Telecommunication & Networking

Lesson Six

ISO Open System Interconnect (OSI)

Application

Presentation

Session

Transport

Network

Data Link

Physical

LLC

MAC

Routing

Move to TCP/IP and VPN

HTTP

Binding Virtual IP AddressesSource IPAddress

DestinationIP Address

Address resolution

Hardware Address

137.45.192.8

0A:07:4B:12:BE:3C

• ARP (Address Resolution Protocol): Who has this IP?– Ultimate communication is by physical

address (Internet/Ethernet card)– Map IP to Physical Address (MAC

address)– NIC replacement or machine replacement

makes easy with same IP address– IP-to-physical binding included in every

ARP broadcast

• RARP: Who knows my IP address?

Network Layer• Routing: end-to-end not node-to-node

• Network Control: status information to determine the best path

• Congestion Control: balancing act

• Collection of Accounting Data

TCP Layers

Application

Transporthost-to-host

NetworkAccess

Physical

Internet

Application

Presentation

Session

Transport

Data Link

Physical

Network

UDPTCP

IP

ConnectionlessNo re-transmissionNo time outsNo ACKs No Naks

FTPSMTP

RPCTFTP

Class A: 126 max network number; 16,777,124 hostsClass B: 16,384; 65,634Class C: 2,097,152; 254

TCP/IP• TCP was responsible for the integrity of the

delivery of information from applications to applications

• It was broken off into two parts (1978): TCP and IP• TCP is responsible for accepting information from

applications and breaking it into pieces (TCP segments) acceptable to the network and reassembling the segments at the other end and deliver to applications as ‘information’

• IP simply forward datagrams from source host to destination host

• IP Data Unit– Version (bit 0-3): IPv4 for example

– H-Length (4-7): Header length; usually is 20 Octets (5 32-bit words)

– Type of Service (8-15): first three bits indicate “precedence,” next bit is the delay bit (1 indicates a short delay request), next bit (bit 4) is the throughput bit (1 indicates a high throughput request), next bit is the reliability bit, bit 6 is the cost bit and bit 7 is not used.

– Total Length (16-31): max length is 65,535 Octets

– Time-to-Live (TTL): 8 bits; the number of hops to traverse in the internet; routers are required to check this field and throw away the datagram if the value is 0

Classes of IP Address Space

• Primary Classes– Class A: 0 + 7-bit prefix/128 networks (24-bit

suffix - 16,777,216 hosts) [0 thru 127]– Class B: 10 + 14-bit prefix/16384 networks (16-

bit suffix - 65536 hosts) [128 thru 191]– Class C: 110 + 21-bit prefix (netid)/2,097,152

networks (8-bit suffix - 256 hosts) [192 thru 223]

– Class D: 11102 multicast address

Weaknesses of IP Addressing

• IP identifies the network and the host (machine)

• If a host computer moves from one network to another, its IP address must change

• Normadic computing makes difficult• Expansion (class C to class B) implies

complete halt of all machines to effect the change

TCP• Sequence number: either the sequence

number of the first octet in the data field or the initial send sequence (ISS) number

• Acknowledgement number: set to a value that acknowledges data previously received and the next expected octet (sequence number)

• Each node (a computer or a printer) has a unique IP address

• Address - assignment could be different; e.g., AppleTalk addresses are chosen randomly at startup by each host

• But each computer performs many functions• Need different sessions, each session is assigned a

logical ‘address’ or port number• There are 65,535 ports• Port 80 is usually for the Web Server• Check out /etc/services file in UNIX

Port Assignments in UNIX

Echo 7 Tcp/udp To verify if twomachines are able toconnect (RFC862)

ftp 21 Tcp

telnet 23 Tcp

Finger 79 Tcp RFC1288

Smtp 25 Tcp Simple Mail TransferProtocol (RFC821)

Sockets

• Datagrams - finite size packets• Header + Payload• A message could be broken down into a number

of payloads with the header indicating the source/destination nodes and port numbers respectively

• The receiving end will rearrange the packets back to a message

• Socket is born

Socket Operations

• Connect to a remote host• Send data• Receive data• Close a connection• Bind to a port• Listen for incoming data• Accept connections from remote machines on the

bound port

• The development of technologies for LANs and WANs were not coordinated and was never considered to be meshed together

• The push of Internet is to transform it from a data-network to multimedia-network using the existing infrastructure which has moved from 9600 kbps to magnitude of Gbps transmission rate

• What are the requirements to support data, voice, and video applications

• Voice & Video have a high tolerance for errors versus Data; loss of video packets demands no resend; loss of voice packets delivers poor fidelity; loss of data packets renders the communication useless.

