Real-time Conversational Applications
Advanced Network Technologies
4G LTE
School of Computer Science
Dr. | Lecturer
1
4G/5G cellular networks
the solution for wide-area mobile Internet
widespread deployment/use:
more mobile-broadband-connected devices than fixed-broadband-connected devices (2019)!
4G availability: 97% of time in Korea, 90% in US
transmission rates up to 100’s Mbps
technical standards: 3rd Generation Partnership Project (3GPP)
wwww.3gpp.org
4G: Long-Term Evolution (LTE) standard
2
4G/5G cellular networks
similarities to wired Internet
edge/core distinction, but both below to same carrier
global cellular network: a network of networks
widespread use of protocols we’ve studied: HTTP, DNS, TCP, UDP, IP, etc.
separation of data/control planes, SDN, tunneling
interconnected to wired Internet
differences from wired Internet
different wireless link layer
mobility
user “identity” (via SIM card)
subscriber identification module
business model: users subscribe to a cellular provider
“home network” versus roaming on visited nets
global access, with authentication infrastructure, and inter-carrier settlement
3
Mobile device
(UE)
Base station
(eNode-B)
Elements of 4G LTE architecture
radio access
network
all-IP Enhanced Packet Core (EPC)
Mobility Management Entity (MME)
Serving Gateway (S-GW)
Home Subscriber Service (HSS)
PDN gateway (P-GW)
to
Internet
…
Mobile device:
smartphone, tablet, laptop, IoT, … with 4G LTE radio
64-bit International Mobile Subscriber Identity (IMSI), stored on SIM (Subscriber Identity Module) card
LTE jargon: User Equipment (UE)
PDN: Packet Data Network
4
Elements of 4G LTE architecture
Mobility Management Entity (MME)
Serving Gateway (S-GW)
Home Subscriber Service (HSS)
PDN gateway (P-GW)
…
to
Internet
Mobile device
(UE)
Base station
(eNode-B)
Base station:
at “edge” of carrier’s network
manages wireless radio resources, mobile devices in its coverage area (“cell”)
coordinates device authentication with other elements
similar to WiFi AP but:
active role in user mobility
coordinates with nearly base stations to optimize radio use
LTE jargon: eNode-B
5
Elements of 4G LTE architecture
Mobility Management Entity (MME)
Serving Gateway (S-GW)
Home Subscriber Service (HSS)
PDN gateway (P-GW)
…
to
Internet
Mobile device
(UE)
Base station
(eNode-B)
Home Subscriber Service
stores info about mobile devices for which the HSS’s network is their “home network”
works with MME in device authentication
6
Elements of 4G LTE architecture
Mobility Management Entity (MME)
Serving Gateway (S-GW)
Home Subscriber Service (HSS)
PDN gateway (P-GW)
…
to
Internet
Mobile device
(UE)
Base station
(eNode-B)
Serving Gateway (S-GW), PDN Gateway (P-GW)
lie on data path from mobile to/from Internet
P-GW
gateway to mobile cellular network
Looks like other internet gateway router
provides NAT services
other routers:
extensive use of tunneling
7
Elements of 4G LTE architecture
Mobility Management Entity (MME)
Serving Gateway (S-GW)
Home Subscriber Service (HSS)
PDN gateway (P-GW)
…
to
Internet
Mobile device
(UE)
Base station
(eNode-B)
Mobility Management Entity
device authentication (device-to-network, network-to-device) coordinated with mobile home network HSS
mobile device management:
device handover between cells
tracking/paging device location
path (tunneling) setup from mobile device to P-GW
8
LTE: data plane control plane separation
control plane
new protocols for mobility management , security, authentication
MME
HSS
base station
P-GW
S-GW
base station
P-GW
S-GW
data plane
new protocols at link, physical layers
IP tunnels
extensive use of tunneling to facilitate mobility
9
IP
Packet Data
Convergence
Radio Link
Medium Access
Physical
LTE data plane protocol stack: first hop
data
plane
P-GW
S-GW
Application
Transport
IP
Packet Data
Convergence
Radio Link
Medium Access
Physical
Link
base station
LTE link layer protocols:
Packet Data Convergence: header compression, encryption
Radio Link Control (RLC) Protocol: fragmentation/reassembly, reliable data transfer
Medium Access: requesting, use of radio transmission slots
10
IP
Packet Data
Convergence
Radio Link
Medium Access
Physical
LTE data plane protocol stack: first hop
Application
Transport
IP
Packet Data
Convergence
Radio Link
Medium Access
Physical
Link
base station
LTE radio access network:
downstream channel: FDM, TDM within frequency channel (OFDM – orthogonal frequency division multiplexing)
“orthogonal”: minimal interference between channels
upstream: FDM, TDM similar to OFDM
