1. Chapter 5 Data Link Layer
CMPT 371 Data Communications and Networking
Chapter 5 Data Link Layer
5: DataLink Layer

2. 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

3. Chapter 5 outline 5.1 Introduction and services
5.2 Error detection and correction
5.3Multiple access protocols
CDMA
Aloha, CSMA/CD
5.4 Ethernet
5.5 Wireless links and LANs
(WiFi)
5: DataLink Layer

4. Link Layer: Introduction
Some terminology:
hosts and routers are nodes
(bridges and switches too)
communication channels that connect adjacent nodes along communication path are links
wired links
wireless links
LANs
Data unit is a frame, encapsulates datagram
data-link layer has responsibility of
transferring datagram from one node
to adjacent node over a link
5: DataLink Layer

5. Where is the link layer implemented !
datagram
link layer protocol
rcving
node
sending
node
frame
frame
adapter
adapter
link layer implemented in “adaptor” (aka NIC) or on a chip
Ethernet card, card/chipset
Combining hardware, software, firmware
5: DataLink Layer

6. Link layer: context transportation analogy
trip from Princeton to Lausanne
limo: Princeton to JFK
plane: JFK to Geneva
train: Geneva to Lausanne
tourist = datagram
transport segment = communication link
transportation mode = link layer protocol
travel agent = routing algorithm
Datagram transferred by different link protocols over different links:
e.g., Ethernet on first link, frame relay on intermediate links, on last link
Each link protocol provides different services
e.g., may or may not provide rdt over link
5: DataLink Layer

7. Link Layer Services Reliable delivery between adjacent nodes
we learned how to do this already (chapter 3)!
Q: why both link-level and end-end reliability?
5: DataLink Layer

8. Link Layer Services (more)
Flow Control (Chap 3):
pacing between adjacent sending and receiving nodes
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
5: DataLink Layer

9. Chapter 5 outline 5.1 Introduction and services
5.2 Error detection and correction
5.3Multiple access protocols
5.4 Ethernet
5.5 Hubs, bridges, and switches
5.6 Wireless links and LANs
5: DataLink Layer

10. 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
5: DataLink Layer

11. Internet checksum
Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer only)
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?
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
5: DataLink Layer

12. Example of checksum - Recall
Segments +=
 1’s complement
5: DataLink Layer

13. Parity Checking Two Dimensional Bit Parity: Single Bit Parity:
Detect single bit errors
Two Dimensional Bit Parity:
Detect and correct single bit errors
5: DataLink Layer

14. Chapter 5 outline 5.1 Introduction and services
5.2 Error detection and correction
5.3Multiple access protocols
5.4 Ethernet
5.5 Hubs, bridges, and switches
5.6 Wireless links and LANs
5: DataLink Layer

15. 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)
traditional Ethernet
Satellite
wireless LAN
Or lecture room
5: DataLink Layer

16. Multiple Access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes: interference
only one node can send successfully at a time
multiple access protocol
algorithm that determines how nodes share channel, i.e., determine when node can transmit
communication about channel sharing must use channel itself!
what to look for in multiple access protocols:
5: DataLink Layer

17. Ideal Mulitple 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

18. 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”
speak only if “token” is at hand
tightly coordinate shared access to avoid collisions
5: DataLink Layer

19. 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
example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle
Problem
unused slots go idle
More ?
6-slot
frame
6-slot
frame 1 3 4 1 3 4
5: DataLink Layer

20. Channel Partitioning MAC protocols: FDMA
FDMA: frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band
example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle
Problem
unused transmission time in frequency bands go idle
More ?
time
frequency bands
FDM cable
5: DataLink Layer

21. Channel Partitioning (CDMA)
CDMA (Code Division Multiple Access)
Analogy
Public Key Encryption
Only the one holing the key can decrypt
Motivation
Sender – Mix information encoded with “codes” of receivers
Receiver – Decode mixed information using its own code, and find that for itself
5: DataLink Layer

22. Channel Partitioning (CDMA)
CDMA (Code Division Multiple Access)
used mostly in wireless broadcast channels (cellular, satellite, etc)
unique “code” assigned to each user; i.e., code set partitioning
all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding: inner-product of encoded signal and chipping sequence
allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)
5: DataLink Layer

23. CDMA Encode/Decode
5: DataLink Layer

24. CDMA: two-sender interference
5: DataLink Layer

25. CDMA: two-sender interference
Assume codes
(1,-1) for recv1
(1,1) for recv2
Data bit 1 for recv 1:
1·(1,-1)
Decoded using code for recv1:
1·(1,-1) ·(1,-1)
=(1,-1) ·(1,-1)=2
Decoded using code for recv 2:
1·(1,-1) ·(1,1)
=(1,-1) ·(1,1)=0
5: DataLink Layer