• Yet network delay is acceptable to data packets, the information just arrived ‘late.’ But for voice & video packets, high network delay renders inefficient communications that may lead to communication breakdown due to annoyance.

• A speaks, reaches B, B listens, B responds, reaches A, A listens

• “Voice and video transmissions require a short queue length at the network nodes in order to reduce delay, or at least to make the delay more predictable. The short voice packet queue lengths can experience overflow occasionally, with the resulting packet loss. However, data packets require longer queue lengths to prevent packet loss in overflow conditions.”

VBR vs CBR• Variable Bit Rate: does not require a constant and

continuous allocation of bandwidth; bursty, asynchronous, no time or timing constraint

• Constant Bit Rate• IGMP (Internet Group Management Protocol):

multicasting• RSVP (Resource Reservation Protocol):

establishes a path (versus connectionless IP) and guarantee bandwidth of the path

UDP• UDP (User Datagram Protocol):

connectionless protocol with no retransmissions, no time outs, no ACKs and NAKs

• NTP (Network Time Protocol): • RTP (Real Time Protocol): sits on top of

UDP to support transmission of data within a defined (very short) time period

• Voice over IP (VoIP)??

Internetworking Devices

• Networking Devices – Repeaters

– Bridges

• Internetworking Devices– Routers

– Gateways

Application

Presentation

Session

Transport

Data Link

Physical

Network Gat

eway

Rou

ter

BridgeRepeater

Repeaters A repeater is a device that regenerates the original bit pattern(before the signals become too weak or corrupted).

Bridges Bridges bring together two segments of the same type ofnetwork together and relay frames across only if necessary.

Bridges

• A complete and valid frame must be received before it forwards to other segment

• At the Data Link Layer, e.g., follows CSMA/CD in a Ethernet; thus isolated collisions and delays within a segment

• Computers are not aware the existence of bridges

Bridges

Routers Routers determine where a transmission should continue bylooking at addresses at the network layer.

Gateways Gateways are typically software installed in a router thatsupports connectivity between two different kinds of network.

Simple Switching Network

Switching Node

Switching Networks

• Transmission between stations is accomplished via the network of switching nodes

• Switched Communication Networks

• Node-to-node links are TDM or FDM

• Circuit-switching vs Packet-switching

Circuit-Switching Networks

• A complete path is established before data is being transmitted

• Transmission can be analog and/or digital depending on the nature of the network

• Path will be discontinued after the completion of the transmission

• Examples are telephone networks and PBX– subscribers (telecommunication device), local loops,

exchanges, and trunks

Switching Concepts

• A Typical Circuit-Switching Node– Control Unit

• establishes connections• maintain connections• shut down connections

• Blocking vs Non-blocking– blocking occurs when a request for connection

cannot be satisfied due to existing connections in the switching node

Switching Concepts (cont’d)

• Space-division Switching – Crossbar switch - costly (n2); not fault-tolerant; non-

blocking

– Multiple-stage switch - less costly; more complex control; blocking; multiple paths between two points

• Time-division Switching– Virtually all modern circuit switches use digital TDM

• TDM bus switching

Routing in Circuit-Switched Networks

• Routing : must be efficiency and resilience• Able to handle the busiest traffic load• Static routing with crossover in a

hierarchical structure to improve fault tolerance

• Dynamic routing - peer relationship at the switches (no hierarchical relationship and all are ‘equal’)

Dynamic Routing in Circuit-Switched Networks

• Two classes:– Alternate Routing

• predefined routes between two switches• select according to predetermined order (order in which

reflects the best use of network resources given historical data)• fixed vs dynamic alternate-routing scheme• fixed offer one route per two nodes; dynamic cycles through a

pre-defined set of routes per two nodes

– Adaptive Routing• reacts to changing patterns in network traffic• one example : Dynamic Traffic Management (DTM) -

developed by Northern Telecom

DTM

• Central controller finds the best alternate routes• Analysis is performed (say, every 10 seconds) to

evaluate the traffic load for all the alternate routes if the direct link is not available

• Central controller uses information such as utilization at the switch (only 2 out of 5 trunks are used), CPU load, overflow traffic.

Packet Switching

• Datagram Approach– The relationship between two or more packets of the

same message, even on the same circuit does not exist.