each active mobile device allocated two or more 0.5 ms time slots over 12 frequencies
scheduling algorithm not standardized – up to operator
100’s Mbps per device possible
11
IP
Packet Data
Convergence
Radio Link
Medium Access
Physical
GTP-U
UDP
IP
link
Physical
LTE data plane protocol stack: packet core
GTP-U
UDP
IP
link
Physical
GTP-U
UDP
IP
link
Physical
P-GW
S-GW
base station
\
tunneling:
mobile datagram encapsulated using GPRS Tunneling Protocol (GTP), sent inside UDP datagram to S-GW
S-GW re-tunnels datagrams to P-GW
supporting mobility: only tunneling endpoints change when mobile user moves
GTP-U: user data
GTP-C: control
12
LTE data plane: associating with a BS
data
plane
P-GW
S-GW
base station
BS broadcasts primary synch signal every 5 ms on all frequencies
BSs from multiple carriers may be broadcasting synch signals
1
mobile finds a primary synch signal, then locates 2nd synch signal on this freq.
mobile then finds info broadcast by BS: channel bandwidth, configurations; BS’s cellular carrier info
mobile may get info from multiple base stations, multiple cellular networks
2
mobile selects which BS to associate with (e.g., preference for home carrier)
3
more steps still needed to authenticate, establish state, set up data plane
4
1
2
3
13
LTE mobiles: sleep modes
data
plane
ZZZZ…
as in WiFi, Bluetooth: LTE mobile may put radio to “sleep” to conserve battery:
light sleep: after 100’s msec of inactivity
wake up periodically (100’s msec) to check for downstream transmissions
deep sleep: after 5-10 secs of inactivity
mobile may change cells while deep sleeping – need to re-establish association
14
Global cellular network: a network of IP networks
…
visited mobile
carrier network
…
public Internet
and
inter-carrier IPX
…
P-GW
P-GW
home mobile
carrier network
Home
Subscriber
Server
in home network
roaming in visited network
SIM card: global identify info in home network
all IP:
carriers interconnect with each other, and public internet at exchange points
legacy 2G, 3G: not all IP, handled otherwise
home network HSS:
identify & services info, while in home network and roaming
15
Mobility manager
Mobility in 4G networks: major mobility tasks
Visited network
P-GW
Streaming server
Home
network
Internet
base station
MME
1
2
3
4
Home
Subscriber
Server
S-GW
P-GW
base station association:
covered earlier
mobile provides IMSI – identifying itself, home network
1
control-plane configuration:
MME, home HSS establish control-plane state – mobile is in visited network
2
data-plane configuration:
MME configures forwarding tunnels for mobile
visited, home network establish tunnels from home P-GW to mobile
3
mobile handover:
mobile device changes its point of attachment to visited network
4
16
Mobility manager
Mobile communicates with local MME via BS control-plane channel
MME uses mobile’s IMSI info to contact mobile’s home HSS
retrieve authentication, encryption, network service information
home HHS knows mobile now resident in visited network
BS, mobile select parameters for BS-mobile data-plane radio channel
Configuring LTE control-plane elements
Visited network
P-GW
Home
network
base station
MME
2
Home
Subscriber
Server
S-GW
P-GW
17
Mobility manager
Configuring data-plane tunnels for mobile
P-GW
Streaming server
Internet
base station
MME
Home
Subscriber
Server
S-GW
P-GW
Visited network
Home
network
S-GW to BS tunnel: when mobile changes base stations, simply change endpoint IP address of tunnel
S-GW to home P-GW tunnel: implementation of indirect routing
tunneling via GTP (GPRS tunneling protocol): mobile’s datagram to streaming server encapsulated using GTP inside UDP, inside datagram
18
Handover between BSs in same cellular network
P-GW
S-GW
MME
source BS
1
3
2
data path before handover
data path after
handover
1
current (source) BS selects target BS, sends Handover Request message to target BS
2
target BS pre-allocates radio time slots, responds with HR ACK with info for mobile
3
source BS informs mobile of new BS
mobile can now send via new BS – handover looks complete to mobile
4
source BS stops sending datagrams to mobile, instead forwards to new BS (who forwards to mobile over radio channel)
target BS
4
19
Handover between BSs in same cellular network
1
6
3
5
5
2
5
target BS informs MME that it is new BS for mobile
MME instructs S-GW to change tunnel endpoint to be (new) target BS
6
target BS ACKs back to source BS: handover complete, source BS can release resources
P-GW
S-GW
MME
source BS
target BS
4
7
7
mobile’s datagrams now flow through new tunnel from target BS to S-GW
20
On to 5G!
