Wireless Communication Protocols and Technologies

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Hans Peter SchwefelWireless Networks II, Lecture 1, Spring 04

Wireless Communication Protocols and Technologies

by Haibo Wang, Petar Popovski, Hans Peter Schwefel

• Mm1 Advanced mobility support (hps)

• Mm2 Ad-hoc networks (pp)

• Mm3 Wireless multi-cast (hw)

• Mm4 Intro to performance modelling (HPS)

• Mm5 Simulation Techniques and Measurements (HPS)

www.kom.auc.dk/~hw/

Page 2 Hans Peter SchwefelWireless Networks II, Lecture 1, Spring 04

Content1. Introduction of mobile

Multicast1.1 What is Multicast?1.2 Why Multicast is needed?1.3 Motivation of using Multicastin wireless network

2. Multicast on the Internet2.1 MBONE2.2 Group MembershipManagement2.3 Multicast Routing2.4 Application Layer Multicast

3. Multicast in wireless Access Network – challenges and solutions3.1 Multicast Routing in mobile scenario3.4 Heterogeneity in Access Networks3.2 Reliable multicast3.3 Bandwidth limitation

4. Summary5. Exercise

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1.1 What Is Multicast?There are three fundamental data transmission methods• Unicast is a type of point-to-point traffic between two users (one-to-

one type of traffic) (PTP)

• Multicast is a type of traffic from one sender to a specific group of users (one-to-many type of traffic) (PTM)

• Broadcast is a type of traffic from one sender to all the users who areable to recieve it.

• Q: Can you think of some examples of them?


Example

Unicast Broadcast Multicast

B C D

CommunicationNetwork

A

B C D


A

B C D


A

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1.2 Why Multicast is needed?• When there is no Multicast, if several users require the same

content, several unicast traffic streams carring the same content have to pass one link several times, which wastenetwork bandwidth!

Source

Internet

Router Router

Router

Router

Router


Why Multicast is needed? -cont• In case of multicast, the content will be deliver along the Multicast

Tree, and only be copied when the two branches split. -> Multicast saves network resources, which is expecially important

for broadband application.

Source

Internet

Router Router

RouterRouter

Router

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1.3 Motivation of using Multicast in wireless network

Bandwidth to end users in wired and wireless networks

Comparing to wired network, bandwidth in wireless networks is much more scarce, hence it is more crucial to ’share’ bandwidth via multicast!

Up to 11.3MWiMAXUp to 30MCable TV + cableModem

384KWCDMAUp to 12MADSL

11M/54M (shared)Wireless LAN10M/100MLAN

Downlink bit rate(bps)

Wireless Access Technology

Downlink bit rate(bps)

Wired Access Technology







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2. Multicast on the InternetReview the three fundamental communication methods for the Internet: • IP Unicast

• IP BroadcastQ: If it is pratical for LAN and WAN?

• IP Multicast- Based on a user group, which users can join by exchangingmessages.- Only group members receive the data stream no matter wherethey are in the Internet.


• Internet Protocol (IP) provides best effortand connectionless service

• Reliability provided by upper levels

IP Multicast

OSI 7 Layers Model

IPTCP UDP

• In IPv4, Class D of IP-addresses is used for multicast groups• From 224.0.0.0 to 239.255.255.255

• Special addresses- 224.0.0.0 is never used

- 224.0.0.1 all multicast hosts on the local networks

- 224.0.0.2 all multicast routers on the local networks

Group Identification01110 21 313

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2.1 MBONE

Router MulticastRouter

MulticastRouter

MulticastRouter

Router

Router

UE

Tunnel

Tunnel

Tunnel

UE

UE

Multicast backbone (MBONE) is a well-known implementationof IP Multicast.


MBONE Features

• Virtual overlay network, operated since 1992• uses User Datagram Protocol (UDP)• Most IP routers in that period didn’t support multicast routing• Forwarding of multicast datagrams between multicast-enabled

subnetworks is handled by “multicast routers” through tunnels• Tunnels are implemented by encapsulating IP packets (destined to

a multicast address) within an IP packet with the unicast address of next multicast capable router along the path [1].

