1. Selected Topics in Modern Networks
Amr El Mougy
C7.207

2. Course Objectives Strengthen your foundation of networking
Build your knowledge of the latest advances in networks
Understand and apply your knowledge of how networking affects software development decisions
Empower your research and practical skills

3. Course Outline Wireless networks - WiFi 2. Internet Technologies
- Mobile networks
2. Internet Technologies
- Content-delivery networks (CDN)
- Autonomous systems, BGP
- Internet exchange points (IXP)
- TOR (onion routing)
- Software-Defined Networks (SDN)
3. Networking for developers
- Browser optimization
- Connection optimization
- Media delivery

4. Grading Policy

5. Important Note
These slides are not meant to be comprehensive lecture notes! They are only remarks and pointers. The material presented here is not sufficient for studying for the course
Your main sources for studying are:
• Research papers
• your own lecture notes

6. Wireless Networks

7. Why Wireless Networks? Easy to install Mobility and collaboration
Better access
Easier network expansion (scalability)
Easy guest access
Low installation cost (CAPEX)
Low maintenance and operation cost (OPEX)

8. Disadvantages of Wireless Networks
Unreliable channels (path loss, multipath fading, Doppler effect)
Vulnerable to interference
Limited range
Lower data rates
Security

9. Classification of Wireless Networks
Infrastructure-based
Infrastructure-less
MAN
WLAN
Single-hop
Multiple-hop
Cellular
(2G,3G,4G,5G)
Wi-Fi
Direct Wi-Fi
Bluetooth
MANET
VANET
WSN
WMN

10. Infrastructure-based Networks

11. The Wi-Fi Family of Standards

12. Wi-Fi
Wi-Fi is the brand name for the family of IEEE Standards known as
Standard
Data Rate
Spectrum
Range
Legacy (1997)
2 Mbps
2.4 GHz
30 ~ 35 m
802.11b (1999)
11 Mbps
802.11a (1999)
54 Mbps
5 GHz
802.11g (2003)
802.11n (2009)
Up to 600 Mbps
2.4 GHz, 5 GHz
60 ~ 70 m
802.11ac (2013)
Up to 7 Gbps
35 m

13. Distribution System (DS)
Wi-Fi Architecture
Two Modes of Operation:
Ad Hoc Mode
Infrastructure Mode
Distribution System (DS)
BSS
BSS1
BSS2
BSS3

14. Wi-Fi Layers Physical Layer:
Transmission and reception of radio signals
MAC Layer:
Interacts with the wired backbone
Addressing
Radio resource management
Mobility management
MAC sends three types of frames: control, data, and management
Application
Transport
Network
Data Link
MAC
802.11
Physical

15. Addressing
Each MAC frame has up to 4 (usually 3) address fields. Their types are:
BSS Identifier (BSSID): Identifies the AP. In the absence of an AP, this is a random number administered locally
Destination Address (DA): the final recipient of the frame
Source Address (SA): Initial source of the frame
Receiver Address (RA): the immediate recipient AP on a wireless DS
Transmitter Address (TA): the AP that transmitted the frame on the wireless DS
To DS
From DS
Addr 1
Addr 2
Addr 3
Addr 4 DA SA
BSSID
N/A 1 RA TA
Recipient’s
Addr
Transmitter’s Addr
Original source or intended destination’s Addr
Final source Addr of a frame transmitted on a wireless DS

16. Medium Access Control (MAC)
Can be classified in 3 main categories:
Channel partitioning
Random Access
Taking-turns

17. Channel Partitioning Divides resources evenly Eliminates collisions
Not very flexible
May waste resources if they are not needed by the device

18. Random Access Each node transmits at the full rate, R, of the channel
Collisions cannot be avoided
Upon collision:
Wait for random period of time
Retransmit frame

19. ALOHA and Slotted ALOHA
Nodes may transmit whenever they want
Upon detecting a collision, either retransmit immediately or wait for one frame time
Slotted ALOHA:
All nodes are synchronized
Nodes can only transmit at beginning of a slot
Upon a collision, the node will retransmit in the next slot with probability p

20. Carrier Sense Multiple Access (CSMA)
Nodes employ carrier sensing before transmission
Collisions occur if two nodes start transmitting at the same time
Upon collision detection, nodes stop transmitting the rest of the packet
Collisions are caused by channel propagation delay

21. Carrier Sense Multiple Access (CSMA)

22. Virtual Carrier Sensing
Challenges imposed by the nature of wireless transmissions
The Hidden Node Problem
Frame
C’s range
Ack C A B
A’s range
The Exposed Node Problem
C’s range
RTS
CTS D A B C
A’s range
Frame
Ack

