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

2. Mobile Networks

3. Mobile Technology Statistics

4. Migration to 3G

5. History Frequency reuse allows more users
Handover as caller moves from cell to another

6. Frequency Allocation Examples

7. Medium Access in Mobile Networks
Ex: 2G

8. Multi-Access Radio Technology

9. Architecture of Mobile Networks
Radio Access Network is responsible for managing radio resources
Directly controls the air interface
Manages handovers
Core network responsible for intra-network and inter-network routing
Manages user accounts and locations

10. 3G Architecture

11. HSDPA/HSUPA

12. 4G Technology Specifications
HSPA+ is still dominant because it achieves high rates without the cost of the parallel infrastructure
LTE is rapidly penetrating the market

13. Architecture Comparison
PSTN
PDNs
PDNs
Core Network
EPC
CS domain
PS domain
GERAN
UTRAN
E-UTRAN

14. Trusted non 3GPP IP Access
LTE Architecture
GERAN
SGSN
GPRS Core
UTRAN
HSS
eNode B S3
S6a Uu
S1-C UE Uu
S1-C
MME
PCRF X2
S11 Uu
S1-U Gx UE S5
PDN-GW Uu
S1-U
eNode B
S-GW
SGi
Trusted non 3GPP IP Access

15. MIMO Support Diversity for more reliable communications
Multiplexing for higher data rates
SDMA for higher cell capacity
Beamforming for improved coverage

16. User Device Categories
HSPA UE categories
The user device is the second half of the picture
UEs have different CPUs, storage capacity, memory, and radio capabilities
The speed depends on the UE
Operating 8x MIMO is similar to operating 8 radios  very high energy consumption
LTE UE categories

17. Radio Resource Controller
Controls all operations between UE and radio tower
Is in full control of the operations of the UE’s radio (even determines its transmit power)
RRC assigns dedicated resources to each UE, depending on its needs
Contention only happens when requesting access
Has a profound impact on latency, throughput, and battery life

18. Energy Efficiency: WiFi vs. RRC
Device chooses the transmit power
Somewhere between mW, depending on distance to AP
Radio is active by default. Can go to sleep when idle
Delivery traffic indication message (DTIM) sent in the beacon to notify the device that it should receive
Saves energy at the expense of latency
RRC chooses the transmit power
Somewhere between mW when transmitting!!
UE can be instructed to go to sleep state by the RRC
Paging messages can alter the state of the UE to high power
More efficient when idle, much less efficient when transmitting

19. LTE RRC State Machine
Idle: low power state (<15 mW). Listening only to control messages
No network context, no assigned resources. Device cannot send or receive data
Needs to synchronize with network, negotiate resources, and switch to active to start sending or receiving.
Needs several roundtrips to synchronize (up to 100ms in LTE)
Active: high power state ( mW). Established network context, allocated resources

20. LTE RRC State Machine
Short discontinuous reception (DRX): established context but no allocated resources
UE only listens to periodic broadcasts and sleeps in between.
However, since context is maintained, transition to active is fast (up to 50ms in LTE)
Long discontinuous reception (DRX): same as above, but the UE sleeps for longer intervals

21. LTE RRC State Machine One-way latency is 5ms in LTE
Transitions from active to short to long DRX is through timers

22. HSPA(+) RRC State Machine
Idle and DCH in RRC are almost identical to idle and active in LTE
FACH is designed for delay-tolerant, non interactive applications
Low rate data transfer can take place in this state
Transition from DCH to FACH to idle is through timers
Transition from FACH back to DCH is based on the amount of data in the buffer
LTE is more power hungry, especially with many MIMO streams

23. The Problem of Periodic Transfers
Consider the case of HSPA+, where the transition from DCH to FACH is after 10 sec of inactivity
Now imagine an application that schedules a transfer every 11sec
High energy waste, the radio remains in high energy state for most of the time, without transferring any data
Best to burst as much data as you can, then go to sleep for as long as possible

