4. Overview Circuit Switching

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Oct 23, 2013 (3 years and 9 months ago)


4. Overview
4.1 Network basics for Multimedia Data 4.2 Network Technologies Considered for Multimedia 4.3 Protocols Considered for Multimedia
4. Overview1
Circuit Switching
Provide a "physical" link:connection A, B established connection C, B blocked
guaranteed bandwidth and delay ·
worldwide available ·
bandwidth not scalable ·
bad efficiency (bandwidth usage) ·
Circuit Switching2
Packet Switching
Handle independent packages:A+C sending packages of different size to B Packet may get lost because of congestion
high efficiency (bandwidth usage) ·
bandwidth is scalable ·
no guaranteed delay (bandwidth) ·
Packet Switching3
Cell Switching
Promises to combine the best of circuit switching and packet switching.All data is segmented into small cells of fixed size.Cells are multiplexed as needed.
high efficiency (bandwidth usage) ·
bandwidth is scalable ·
guaranteed bandwidth and delay ·
rare availability ·
Cell Switching4
4.1.2. ISO / OSI Reference Model (1)
4.1.2. ISO / OSI Reference Model (1)5
ISO / OSI Reference Model (2)
1The physical layer defines electric signaling on the transmission channel; how bits are converted into electric current, light pulses or any other physical form. Serial_line is an example of the physical layer. A network device for this layer is called a repeater.
2The data link layer defines how the network layer frames are transmitted as bits. An example of a data link layer protocol is Ethernet. A network device for this layer is called a bridge.
(usually hardware)
(usually software)
3The network layer defines how information from the transport layer is sent over networks and how different hosts are addressed. An example of a network layer protocol is the Internet Protocol. A network device for this layer is called a router.
ISO / OSI Reference Model (2)6
ISO / OSI Reference Model (3)
4The transport layer takes care of data transfer, ensuring the integrity of data if desired by the upper layers. TCP and UDP are operating at this layer.
5The session layer establishes and terminates connections and arranges sessions to logical parts.TCP and RPC provide some functions at this layer.
6The presentation layer takes care of data type conversion. Protocols residing at this layer are used to provide interoperability between heterogeneous computer systems.
7The application layer defines the protocols to be used between the application programs.Examples of protocols at this layer are protocols for WWW (http) electronic mail (e.g. SMTP) and file transfer (e.g. FTP).
ISO / OSI Reference Model (3)7
4.1.3. Network QoS Mechanisms
Network Device QoS Mechanisms
Classification: type of incoming data ·
Shaping & Policy: keep / monitor traffic characteristic
Queueing: determine output schedule ·
Network Mechanisms related to QoS
Congestion control / avoidance ·
Routing ·
SLA / QoS Signaling ·
Media transport & usage ·
4.1.3. Network QoS Mechanisms8
Traffic Classification
In order to provide QoS in a packet switched network, a network device has to classify each incoming packet
distinguish flows and aggregations (terms: flow-based vs class-based) ·
Classification criteria
physical port of incoming data 
frame/packet addresses (MAC, IP-Address, TCP/UDP Port-Number) 
protocol interpretation 
Obtaining classification info
static: by (manual) configuration 
dynamic: by signaling 
Traffic Classification9
Shaping vs Policing
(image source: Cisco.com)
shaping: keep a traffic characteristic, increases delay!·
policing: monitor the traffic characteristic, increases loss rate!·
Shaping vs Policing10
Packet Scheduling / Queueing 1
best effort service only ·
prior admission control and policing may improve fairness
Packet Scheduling / Queueing 111
Packet Scheduling / Queueing 2
(Strict) Priority Queueing
different services according to bandwidth and delay
unfair, because starvation of low priority flows possible
Packet Scheduling / Queueing 212
Packet Scheduling / Queueing 3
Weighted Fair Queueing (WFQ)
each queue receives a portion of the available bandwidth resources
round robin according to weight of queues, guarantees fairness ·
Packet Scheduling / Queueing 313
An end-to-end network connection is called isochronous if the bit rate and the jitter over the connection life time is guaranteed and the jitter is also small Isochronism therefore simply defines the requirements of continuous media streams.Remark:
The property of media like audio and video, that must be sampled and played in regular intervals is also called isochrony
Isochronism does not define quantitative values for jitter or probabilities refereed to by guaranteed.·
If a jitter may be considered small depends on the application.·
Multicasting is the capability to replicate data at certain internal points. Replicated data is forwarded to endsystems which are part of a multicast group.
multicast avoids or minimizes the multiple transport of the same data over the same network segments ·
broadcast is a special case of multicast ·
data duplication must be supported by forwarding engines in switches ·
multicast types
one-to-many unidirectional 
one-to-many bi-directional 
many-to-many 
Note: The multicast or broadcast capability on OSI layer 2 is usually a prerequisite for the realization of multicast on layer 3.
4.2. Networks
Different network characteristics lead to different usability for the transport of multimedia data:
QoS guarantees:
bandwidth 
delay 
delay variation 
Isochronism ·
Multicast capability ·
bandwidth 
traffic types 
distance (LAN, WAN) 
physical media 
Efficiency/Utilization of physical media ·
Costs ·
4.2. Networks16
4.2.1. Ethernet
Xerox Corp.: R. Metcalfe (PHD at the M.I.T.) and D. Boggs ·
Standardized by IEEE 802.3
there are vendor specific Ethernet variants, e.g. Ethernet V2 
The original article from Robert M. Metcalfe and David R. Boggs Xerox Palo Alto Research Center
4.2.1. Ethernet17
Original: Bus topology ·
Bandwidth: 10 Mbit/s half-duplex ·
Several physical media: coax cable, twisted-pair, fiber ·
Access protocol CSMA/CD (Carrier Sense Multiple Access with Collision Detection) ·
Carrier sense: check if there is traffic on the net before sending 
Multiple access: each station "listens" simultaneously to the net and tries to send 
Collision detection: if multiple stations are sending, data will be corrupted, wait and try again 
⇒ demands a minimum frame size!Evolution ·
Using switches, leads to star topology ⇒ CSMA/CD no longer necesarry ⇒ enables full-duplex