goal: 10x increase in peak bitrate, 10x decrease in latency, 100x increase in traffic capacity over 4G
5G NR (new radio):
two frequency bands: FR1 (450 MHz–6 GHz) and FR2 (24 GHz–52 GHz): millimeter wave frequencies
not backwards-compatible with 4G
MIMO: multiple directional antennae
millimeter wave frequencies: much higher data rates, but over shorter distances
pico-cells: cells diameters: 10-100 m
massive, dense deployment of new base stations required
21
Advanced Network Technologies
Mobile Network Analysis, Erlang B, Erlang C
School of Computer Science
Dr. | Lecturer
22
λ
λ
λ
λ
λ
2μ
kμ
(S −1)μ Sμ
3μ
λ λ
λ λ
… …
M/M/1
M/M/m/m
…
…
μ
μ
μ
μ
μ
(m=S)
Review of M/M/1 and M/M/m/m Queue
23
M/M/m/m Queue Model
The cell can serve S users (S channels)
User arrival follows a Poisson process (arrival rate λ)
User will use the channel for t, where t follows exponential distribution (mean 1/μ)
t is called “channel holding time”
New arrivals will be dropped if all channels are occupied.
λ
λ
λ
λ
λ
2μ
kμ
(S −1)μ Sμ
3μ
M/M/m/m
…
…
μ
(m=S)
M/M/m/m Queue Model
Q: What is the probability if a new arrival is dropped?
PS
Poisson arrival sees time average.
25
∑
S n=0
(λ / μ)n / n!
(λ / μ)S / S!
Erlang B Formula
Erlang B formula
PS =
Erlang B formula, or Erlang loss formula, the formula for the blocking probability
PS is also called grade of service (GOS).
Erlang B formula for system design
Q1 (Performance Evaluation): Given traffic load (λ / μ), number of channels (S), calculate blocking probability PS
A: Directly apply Erlang B formula
Q2 (Traffic Shaping) : Given blocking probability, number of channels, calculate max traffic load
A: Solve the value of λ / μ by Erlang B formula; using Erlang table/chart.
Q3 (Channel Reservation) : Given blocking probability, traffic load, calculate the number of channels needed
A: Solve the value of S by Erlang B formula; using Erlang table/chart.
Erlang: a dimensionless unit used as a measure of offered load
The average number of concurrent calls measured over a time unit.
https://www.callcentrehelper.com/what-are-erlangs-154459.htm
Erlang
28
Erlang: In our model
λ: average number of arrivals per unit time.
1/μ: average channel holding time.
Traffic load is λ/μ Erlang.
Erlang
29
Traffice Intensity in Erlangs
Number of Channels
Erlang B chart
log-log chart
Assume there are 10 channels.
Assume each user uses the channel for 6 minutes (0.1 hour).
What is the arrival rate can be supported for 0.5% blocking probability?
Example A
Traffice Intensity in Erlangs
Number of Channels
log-log chart
λ/μ=4
Example A
λ/μ=4
and 1/μ = 0.1 hour
λ=40 units/hour
Example A
Consider a base station
An average call lasts two minutes
1500 calls per hour on average
the probability of blocking is to be no more than 1%.
How many channels do we need?
λ=1500 units/hour
1/μ = 1/30 hour
λ /μ = 50 Erlang
Example B
Traffice Intensity in Erlangs
Number of Channels
Example B
Between 60 channels and 70 channels;
60 is not enough; 70 is the number of channels needed.