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MBONE Multimedia Applications

• Video conferencing– uses RTPv2 (Real Time Protocol)– allows slow frame rates

• Audio conferencing– Freephone, uses special coding

• Internet Radio and TV• Other free applications for Audio, Video and Whiteboard


2.2 Group Membership Management- IGMP

• Internet Group Management Protocol• When a hosts joins a group, sends IGMP to all neighboring

multicast routers and other multicast-enabled hosts on local network.

• Multicast routers listen to IGMP messages and periodically send out queries to discover which groups are active or inactive on aparticular subnet.[2]

• If there is no reply of its queries, the router times out the group and stops forwarding traffic directed toward that group.

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IGMP Message Format

• Type = Query or Response

0 8 164 31

CheckSumUnusedTypeVersion

Group Address (0 in query)


2.3 Multicast Routing- Multicast Distribution Trees• Multicast-capable routers create distribution trees as the path to

deliver traffic to all receivers.

• The two basic types of multicast distribution trees [2]

- Source TreesThe tree root is simply the traffic source. This tree uses the shortest path through the network, so it is also named as a shortest path tree (SPT).

- Shared TreesShared trees use a single common root at a chosen point in the network. This shared root is called the Rendezvous Point (RP).

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ExamplesSource

Router Router

RouterRouter

Router

Source

Router Router

RouterRouter

Router

RP

Source tree Shared tree


Comparison of Source Trees & SharedTrees

-the paths between the source and receivers might not be the optimal path;- longer latency

The routers must maintain path information for each source. (What problem could it cause in a large scale network?)

Disadvantages

- minimum amount of state need to be kept in each router, which lowers the overall memory requirements (for a network that allows only shared trees).

- optimal path between the source and the receivers;- minimum transmission delay

Advantages

Shared Trees Source Trees

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Multicast Routing - Reverse Path Forwarding (RPF)

Source

RouterRouter

Upstream

Downstream

Router

Router

- In multicast forwarding, the router must identify direction is upstream and whichis downstream.

- To forward multicast traffic away from the source, rather than to the receiver, is called reverse path forwarding (RPF).


- Reverse Path Forwarding (RPF) cont.

When a multicast packet arrives at arouter, an RPF check is performed

on the packet.Step 1 Router checks the source

address in the unicast routing table -> whether it was received on the

interface that is on the reverse path back to the source(upstream).

Step 2 If packet is from upstream, the RPF check is successful and thepacket is forwarded.

Step 3 If not from upstream, the RPF check fails and the packet is dropped.

Router

Upstream

DownstreamS0 S2

S1

S1198.14.32.0/24S2204.1.16.0/24

S0151.10.0.0/16InterfaceNetwork

Multcast route table

Source: 151.10.5.21

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Multicast Routing Protocols

Sparse and Dense mode

From unicastrouting table

Source trees orshared trees

PIM

Not scalableLink statedatabase

Source treesMOSPF

Not scalableUsed by MBONE

Messageexchangebetween routers

Source treesDVMRP

MemoInformation Collection

Tree TypeProtocol

DVMRP: Distance Vector Multicast Routing Protocol

MOSPF: Multicast Open Shortest Path First

PIM: Protocol Independent Multicast


Protocol-Independent Multicast (PIM)• PIM does not depend on any specific IP routing protocols.• PIM uses existing unicast routing information to perform the

multicast forwarding function, such as the reverse path forwarding (RPF) check.

• PIM does not build up a completely multicast routing table at all.• PIM does not exchange multicast routing updates between routers

like other routing protocols do.

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PIM - Dense Mode & Sparse Mode• PIM Dense Mode (PIM-DM)

PIM-DM initially push multicast traffic to all the subnet. Routers that do not have any downstream receivers will prune back the unwanted traffic.

- In this Push and Prune process routes collect their state information and build up their multicast forwarding tables.

- PIM-DM can support only source trees.


PIM - Dense Mode & Sparse Mode (cont)

• PIM Sparse Mode (PIM-SM)A pull model is used in PIM-SM to deliver multicast traffic. Only active receivers who have explicitly requested the data will be forwarded the traffic to their network.

- PIM-SM uses a shared tree for multicast traffic distribution.

