Title: Validating%20the%20Resilience%20Mechanisms%20for%20the%20Packet%20Switched%20Domain%20in%203G%20Networks
1Validating the Resilience Mechanisms for the
Packet Switched Domain in 3G Networks
- Jari Hietanen
- Nokia Networks
- Helsinki
- Supervisor Professor Raimo Kantola
- Instructor Juhani Helske (M.Sc.)
2Agenda
- Background for the Thesis
- The Objective and the Scope
- Packet Switched Domain in 3G Networks
- Resilience Mechanisms in Gn Interface
- Validation of the Existing Resilience Mechanism
in Gn Interface - Validation of New Resilience Mechanisms
- Validation Results and Comparison of Resilience
Mechanisms - Conclusions
3Background for the Thesis
- Mobile networks are evolving from 2G towards 3G.
- So far the most of traffic has been voice
traffic. - Now amount of data traffic is growing faster than
traditional voice traffic. - New packet based services
- Multimedia messaging
- Wireless Internet browsing
- Advertising
- Entertainment services
4Voice and data traffic volume evolution in
Western Europe
5New Challenges for Telecom Vendors and Operators
- Amount of IP based data traffic is increasing in
3G networks. - Traditionally people are used to high
availability and service quality in circuit
switched PSTN and GSM netwoks. - Now people are expecting the same level of
availability and quality also in packet switched
3G networks. - This causes huge challenge for vendors and
operators to develop and build fault tolerant and
resilient networks, which quarantee high
availability of the networks. - 3G networks are complex
- Several physical transmission mediums (e.g. IP,
ATM, SS7, GTP..) - Many protocols and network layers
- Multi-vendor HW
6Terms
- What is the difference between terms resilience
and redundancy? - RESILIENCE
- Ability of the system to function seamlessly in
the event of the failure of any single item of
hardware or failure of the software package. - REDUNDANCY
- According to Collins English dictionary, the term
redundancy means duplication of components in
electronic or mechanical equipment so that
operations can continue following failure of a
part or repetition of information or inclusion of
additional information to reduce errors in
telecommunication transmissions and computer
processing.
7The Objective and the Scope of the Thesis
- The objective
- The main objective was to study and validate
existing and new resilience mechanisms for the Gn
interface in 3G networks. - The scope
- Thesis concentrated only on resilience mechanisms
of data link layer (L2) and network layer (L3).
Upper and lower level resilience solutions were
out of the scope. - The implementation of Gn interface architecture
and resilience mechanisms are not standardized.
Thesis concentrated on validating Nokias
implementation to build the resilient Gn
interface between 3G SGSN and GGSN network
elements.
8The UMTS Network Architecture
9The 3G SGSN
- Main functions
- Subscriber authentication authorization.
- User data tunneling and routing (acts as a
gateway for user data tunneling between RNC and
GGSN, separate tunnels towards RNC and GGSN for
each connection). - Mobility management (controls the location, state
and security of UE). - Session management (managed through resource
monitoring, admission control and PDP context
creation, modification and deletion). - Traffic management (performs packet classifying,
policing, buffering, shaping, marking and
scheduling to ensure that all connections receive
appropriate Quality of Service). - Short message delivery.
- Collection of charging data and traffic
statistics.
10The 3G GGSN
- Main functions
- Signalling towards access networks. There is
signalling, which is required for creating,
modifying and deleting the PDP contexts. The
request for PDP context creation comes always
from an external network equipment. - Signalling towards data networks. Some of the
signalling is required for configuring the PDP
context. Most of this signalling happens when the
PDP context is created. It is used e.g.
allocating a IP address for User Equipment (UE). - Charging. The GGSN analyses user plane traffic
and reports the metering results to the charging
system via signalling interfaces. - Subscription management, authentication and
session control. GGSN may need to authenticate
mobile subscribers before PDP context can be
created. In addition GGSN may need to know what
services the mobile subscriber is allowed to use. - Lawful interception. In many countries local
authorities require possibility for monitoring
the traffic for certain mobile subscribers.
11The GPRS Tunneling Protocol (GTP)
- GPRS Tunnelling Protocol (GTP) is used in Gn
interface between GPRS Support Nodes (GSNs) in
UMTS and also in GPRS backbone networks. - GTP allows multi-protocol packets to be tunnelled
through the UMTS or GPRS backbone between GSNs
and UMTS Terrestrial Radio Access Network
(UTRAN). - GTP protocol is divided in to GTP Control Plane
(GTP-C) and GTP User Plane (GTP-U) procedures.
12GTP Path Management Messages and timers
- Echo Request Interval
- Echo Response Interval
- The timer T3-RESPONSE holds the maximum wait time
for a response of a request message. - The counter N3-REQUESTS holds the maximum number
of attempts made by GTP to send a request
message. The recommended value is 5.
13Default Parameter values used in Nokia 3G SGSN
and GGSN
This means that GTP considers a path between GSNs
to be down from 1 to 600 seconds depending on
configuration of GTP parameters.
For example if Echo reply waiting time (T3) is
set to be 5 seconds and Echo request
retransmission (N3) is set to be 3 times, then
the time that a GTP tunnel is declared to be down
is T3 x N3 5 s x 3 15 seconds.