24. Latency Due to Handovers
Each tower covers a certain range. UEs have to be “handed over” to a new tower if they move
This process takes up to hundreds of ms and may disrupt the application
LTE does it in an energy-efficient way
Every group of eNodeB’s form a logical tracking area
If the UE is in idle mode and does a handover, there is no need to wake it up
The eNodeB’s will manage the handover without informing the core
Once the UE becomes active, the core is informed of the new location
Problem: the core knows the tracking area of every UE all the time, but the actual tower only some of the time  latency for the sake of energy efficiency

25. Connectivity Issues
Within a mobile network, the PGW assigns IP addresses to UEs
Since IP addresses are scarce, the PGW may assign the same IP address to many devices (same as NAT)
The differentiation is done using unique IDs within the network and port numbers
PGW is also the connection termination point in the network
When radio resources are torn down, this does not affect the TCP/UDP state
Thus a TCP connection will remain open when the UE goes to idle state

26. Inbound vs. Outbound Packet Flow
Initial latency maybe high if transition from idle to active
Core latency is not guaranteed
MME will be contacted to determine the tracking area
All towers may send pages to find the UE

27. Where is the Bottleneck
Only the air interface is guaranteed (state transition times and one-way latencies)
Delay in the core is up to the carrier
Every tower needs a fiber optic link to support the large number of users and high rates (10+ Mbps per user for a large number of users)
Massive infrastructure cost

28. Optimizing Performance over Mobile Networks

29. Battery Power Radio is second highest consumer of energy
Using radio at full power can drain battery life completely in a few hours
Radio requirements keep increasing with newer mobile generations
Energy consumption has a nonlinear relationship with data transfer
Transmit more when the radio as active
Extend idle times

30. Periodic Data Transfer
The energy cost of a packet cannot be measured only by its size
Polling, real-time analytics, measurement pings are expensive when done periodically  avoid at all cost
Aggregate transfers, preferably to the time when the radio is already active
Google cloud messaging (GCM) offers an API for servers that detects when a mobile is active
Use push delivery whenever possible. However, periodic pushes can be just as harmful
Ask yourself, is this transmission critical? Most of them aren’t!! Even real-time analytics often can be replaced with periodic logs that are transferred when radio is active

31. Keepalive Connections
TCP and NAT connections have an independent termination lifetime from the radio
The radio timeout (transition to idle) is typically much shorter (NAT timeouts can be anywhere between 5 – 30 mins)
Thus, keepalive pings need to take much longer periods to avoid waking up the radio needlessly

32. End-to-End Delay
Many roundtrips are typically required until you get a response for your HTTP request
Best case scenario is when the radio is already active, DNS is already resolved, and previous TCP connection already exists and can be reused
Otherwise, many additional roundtrips are required

33. End-to-End Delay
Example: assume RTT = 100ms for LTE and RTT = 200ms for 3G
You may need extra roundtrips to:
Renegotiate context with control plane
TCP handshake
TLS handshake
HTTP exchange

34. The Illusion of Instant Feedback
Decouple UI feedback from network communications
When a user submits a request that requires network communications, provide instant feedback from the UI and initiate the request in the background
Latency cannot be controlled
Smart UI design can make the application “seem” as if the network is fast

35. Adapt to Variable Availability of Resources
Never assume that the network interface will be always available, even if a high quality initial connection is made
Monitor network status continuously
Use a set of set of strategies to recover from failures that include
Retry attempts
Backoff strategy (do not loop forever)
Caching

36. Data Pre-Fetching
Whenever possible, download data that may be needed in the future
Ex: download entire music file for streaming ahead of time. Or download ads ahead of time to be displayed when needed
This policy collides with another one: limiting data transfer on mobile connections
Balance three constraints:
Number of transferred bytes
Impact on battery
Variable air link quality
Decide what are the primary goals of the application and plan accordingly