26. Frequency Hopping (origin of CDMA)
Frequency Hopping Spread Spectrum:
transmitting radio signal by rapidly switching among many frequency
channels, using a sequence known to both transmitter and receiver
Google “Mother of CDMA”
5: DataLink Layer

27. 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 (Ethernet), CSMA/CA ( WLAN or Wi-Fi)
5: DataLink Layer

28. Slotted ALOHA Assumptions all frames same size
time is divided into equal size slots, time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot, all nodes detect collision
Operation
when node obtains fresh frame, it transmits in next slot
no collision, node can send new frame in next slot
if collision, node retransmits frame in each subsequent slot with prob. p until success
5: DataLink Layer

29. Slotted ALOHA 1 2 3
node 1
node 2
node 3 C S E
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 – but sill waste a slot
clock
5: DataLink Layer

30. Slotted Aloha efficiency
Efficiency: long-run fraction of successful slots (many nodes, each with many frames to send)
For max efficiency with N nodes, 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: max efficiency 1/e = .37
Suppose a node transmits a frame in a slot with probability p
prob that a node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
At best: channel
used for useful
transmissions 37%
of time!
5: DataLink Layer

31. CSMA (Carrier Sense Multiple Access)
CSMA: listen before transmit:
If channel sensed idle: transmit entire frame
If channel sensed busy, defer transmission
No time slot – fully distributed
Human analogy: listen and don’t interrupt others
Seems perfect – no collision ! (not really though)
5: DataLink Layer

32. “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! – but practically not
5: DataLink Layer

33. “Taking Turns” MAC protocols
Type 1: Polling:
master node “invites” slave nodes to transmit in turn
Ask round-robin: do you have frame to send ? (e.g., USB polling)
concerns:
polling overhead
latency
single point of failure (master)
data
poll
master
data
slaves
5: DataLink Layer

34. “Taking Turns” MAC protocols
Type 2: 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

35. Chapter 5 outline 5.1 Introduction and services
5.2 Error detection and correction
5.3Multiple access protocols
5.4 Ethernet
5.5 Wireless links and LANs
5: DataLink Layer

36. Ethernet “dominant” LAN technology: cheap $20 for 100Mbs!
first widely used LAN technology
Simpler, cheaper than competitors
Kept up with speed race: 10, 100, 1000 Mbps …
Metcalfe’s Ethernet
sketch
5: DataLink Layer

37. (Traditional) Ethernet uses CSMA/CD
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 transmitting, that is, collision detection
Before attempting a retransmission, adapter waits a random time, that is, random access
5: DataLink Layer

38. Manchester encoding 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!
5: DataLink Layer

39. Chapter 5 outline 5.1 Introduction and services
5.2 Error detection and correction
5.3Multiple access protocols
5.4 Ethernet
5.5 Wireless links and LANs
5: DataLink Layer

40. IEEE 802.11 Wireless LAN 802.11b 802.11a 802.11g 802.11n (2009)
2.4-5 GHz unlicensed radio spectrum
up to 11 Mbps
direct sequence spread spectrum (DSSS) in physical layer
all hosts use same chipping code
802.11a
5-6 GHz range
up to 54 Mbps
802.11g
2.4-5 GHz range
802.11n (2009)
2.4/5GHz, MIMO
All use CSMA/CA for multiple access
All have base-station and ad-hoc network versions
5: DataLink Layer

41. Base station approach Wireless host communicates with a base station
base station = access point (AP)
Basic Service Set (BSS) (a.k.a. “cell”) contains:
wireless hosts
access point (AP): base station
5: DataLink Layer

42. Ad Hoc Network approach
No AP (i.e., base station)
wireless hosts communicate with each other
to get packet from wireless host A to B may need to route through wireless hosts X,Y,Z
Applications:
“laptop” meeting in conference room, car
interconnection of “personal” devices
battlefield
IETF MANET (Mobile Ad hoc Networks) working group
5: DataLink Layer

43. IEEE : multiple access
Collision if 2 or more nodes transmit at same time
CSMA makes sense:
get all the bandwidth if you’re the only one transmitting
try to avoid collision if you sense another transmission
5: DataLink Layer

44. Enhancement: CSMA/CD ?
Collision detection doesn’t work: hidden terminal problem
5: DataLink Layer

45. Collision avoidance mechanisms
Problem:
two nodes, hidden from each other, transmit complete frames to base station
wasted bandwidth for long duration !
Solution:
small reservation packets
nodes track reservation interval with internal “network allocation vector” (NAV)
5: DataLink Layer

46. Collision Avoidance: RTS-CTS exchange
sender transmits short RTS (request to send) packet: indicates duration of transmission
receiver replies with short CTS (clear to send) packet
notifying (possibly hidden) nodes
hidden nodes will not transmit for specified duration: NAV
5: DataLink Layer