(More accurate calculation is 64)
Queue Model with Handover
The cell can serve S users (S channels)
Intra-cell new arrival follows a Poisson process (arrival rate λn)
Inter-cell handover arrival follows a Poisson process (arrival rate λh)
User will use the channel for t, where t follows exponential distribution (mean 1/μ)
S channels.
How to block/drop calls?
Block intra-cell new arrival. Penalty
Drop Inter-cell handover arrival. Higher penalty
Guard channel approach!
S-L guarded channels
[0,L-1] active users, new arrivals and handover arrivals are accepted.
[L, S-1] active users, new arrivals are blocked, handover arrivals are accepted.
S or more active users. new arrivals are blocked, handover arrivals are dropped.
Guard channel approach
λn + λh
2μ
Lμ
(S −1)μ Sμ
3μ
λn + λh
…λn + λh
…λh
λh
S
μ
All arrivals are accepted
Only handover arrivals are accepted
None is accepted
New arrival blocking probability Pb=PL +PL+1 +…+PS
Handover dropping probability Pd= PS
∑
L n=0
((λn + λh ) / μ)i / i!
Pi =
((λn + λh ) / μ)n / n!
+∑
((λn + λh ) / μ)L (λh / μ)n-L / n!
S n=L+1
0≤ i ≤ L
Calculate the L< i ≤ S case by yourself
38
System design problem
Q1 (Performance Evaluation): Given traffic load (λn , λh , and μ), number of channels (L and S), calculate probabilities (Pb and Pd )
Q2 (Traffic Shaping) : Given required probabilities (Pb and Pd ) number of channels (L and S), calculate the traffic load
Q3 (Channel Reservation) : Given required probabilities (Pb and Pd ) , traffic load, calculate the number of channels needed
S is also given, calculate the optimal L
40
Erlang C
An arriving unit will need to wait if all servers are busy (it is not blocked)
Examples: Call centers
M/M/m queue
λ
λ
λ
λ
λ
2μ
kμ
(S −1)μ Sμ
3μ
M/M/m
…
…
μ
(m=S)
S+1
S+2
…
Sμ
Sμ
λ
λ
λ
Sμ
41
Erlang C
Pw: The probability that a new arrival has to wait (cannot be served immediately).
λ
λ
λ
λ
λ
2μ
kμ
(S −1)μ Sμ
3μ
M/M/m
…
…
μ
(m=S)
S+1
S+2
…
Sμ
Sμ
λ
λ
λ
Sμ
Pw =
∑
∞ n=S
Pn
42
Erlang C
Pw: The probability that a new arrival has to wait (cannot be served immediately). Erlang C formula
Pw =
∑
∞ n=S
Pn
Pw =
∑
S-1 n=0
(λ / μ)n
(λ / μ)S
S!
S
S- λ / μ
n!
(λ / μ)S
S!
S
S- λ / μ
+
Pw is grade of service (GOS).
43
Erlang C Chart
Advanced Network Technologies
Mobile Network Analysis, Bit Error Detection and Correction
School of Computer Science
Dr. | Lecturer
45
Bit Error Detection and Recovery
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
f(D)=EDC
f(D’)=EDC’?
46
Modulo 2 operation
Binary, modulo 2 domain
addition, subtraction
1+1=0, 1-1=0,
1+0=1, 1-0=1
0+1=1, 0-1=1
0+0=0, 0-0=0
“-”, “+”, are equivalent to, XOR,
11+11=00: no carry over
Multiplication
11*100=11*22=1100 (left shift 2 bits)
11*11=11*10+11*1=110+11=101
47
Parity Checking
Single Bit Parity:
Detect single bit errors
110100
1
d data bits
parity bit
odd number of ‘1’s -> 1
even number of ‘1’s -> 0
total number of ‘1’s -> even
48
Bit Check in Matrix format
Parity Checking
dataword
generator matrix
codeword
identity submatrix
parity submatrix
49
Bit Check in Matrix format
Linear Block Code: Generalized Parity Check
dataword
generator matrix
codeword
identity submatrix
parity submatrix
dG=c
1*k vector
k*n matrix
1*n vector
50
Linear Block Code: Generalized Parity Check
k data bits
n-k parity bits
code rate: k/n
51
Linear Block Code: Decoding
paritycheck matrix
Transpose of parity matrix
Identity submatrix
H
GHT=0
52
Linear Block Code: Decoding
cHT=dGHT =0
Not 0? Error detected.
53
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