Bandwidth: 100 Mbit/s, 1 Gbit/s, 10 Gbit/s 
Several extensions liek: auto negotiation, flow-control, burst modes, VLAN-tagging (IEEE 802.3Q), priorities (IEEE 802.3p), link aggregation (IEEE 802.3ad)

Ethernet: Frame
MAC=Medium Access ControlLLC=Logical Link control
SNAP=Sub-network Access ProtocolDSAP=Destination Service Access Point
SSAP=Source Service Access Pointcntl=control
org code=organization codeCRC=Cyclic Redundancy Check
Ethernet: Frame19
4.2.2. ISDN:
Integrated Services Digital Network
ITU recommendations (former CCITT) ·
ETSI and ANSI standards ·
Public, digital, end-to-end network ·
Implements digital bit pipe
Based on 64 Kbit/s data rate 
Multiple full duplex data channels 
Support for multiple media and services within one network:
Voice, low quality video, image data, text data,
supplementary services 
Common signaling channel with common set of signaling protocols ·
Circuit switching ·
Fixed bandwidth channel assignment ·
4.2.2. ISDN:Integrated Services Digital Network21
ISDN: Usability for Multimedia Data
QoS parameters:guaranteed bandwidth low delay and low delay variation (not guaranteed, e.g. Satellite links with significantly higher delay)
Isochronism:guaranteed by design principles
Multicast capability:
no multicast capabilities
Flexibility:fixed bandwidth although ISDN is used for end-to-end communications, it is mainly a WAN technology independent of physical media
Efficiency:low bandwidth utilization
Costs:expensive bandwidth (Telecom ports include WAN connectivity)
ISDN: Usability for Multimedia Data22
5 Protocols (Layer 3-7)
A protocol defines the rules and formats for the exchange of data.
Examples for typicall proctocoll tasks:
Layer 3: End-to-end connectivity (host-to-host) ·
Layer 4: Process-to-Process connectivity ·
Reliable communication
Error detection 
Error recovery, e.g. forward error correction or retransmission 
Resource management
avoid congestion, by flow control
within the network 
within end systems 

Priorisation 
Resource reservation 
Support for specialized media types
Content description 
Timing / Synchronization Information 
And more ...·
5 Protocols (Layer 3-7)23
TCP/IP Suite
TCP/IP Suite24
4.3.2. IP Protocol (RFC 791)
Development of IP
DARPA: Defense Advanced Research Projects Agency
The research aim was to build a network that is tolerant to extensive damage, e.g. by a nuclear strike