14Existing Resilience Mechanisms in the Gn
Interface
- Existing solution is to use dynamic OSPF routing
protocol in 3G networks. - 3G SGSN build on an IPSO router platform.
15Gn Interface Resilience in 2G networks
- Gn interface topology for host based elements in
2G networks. - 2G SGSN has not any routing functionality.
16New Resilience Mechanisms for the Gn Interface
- The problem of the existing OSPF based resilience
mechanism for Gn backbone is that convergence
time is not necessarily fast enough. - The worst case scenario is that convergence time
can be even 40 seconds, if OSPF hello protocol is
only mechanism to detect a network failure. - Also some operators are not willing to start to
use a dynamic routing protocol in their Gn
backbone network. - Solution would be to find a suitable data link
layer (L2) protocol to be used as Gn interface
resilience mechanism. - Advantages of L2 mechanisms
- Simple and flexible network architecture using L2
resilience mechanisms. - Similarity with 2G solution.
- No need for two separate Gn interface Virtual
LANs (VLANs). - Fast convergence from error situations compared
to OSPF.
17Link Layer Resilience Mechanism validated
- Link aggregation (IEEE 802.3ad)
- Proxy ARP
- Virtual Router Redundancy Protocol (VRRP)
- Hot Standby Router Protocol (HSRP)
- Virtual MAC address based method (Nokias own
solution) - Bidirectional Forwarding Detection
18Validating Existing Resilience Mechanism
- First existing OSPF based (L3) resilience
mechanism was validated building a Gn test
network, which included 3G SGSN and GGSN nodes. - The conclusions from OSPF validation test
results - With default parameter values OSPF network
convergence times are too slow. Convergence time
can be even 40 seconds, if network includes hubs
or switches. - It is possible to improve the performance of OSPF
convergence, using shorter Hello and Router Dead
intervals. - Minimum value for Hello interval is 1 second and
for Router Dead interval 4 seconds.
19Validating New Resilience Mechanisms
- Some of the link layer techniques were validated
building a test network and measuring convergence
times from link failure situations. - Some techniques were analyzed with other methods
- using literature sources
- interviewing
20Link Aggregation Test Results
- Failover time from a link failure about 500 ms.
21VRRP Test Results
- Failover time from a link failure about 2.8
seconds.
22Test results of Virtual MAC based Mechanism
- Failover time from a link failure about 600 ms.
23Comparison of Resilience Mechanisms
- Existing OSPF based solution
- A dynamic routing protocol is easy to configure
and administrate for network operator. - OSPF is common protocol, which is supported by
also by other vendors products. - - OSPF protocol not suitable to all networks
topologies, e.g. if network includes switches - - Convergence time from error situation rather
slow. - - Hello protocol based mechanism is not fast
enough if the default parameter values are used.
24Link aggregation
- Does not require new hardware
- Does not waste extra interface or line card
capacity in router. - Fast recovery time from failure situation
(about 500 ms) - Standardized solution
- Economic and flexible method to increase
network capacity
25Proxy ARP
- It can be added to a single router on a network
without disturbing the routing tables of the
other routers on the network. - IP hosts can be used without configuring
default gateway. - IP network does not need to have any routing
intelligence. - Fast recovery time from failure situations.
- Standardized solution.
- It is easy to implement (only the gateway
router has to be updated to support Proxy ARP). - - Hosts need larger ARP tables to handle
IP-to-MAP mappings. - - The amount of ARP traffic increases.
- - This does not work with all network topologies
(e.g. more than one router connecting two
physical networks).
26VRRP
- It offers higher availability of the default
path without requiring configuration of dynamic
routing protocol or router discovery protocols on
every end-host. - Fast recovery from failure situations (average
about 2.8 s). - Simple and flexible protocol.
- Standardized solution.
- - Not feasible to use with all network node
architectures.
HSRP
The protocol offers similar functionality with
VRRP. - It is a patented solution, which is not
possible to be utilized in free of charge. - It
does not offer anything superior compared to
VRRP.
27Bidirectional Forwarding Detection
- OSPF alone offers minimum convergence time of
1-2 second. With BFD protocol OSPF can provide
sub-second failure detection time. According to
measurements made by Marko Luoma (Lic.Tech.) in
HUT Networking Laboratory convergence time can be
even 75 ms. - Because BFD is not tied to any particular
routing protocol, it can be used as a generic and
consistent failure detection mechanism for e.g.
OSPF, IS-IS, EIGRP, and BGP routing protocols. - CPU usage is minimal for route processor.
- - BFD can potentially generate false alarms and
signaling a link failure when one does not exist.
Because the timers used for BFD are so tight, a
brief interval of data corruption or queue
congestion could potentially cause BFD to miss
enough control packets to allow the detect-timer
to expire
28Conclusions
- OSPF based layer 3 resilience mechanism is
suitable for Gn network alone, but in the future
it will be too slow mechanism to detect network
failures. - For a greenfield operator who does not have
existing 2G network, it might be reasonable to
implement resilience using OSPF. A dynamic
routing protocol based solution offers advantages
compared to link layer solution. For example
configuration of a network is simpler - However, the performance of Gn network can be
improved using some L2 mechanism under OSPF. - Bidirectional Forwarding Detection seems to be
most suitable L2 resilience mechanism to be used
with OSPF.
29Questions?