers maintain routing tables, implement routing
algorithms
 switches maintain switch tables, implement
filtering, learning algorithms
5: DataLink Layer
5-115
Summary comparison
hubs
swithches
routers
traffic
isolation
no
yes
yes
plug & play
yes
no
yes
optimal
routing
cut
through
no
no
yes
yes
yes
no
5: DataLink Layer
5-116
Link Layer
 5.1 Introduction and




services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet
 5.6 Hubs and switches
 5.7 PPP
 5.8 Link Virtualization:
ATM
5: DataLink Layer
5-117
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!
5: DataLink Layer
5-118
PPP Design Requirements [RFC 1557]
 packet framing: encapsulation of network-layer




datagram in data link frame
 carry network layer data of any network layer
protocol (not just IP) at same time
 ability to de-multiplex 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
5: DataLink Layer
5-119
PPP non-requirements
 no error correction/recovery
 no flow control
 out of order delivery OK
 no need to support multipoint links (e.g., polling)
Error recovery, flow control, data re-ordering
all relegated to higher layers!
5: DataLink Layer 5-120
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 (e.g., PPP-LCP, IP, IPCP, etc)
5: DataLink Layer 5-121
PPP Data Frame
 info: upper layer data being carried
 check: cyclic redundancy check for error detection
5: DataLink Layer 5-122
Byte Stuffing
 “data transparency” requirement: data field
must be allowed to include flag pattern <01111110>
 Q: Is received <01111110> data or flag?
 Sender: adds (“stuffs”) extra < 01111110> byte
after each < 01111110> data byte
 Receiver:
 two 01111110 bytes in a row: discard first byte,
continue data reception
 single 01111110: flag byte
5: DataLink Layer 5-123
Byte Stuffing
flag byte
pattern
in data
to send
flag byte pattern plus
stuffed byte in
transmitted data
5: DataLink Layer 5-124
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) messages
(protocol field: 8021) to
configure/learn IP
address
5: DataLink Layer 5-125
Link Layer
 5.1 Introduction and




services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet
 5.6 Hubs and switches
 5.7 PPP
 5.8 Link Virtualization:
ATM and MPLS
5: DataLink Layer 5-126
Virtualization of networks
Virtualization of resources: powerful abstraction in
systems engineering
 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
5: DataLink Layer 5-127
The Internet: virtualizing networks
1974: multiple unconnected nets
 ARPAnet
… differing in:
 addressing
 data-over-cable
networks
 packet satellite network (ALOHA)
 packet radio network
ARPAnet
"A Protocol for Packet Network Intercommunication",
V. Cerf, R. Kahn, IEEE Transactions on Communications,
May, 1974, pp. 637-648.
conventions
 packet formats
 error recovery
 routing
satellite net
5: DataLink Layer 5-128
The Internet: virtualizing networks
Gateway:
Internetwork layer (IP):
 “embed internetwork packets in
 addressing: internetwork
local packet format or extract
appears as single, uniform
them”
entity, despite underlying local  route (at internetwork level) to
next gateway
network heterogeneity
 network of networks
gateway
Satellite net
ARPAnet
5: DataLink Layer 5-129
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
 56K telephone modem
 today: ATM, MPLS
… “invisible” at internetwork layer. Looks like a link
layer technology to IP!
5: DataLink Layer 5-130
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)
 ATM, MPLS: of technical interest in their
own right
5: DataLink Layer 5-131
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
5: DataLink Layer 5-132
ATM architecture
AAL
AAL
ATM
ATM
ATM
ATM
physical
physical
physical
physical
end system
switch
switch
end system
 ATM Adaptation layer: only at edge of ATM network
data segmentation/reassembly
 roughly analogous to Internet transport layer
 ATM layer: treat as “network” layer
 cell switching, routing
 physical layer

5: DataLink Layer 5-133
ATM: network or link layer?
Vision: end-to-end
transport: “ATM from
desktop to desktop”
 ATM
is a network
technology
IP
network
ATM
network
Reality: used to connect
IP backbone routers


“IP over ATM”
ATM as switched
link layer,
connecting IP
routers
5: DataLink Layer 5-134
ATM Adaptation Layer (AAL)
 ATM Adaptation Layer (AAL): “adapts” upper
layers (IP or native ATM applications) to ATM
layer below
 AAL present only in end systems, not in switches
 AAL layer segment (header/trailer fields, data)
fragmented across multiple ATM cells
 analogy: TCP segment in many IP packets
AAL
AAL
ATM
ATM
ATM
ATM
physical
physical
physical
physical
end system
switch
switch
end system
5: DataLink Layer 5-135
ATM Adaptation Layer (AAL) [more]
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 (e.g., IP datagrams)
User data
AAL PDU
ATM cell
5: DataLink Layer 5-136
ATM Layer
Service: transport cells across ATM network
 analogous to IP network layer
 very different services than IP network layer
Network
Architecture
Internet
Service
Model
Guarantees ?
Congestion
Bandwidth Loss Order Timing feedback
best effort none
ATM
CBR
ATM
VBR
ATM
ABR
ATM
UBR
constant
rate
guaranteed
rate
guaranteed
minimum
none
no
no
no
yes
yes
yes
yes
yes
yes
no
yes
no
no (inferred
via loss)
no
congestion
no
congestion
yes
no
yes
no
no
5: DataLink Layer 5-137
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-destination path maintain “state” for
each passing connection
 link, switch resources (bandwidth, buffers) may be allocated to
VC: to get circuit-like performence.
 Permanent VCs (PVCs)
long lasting connections
 typically: “permanent” route between to IP routers
 Switched VCs (SVC):
 dynamically set up on per-call basis