1973/1974 development of TCP/IP, a replacement of NCP (Network Control Protocol) 
Since 1975 the ARPANET was controlled by the DoD 
In the early 80'ies the military part was extracted from the ARPANET 
Since 1983 exclusive use of TCP/IP, defining the term Internet 
IP is specified in RFC 791 ·
"This document is based on six earlier editions of the ARPA Internet Protocol Specification ..."·
IP characteristics
Provides end-to-end communication ·
Connection less, i.e. state less protocol ·
Provides unreliable transfer of packets ·
Packets may be reordered during transmission ·
Error messages are handled by the separate protocol ICMP (Internet Control Message Protocol) ·
4.3.2. IP Protocol (RFC 791)25
IP Header
Version:version of IP headerFlags, Frag.Offset:
used for fragmentation
IHL:IP header length in 32 bit words (5+ no. of options)
TTL:Time To Live, decremented by each machine to pass the packet
Type Of Service precedence ~ priority; D,T,R if set optimize for Delay, Throughput, Reliability
Protocol:layer 4 protocol, e.g. 1=ICMP,6=TCP, 17=UDP
Length:length in bytes including the IP headerChecksum:checksum for the IP header
ID:serial numberOptions:security, record route, timestamp,source routes
IP Header26
4.3.3. UDP (RFC 768) / TCP (RFC 793)
Transport protocols (Layer 4)
Provides process to process connectivity ·
Uses port number to identify processes. An IP address and a port number is a unique identifier for a service.
Closely related to IP ·
UDP offers a connectionless and unreliable transport service
Nearly the same service as IP 
Data unit name: datagram 
TCP offers a connection oriented and reliable transport service
Recognition of lost data 
Retransmission of lost data 
Reordering of data 
Delete duplicate data 
Flow control
With respect to network congestion 
With respect to buffer overflow at the receiver side 

User data is handled as a stream of bytes
User data is split into segments 

Data unit name: segment 
4.3.3. UDP (RFC 768) / TCP (RFC 793)27
TCP Flow Control (2)
Avoid network congestion If multiple identical ACKs indicate packet loss, then slow start + congestion avoidance:If timeout indicate packet loss, then slow start + restart slow start + congestion avoidance:
TCP Flow Control (2)28
TCP Flow Control (2)29
4.3.4. IPv6
Development of IPv6
1993 the IETF called for the development of an IP next generation IPng (RFC 1550)
Improvements required
Larger address space 
Reduce size of routing tables 
Simplification of the protocol, to allow routers to process packets faster 
Better security 
Pay more attention to Type of Service 
Aid multicasting 
Support roaming 
Easier extension of the protocol 
Coexistance with the old IPv4 
1995 the IETF agreed to specification named IPv6 ( RFC1883 )
Changes to other protocols of the TCP/IP suite are specified in RFC 1884-1887

4.3.4. IPv631
IPv6 Header
Version:version of IP header
Priority:0-7 for non real time data, 8-15 for real-time data
Flow Label:may be used to identify a flow, RFC 1809 discusses how the flow label could be used
Payload length:length of the datagram without the header
Next header:options are placed in separate extension header; next header identifies an option or the protocol above IPv6
Hop limit:same as Time to Live of IPv4
Addresses:there are 7*10
IPv6 addresses per square meter of the world enabling well
structured addresses
Support of provider based addresses ·
and geographic based addresses ·
IPv6 Header32
QoS in Data Networks
IP Service Models
Network Technology
Best EffortDiffServIntServMPLSATM
noaggregatedflow basedflow based and
flow based and
nolong term
within a domain
per flow
support for:
DiffServ ·
IntServ ·
per flow (channel) or
per path
dynamic or static
end-to-end or
within a domain
QoS in Data Networks33
5.8 RTP - Real-Time Transport Protocol
RFC 1889
Consists of two closely-linked parts:
the real-time transport protocol (RTP), carries data with real-time properties ·
the RTP control protocol (RTCP), monitors QoS and distributes this information to all participants of a session
RTP makes no reservations and does not guarantee any service RTP is a protocol framework, not a complete protocol
a profile specification defines payload types and may extend RTP ·
a payload specification defines payload formats and encoding types must be specified ·
therefore RTP will typically be part of an application ·
The purpose of RTP is to provide additional information for real-time media streams
payload type, may change dynamically ·
sequence number, to determine the order (and loss) of the incomming data ·
timestamp, to enable synchornized and constant output of the data ·
contributer identifier, distinguish different contributers ·
5.8 RTP - Real-Time Transport Protocol RFC 188934
RTCP - RTP Control Protocol
RTP enables receiver to monitor the QoS:
Delay, jitter, PDU loss rate ·
RTCP periodically transmits control packets between all participants of an RTP session:
the primary function is to provide feedback about the QoS ·
carries transport-level identifiers for RTP sources, the canonical name (the SSRC may change over the time; the canonical name is fixed, e.g. a user name)
the rate of sent RTCP packets depends on the number of participants in order to make RTCP scalable ·
optionally, further information about the participants could be distributed to realize a simple session control
RTCP - RTP Control Protocol35
Interesting Links
Routing Basics
Size of BGB Tables
Internetworking Technology Handbook
Its Latency
TCP - How it works
Interesting Links36