47. Collision Avoidance: RTS-CTS exchange
RTS and CTS short:
collisions less likely, of shorter duration
end result similar to collision detection
IEEE allows:
CSMA
CSMA/CA: reservations
polling from AP
5: DataLink Layer

48. A word about Bluetooth
Low-power, small radius, wireless networking technology
meters
omnidirectional
not line-of-sight infrared
Interconnects gadgets
GHz unlicensed radio band
up to 721 kbps
Interference from wireless LANs, digital cordless phones, microwave ovens:
frequency hopping helps
MAC protocol supports:
error correction
ARQ
Each node has a 12-bit address
5: DataLink Layer

49. Chapter 5: Summary principles behind data link layer services:
error detection, correction
sharing a broadcast channel: multiple access
link layer addressing, ARP
link layer technologies: Ethernet, IEEE LANs
journey down the protocol stack now OVER!
future stops: multimedia, security, network management
5: DataLink Layer

50. Finally: a day in the life of a web request
journey down protocol stack complete!
application, transport, network, link
putting-it-all-together: synthesis!
goal: identify, review, understand protocols (at all layers) involved in seemingly simple scenario: requesting www page
scenario: student attaches laptop to campus network, requests/receives
Link Layer

51. A day in the life: scenario
browser
DNS server
Comcast network
/13
school network
/24
web page
web server
Google’s network
/19

52. A day in the life… connecting to the Internet
DHCP
UDP IP
Eth
Phy
DHCP
DHCP
connecting laptop needs to get its own IP address, addr of first-hop router, addr of DNS server: use DHCP
router
(runs DHCP)
DHCP
DHCP
DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in Ethernet
DHCP
UDP IP
Eth
Phy
DHCP
Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server
Ethernet demuxed to IP demuxed, UDP demuxed to DHCP

53. A day in the life… connecting to the Internet
DHCP
DHCP
UDP IP
Eth
Phy
DHCP server formulates DHCP ACK containing client’s IP address, IP address of first-hop router for client, name & IP address of DNS server
router
(runs DHCP)
encapsulation at DHCP server, frame forwarded (switch learning) through LAN, demultiplexing at client
DHCP
UDP IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP client receives DHCP ACK reply
Client now has IP address, knows name & addr of DNS
server, IP address of its first-hop router

54. A day in the life… ARP (before DNS, before HTTP)
before sending HTTP request, need IP address of DNS
DNS
UDP IP
Eth
Phy
DNS
router
(runs DHCP)
ARP
ARP query
DNS query created, encapsulated in UDP, encapsulated in IP, encapsulated in Eth. To send frame to router, need MAC address of router interface: ARP
Eth
Phy
ARP
ARP reply
ARP query broadcast, received by router, which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router, so can now send frame containing DNS query

55. A day in the life… using DNS
UDP IP
Eth
Phy
DNS
DNS server
DNS
UDP IP
Eth
Phy
DNS
router
(runs DHCP)
DNS
DNS
DNS
DNS
Comcast network
/13
IP datagram forwarded from campus network into comcast network, routed (tables created by RIP, OSPF, IS-IS and/or BGP routing protocols) to DNS server
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
demux’ed to DNS server
DNS server replies to client with IP address of

56. A day in the life…TCP connection carrying HTTPIP
Eth
Phy
router
(runs DHCP)
SYN
SYNACK
SYN
to send HTTP request, client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP IP
Eth
Phy
SYNACK
SYN
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
web server
TCP connection established!

57. A day in the life… HTTP request/reply
web page finally (!!!) displayed
HTTP
HTTP
TCP IP
Eth
Phy
router
(runs DHCP)
HTTP
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to
HTTP
TCP IP
Eth
Phy
HTTP
HTTP
web server responds with HTTP reply (containing web page)
web server
IP datagram containing HTTP reply routed back to client

58. Data Center Networks (some hot topics in recent years; FYI only)
10’s to 100’s of thousands of hosts, often closely coupled, in close proximity:
e-business (e.g. Amazon)
content-servers (e.g., YouTube, Akamai, Apple, Microsoft)
search engines, data mining (e.g., Google)
challenges:
multiple applications, each serving massive numbers of clients
managing/balancing load, avoiding processing, networking, data bottlenecks
Inside a 40-ft Microsoft container,
Chicago data center

59. Data Center Networks load balancer: application-layer routing
receives external client requests
directs workload within data center
returns results to external client (hiding data center internals from client)
Internet
Border router
Load
balancer
Load
balancer
Access router
Tier-1 switches B A C
Tier-2 switches
TOR switches
Server racks 1 2 3 4 5 6 7 8
5: DataLink Layer

60. Data Center Networks rich interconnection among switches, racks:
Server racks
TOR switches
Tier-1 switches
Tier-2 switches 1 2 3 4 5 6 7 8
rich interconnection among switches, racks:
increased throughput between racks (multiple routing paths possible)
increased reliability via redundancy
5: DataLink Layer