- PIM-SM scales well to different sizes of networks.

Q: Can you think in which case PIM-DM will perform better and in which case PIM-SM will perform better?

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Tip: Multicast provisioning in IPv6• In IPv6, multicast is a part of the protocol and supported by all IPv6

nodes.-> No specific Multicast router needed!

• A multicast address format starts with FF (the first 8 bits of the total 128 bits IP address).

• The IGMP functionality is included in ICMPv6 and named as Multicast Listener Discovery (MLD).


2.4 Application Layer Multicast (ALM)

How does ALM work?• Hosts self-organize into an overlay network and deliver data like routers.• The multicast functions like Group Management and Distribution

Tree building are shifted from network layer (IP layer) to application layer (end hosts) [3] .

Why ALM?• The multicast schemes talked before all rely on multicast-enabled routers.• However, many networks are not designed to supportmulticast communications

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• Mobility- Frequent changes on the multicast tree due to the groupmembers moving to different subnet.

• Heterogeneity in Access networks- Different Bandwidth for group members in the same group(WLAN, GPRS, 3G)

• Wireless transmission is unreliable comparing to wiredtransmission- High bit error rate (i.e., 10%), packet loss, delay and jitter

• Limited and asymmetrical bandwidth in wireless link [4].

3. IP Mobile Multicast – Challenges & possible solutions

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3.1 Multicast Routing for Mobile HostsMIP solutions• Solution 1: Remote Subscription

MC Source

Network 1

HA

Router

Network 2

FA


MC Source

Network 1

HA

Router

Network 2

FA

• Solution 2: Bi-directional tunneled multicast

Tunnel

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3.2 Heterogeneity in Access networksMulticastsource

IP based core network

UMTSWLANGPRS

64K

384K

11M


Heterogeneity in Access networks -cont

• Adapt QoS to access network capacitysuch as to us multiple flow / multi-layer coded video traffic streamfor different QoS level [5]

Laye

red

Cod

er D

++

D

D

Layer 0

Layer 1

Layer 2

64kbps

1Mbps

256kbpsVideo

receivedvideo quality

Low

Medium

High

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3.3. Reliable multicast• Power control• Error control coding• Retransmission, possiblely from the neighbour users (Co-operative

retransmission)

WirelessAP

Router

MulticastTraffic

The groupmember has good signal help to retransmit.

Membersin poorsignal condition.


- Protocols to adapt membership management and routingupdates to the bandwidth available, with reduced overhead(such as header compression)

3.4 Bandwidth limitation

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4. Summary1. Introduction of mobile




References1. Mikko Mustakallio, Lecture Notes: Multicasting – Issues and Networking

Support, 20022. Internetworking Technologies Handbook, chapter 43, www.cisco.com3. A.Garyfalos, K.Almeroth, J.Finney, A Comparison of Network and

Application Layer Multicast for Mobile IPv6 Networks, MSWiM’03, September 19, 2003, San Diego, California, USA.

4. Upkar Varshney, Multicast over wireless Networks, Communications of the ACM, Dec 2002/Vol45, No12.

5. Dapeng Wu, Yiwei Thomas Hou and Ya-qin Zhang, Scalable Video Coding and Transport over Broad-Band Wireless Networks, proceedings of the IEEE, vol. 89, No. 1, January 2001

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Exercise1. In section 3, assume the multicast source is a mobile host, and it moves from its home

network (network 1) to another network(network 2), which connect to a differentinterace of the router which is the root of a shared tree to a group of users. What willhappen when a MC packet from the new source position arrive at the rootrouter? Discuss which solution from MIP can solve this problem better? Canyou think of any other proposal to adapt the MC tree to mobile scenario?

Network 2

MC Source

Network 1

HA

FA

interface1

MC Source

interface2 Router

Root

Router

Router


2. If in a WLAN cell there are 8 mobile hosts demanding the same video streaming service, which requires a 1Mbps data rate transmission. Calculate how much cell capacity (in terms of Mbps) could be saved by using one multicast instead of 8 unicasttransmission.

3. Discuss whether multicast is applicable in your project scenario, if itis, what benefits and challenge will it bring to your scenario?

Wireless Communication Protocols and Technologies

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