5: DataLink Layer 5-138
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/destination pair)
does not scale (N*2 connections needed)
 SVC introduces call setup latency, processing
overhead for short lived connections

5: DataLink Layer 5-139
ATM Layer: ATM cell
 5-byte ATM cell header
 48-byte payload
Why?: small payload -> short cell-creation delay
for digitized voice
 halfway between 32 and 64 (compromise!)

Cell header
Cell format
5: DataLink Layer 5-140
ATM cell header
 VCI: virtual channel ID
will change from link to link through net
 PT: Payload type (e.g. RM cell vs. 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

5: DataLink Layer 5-141
ATM Physical Layer (more)
Two pieces (sublayers) of physical layer:
 Transmission Convergence Sublayer (TCS): adapts
ATM layer above to PMD sublayer below
 Physical Medium Dependent: depends on physical
medium being used
TCS Functions:
 Header checksum generation: 8 bits CRC
 Cell delineation
 With “unstructured” PMD sublayer, transmission
of idle cells when no data cells to send
5: DataLink Layer 5-142
ATM Physical Layer
Physical Medium Dependent (PMD) sublayer
 SONET/SDH: transmission frame structure (like a
container carrying bits);
 bit synchronization;
 bandwidth partitions (TDM);
 several speeds: OC3 = 155.52 Mbps; OC12 = 622.08
Mbps; OC48 = 2.45 Gbps, OC192 = 9.6 Gbps
 TI/T3: transmission frame structure (old
telephone hierarchy): 1.5 Mbps/ 45 Mbps
 unstructured: just cells (busy/idle)
5: DataLink Layer 5-143
IP-Over-ATM
Classic IP only
 3 “networks” (e.g.,
LAN segments)
 MAC (802.3) and IP
addresses
IP over ATM
 replace “network”
(e.g., LAN segment)
with ATM network
 ATM addresses, IP
addresses
ATM
network
Ethernet
LANs
Ethernet
LANs
5: DataLink Layer 5-144
IP-Over-ATM
app
transport
IP
Eth
phy
IP
AAL
Eth
ATM
phy phy
ATM
phy
ATM
phy
app
transport
IP
AAL
ATM
phy
5: DataLink Layer 5-145
Datagram Journey in IP-over-ATM Network
 at Source Host:
 IP layer maps between IP, ATM destination address (using
ARP)
 passes datagram to AAL5
 AAL5 encapsulates data, segments cells, passes to ATM layer
 ATM network: moves cell along VC to destination
 at Destination Host:
AAL5 re-assembles cells into original datagram
 if CRC OK, datagram is passed to IP

5: DataLink Layer 5-146
IP-Over-ATM
Issues:
 IP datagrams into
ATM AAL5 PDUs
 from IP addresses
to ATM addresses
 just like IP
addresses to
802.3 MAC
addresses!
ATM
network
Ethernet
LANs
5: DataLink Layer 5-147
Multi-Protocol Label Switching (MPLS)
 initial goal: speed up IP forwarding by using fixed
length label (instead of IP address) to do
(matching) & 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
5: DataLink Layer 5-148
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
 forwarding possible along paths that IP alone would
not allow (e.g., source-specific routing) !!
 use MPLS for traffic engineering
 must co-exist with IP-only routers
5: DataLink Layer 5-149
MPLS forwarding tables
in
label
out
label dest
10
12
8
out
interface
A
D
A
0
0
1
in
label
out
label dest
out
interface
10
6
A
1
12
9
D
0
R6
0
0
D
1
1
R3
R4
R5
0
0
R2
in
label
8
out
label dest
6
A
out
interface
in
label
6
outR1
label dest
-
A
A
out
interface
0
0
5: DataLink Layer 5-150
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
5: DataLink Layer 5-151
Chapter 5: let’s take a breath
 Journey down protocol stack complete
(except PHY)
 Solid understanding of networking
principles, practice
 ….. could stop here …. but lots of interesting
topics!
Wireless
 Multimedia
 Security
 Network management

5: DataLink Layer 5-152