Week 10 4G and mobile network analysis
Advanced Network Technologies
4G LTE
School of Computer Science
Dr. Wei Bao | Lecturer
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
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
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
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
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
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
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
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-GWS-GW
data plane
§ new protocols at link,
physical layers
IP tunnels
§ extensive use of tunneling to
facilitate mobility
IP
Packet Data
Convergence
Radio Link
Medium Access
Physical
LTE data plane protocol stack: first hop
data
plane
P-GWS-GW
Application
Transport
IP
Packet Data
Convergence
Radio Link
Medium Access
Physical
Li
nk
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
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
Li
nk
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
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-GWS-GWbase 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
movesGTP-U: user data
GTP-C: control
LTE data plane: associating with a BS
data
plane
P-GWS-GWbase 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 plane4
1
2
3
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
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
Mobility
manager
Mobility in 4G networks: major mobility tasks
Visited
network
P-
GW
Streamin
g server
Home
network
Internet
base station
MME
12
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
Mobility
manage
r
› 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
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
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
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
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
Advanced Network Technologies
Mobile Network Analysis, Erlang B, Erlang C
School of Computer Science
Dr. Wei Bao | Lecturer
λ λ λ λ λ
2μ kμ (S －1)μ Sμ3μ
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
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 Sμ3μ
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.
∑Sn=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
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
Traffice Intensity in Erlangs
Number ofChannels
Erlang B chart
log-log chart
� Assumethereare10channels.
� Assume each user uses the channel for 6 minutes (0.1
hour).
� What is thearrival ratecan 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 basestation
� An average call lasts twominutes
� 1500 calls per hour on average
� the probability of blocking is to be nomore than 1%.
� How many channels do we need?
λ=1500units/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 Sμ3μ
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
∑Ln=0
((λn +λh ) /μ)i / i!
Pi =
((λn +λh ) /μ)n / n!+∑ ((λn +λh ) /μ)L (λh /μ)n-L / n!Sn=L+1 0≤i≤ L
Calculate theL
even number of ‘1’s -> 0
total number of ‘1’s -> even
𝑐! 𝑐” 𝑐#
1 0 0 1
0 1 0 1
0 0 1 1
Bit Check in Matrix format
Parity Checking
dataword generator matrix
= 𝑐! 𝑐” 𝑐” 𝑐! + 𝑐” + 𝑐#
codeword
identity submatrix parity submatrix
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
𝑐! 𝑐” 𝑐#
1 0 0 1 1 1 0
0 1 0 0 1 1 1
0 0 1 1 1 0 1
= 𝑐! 𝑐” 𝑐” 𝑐! + 𝑐# 𝑐! + 𝑐” + 𝑐# 𝑐! + 𝑐” 𝑐” + 𝑐#
Linear Block Code: Generalized Parity Check
› k data bits
› n-k parity bits
› code rate: k/n
Linear Block Code: Decoding
1 0 1 1 0 0 0
1 1 1 0 1 0 0
1 1 0 0 0 1 0
0 1 1 0 0 0 1
paritycheck matrix
1 0 0 1 1 1 0
0 1 0 0 1 1 1
0 0 1 1 1 0 1
Transpose of parity matrix Identity submatrix
H
GHT=0
Linear Block Code: Decoding
1 0 0 1 1 1 0
0 1 0 0 1 1 1
0 0 1 1 1 0 1
1 1 1 0
0 1 1 1
1 1 0 1
1 0 0 0
0 1 0 0
0 0 1 0
0 0 0 1
=
0 0 0 0
0 0 0 0
0 0 0 0
cHT=dGHT =0
Not 0? Error detected.