23. NAV – neighbor of sender NAV – neighbor of receiver
Network Allocation Vector and IFS
Sender
RTS
Frame
SIFS
SIFS
SIFS
Receiver
CTS
Ack
DIFS
NAV – neighbor of sender
NAV
NAV – neighbor of receiver
Access to medium deferred
DIFS
PIFS
SIFS
Busy ….
frame
Contention Window

24. Distributed Coordination Function (DCF)
To transmit a frame:
1- Perform carrier sensing
2- If the medium is busy, defer transmission for a random period of time
3- If the medium is idle for at least DIFS seconds, begin transmitting
4- Once the medium is captured, subsequent frames can be transmitted after waiting for only SIFS seconds
Backoff slots are used to resolve contention between stations
DIFS
Contention Window = 31 slots
Busy ….
DIFS
Contention Window = 63 slots
Busy ….
DIFS
Contention Window = 127 slots
Busy ….
DIFS
Contention Window = 255 slots
Busy ….
DIFS
Contention Window = 511 slots
Busy ….
DIFS
Contention Window = 1023 slots
Busy ….

25. Fragmentation To transmit a frame:
1- Long frames can be broken down in fragments
2- Fragments are prioritized by having to wait only SIFS
Sender
RTS
Fragment 0
Fragment 1
SIFS
SIFS
SIFS
SIFS
SIFS
Receiver
CTS
Ack 0
Ack 1
NAV – RTS
NAV – Fragment 0
NAV
NAV – CTS
NAV – Ack 0

26. CF-Poll (to Stn 2) +Ack to 1
Point Coordination Function (PCF)
Only implemented in networks with an AP
The AP is in full control
Contention-free repetition interval
Contention-free period
Contention period
SIFS
PIFS
CF-Poll
(to Stn 1)
CF-Poll (to Stn 2) +Ack to 1
Data to Stn 4
+ CF-Poll
CF-End AP
Beacon
SIFS
SIFS
SIFS
SIFS
Frame from 1
+ CF-Ack
CF-Ack
Others
Released by CF-end
NAV
Set by Beacon
CFP maximum duration

27. QoS Support with IEEE e
Introduction of Transmission Opportunities (TXOP)
Every station maintains several Traffic Categories (TCs)
Every station waits for Arbitrary Interframe Spacing (AIFS)
AIFS can have a minimum value of DIFS and can be enlarged for individual TCs
AIFS[TC] ….
Low priority
AIFS[TC] ….
Medium priority
AIFS[TC] = DIFS
High priority
Ack ….
RTS

28. Hybrid Coordination Function

29. MAC Improvements in 802.11n Frame Aggregation
802.11n allows stations to aggregate frames in order to transmit larger amounts of data
Frames belonging to the same service category and heading to the same receiver can be aggregated
Aggregation occurs over two steps: at the top of the MAC and at the bottom of the MAC
At the top of the MAC several packet data units (PDU) are aggregated into MAC service data units (MSDU)
At the bottom of the MAC several MSDUs are aggregated into one MAC packet data unit (MPDU)
MSDU aggregation
** article in
MPDU aggregation

30. Virtual Carrier Sensing
MAC Improvements in n
Virtual Carrier Sensing
802.11n introduces block acknowledgment requests and block acknowledgments, where several frames can be acknowledged at once
A new type of spacing is also introduced, called Reduced Interframe Spacing (RIFS), to reduce wait time between aggregated frames in one TXOP
** article in

31. The New Kid on the Block: 802.11ac
6 times as fast as n
Speed comparable to Gigabit Ethernet
First wave of products is already out. Second wave expected soon
Backwards compatible with n
Major modifications to the PHY layer. Some modifications to the MAC as well
**Cisco, “802.11ac: the fifth generation of WiFi”, Technical white paper

32. Modifications at the PHY Layer
More bandwidth
Higher modulation rates (more efficient use of the bandwidth)
Higher MIMO streams
20, 40, 80,
or 160 MHz BW
Up to 8 spatial streams
More bits per subcarrier
** Aruba Networks, “802.11ac in-depth”, technical white paper

33. Modifications at the PHY Layer
Introduction of multi-user MIMO (MU-MIMO) in Phase 2
AP can send multiple streams to multiple users in the same transmission
** Aruba Networks, “802.11ac in-depth”, technical white paper

34. Modifications at the MAC Layer
Frame aggregation (A-MPDU) is now mandatory for every frame
The new version needs to coexist with n, while having more bandwidth options up to 160MHz
Thus, hidden and exposed terminal problems are now possible at multiple frequency channels
Thus, RTS/CTS is now extended to multiple channels
**Cisco, “802.11ac: the fifth generation of WiFi”, Technical white paper