• Virtual Circuit Approach– Relationship between all packets belonging to a

message is preserved– Switched Virtual Circuit (SVC)

• circuit creates for the exchange and exits when the exchange is complete

– Permanent Virtual Circuit (PVC)• circuit creates for the exchange remains the same

Performance

• Propagation Delay : time it takes a signal to propagate from one node to the next

• Transmission Time : time it takes for a transmitter to send out a block of data

• Node delay : time it takes for a node to perform the necessary processing as it switches data

Characteristics of Packet Switching

• Connection-oriented service– a logical connection is set up between two

stations– all packets for that connection are numbered

and out in the sequence – all packets are received in the same sequence

(order)– external vs internal virtual-circuit service

Characteristics of Packet Switching

• Connectionless service– No logical connection is set up between two

stations– all packets for that connection are numbered

and go out independent of each other – all packets are received in the same sequence

(order)– external vs internal datagram service

Routing

• What do we look for ?– Correctness, simplicity, robustness, stability,

fairness, optimality and efficiency– Elements of routing techniques : how many

hops, cost, delay, distributed vs central vs originating decision place;

Routing Considerations

• Decision Time – Routing decision made per packet or per virtual

circuit

• Decision Place– Distributed routing – Centralized routing by some network control

center (NCC)– Source routing

Routing with no new information

• Flooding– send from a source node to every one of its neighbors

– this repeats at other nodes - to all outgoing links other than the sender link

– use diameter, if known, of the network to control packets propagation

– ADVANTAGES• all possible routes are tried

• high probability of a packet getting to its destination in a timely manner

Routing with no new information

• Random Routing– selects only one outgoing path for

retransmission of an incoming packet– variation: probability to each outgoing link

Routing with network information

• Adaptive Routing - routing decisions made as conditions on the network change– state of network must be exchanged between

nodes– the more frequent the exchange the better the

routing decisions– information exchange put load on network, may

affect overall performance

• Fixed Routing - next slide

Packet Switched Network

2 3

6

54

1

1

1

11

1

5

8

3

2

3

65

8

2

47

2 2

3

3

Fixed RoutingFromNode

- 1 5 1 4 52 - 2 2 4 5

ToNode 4 3 - 5 3 5

4 4 5 - 4 54 4 5 5 - 54 4 5 5 6 -

Minimum Hops

• or least-cost routing…

• Dijkstra’s Algorithm– Find the shortest paths from a given source node to all

other nodes by developing the paths in order of increasing path length

• Bellman-Ford Algorithm– Find the shortest paths from a given source node

subject to the constraint that the paths contain, at most, one link, then two links and so on

Frame RelayPacket Switching with Acknowledgement

Frame Relay

1

2

3

4

5

6

7

8

1 2 3 4

Control and data packets are carried on the same circuit

Control packet is carried on a separate logical connection from data packet; no hop-to-hop flow control and errorcontrol; end-to-end flow and error controls are handled at a higher layer (transport)

ISDN

• Integrated Services Digital Network

• Narrow ISDN (N-ISDN) - 64 Kbps to 1.544 Mbps

• Broadband ISDN (B-ISDN) - up to 600 Mbps; fiber-optic cable

• Allows users to send data, voice, image, facsimile and whatever on the same wires

FDDI

• Fiber Distributed Data Interface [Copper Distributed Data Interface (CDDI) using twisted-pair wires]

• Uses pulses of light to encode data for transmission

• Uses token-ring technology to achieve a speed of 100 Mbps over a distance of 200 km of fiber optics medium

ATM (Asynchronous Transfer Mode)

• Based on Broadband ISDN (B-ISDN), 1986

• ATM Forum, 1991

• First ATM specification released, 1992

• ATM Forum approved the Anchorage Accord that brings together about 60 different ATM specifications, 1996

Challenges for ATM

• Transmission system to optimize the use of high data-rate transmission media

• Interface with existing system - LAN & WAN• Inexpensive to implement• Support existing telecommunication hierarchy• Connection-oriented• Move functions to hardware

Cells

• 48 octets + 5 octets

Cell Format• GFC - generic flow control (4 bits); exists

only in UNI; for NNI, it collapses with the next field

• VPI - Virtual Path Identifier (8 bits)• VCI - Virtual Connection Identifier (16

bits): for routing to and from the end user• PT - Payload Type (3 bits)• CLP - Cell Loss Priority (1 bit)• HEC - Header Error Control (8 bits)

Cell Fields• Generic Flow Control - provides flow

control at the UNI level• Payload Type -

0 or 1

Management Data

User Data

Congestion bit0 : no congestion1 : congestion

Congestion bit Signaling bit

Management bits

00: link-associated management01: end-to-end management10: resource management11: reserved

ATM Layers

• AAL (Application Adaptation Layer)– allows existing networks (e.g., packet) to

connect to ATM facilities– map packet into cells– cells are reassembled at the receiver's end to the

original packet

Category of Services

• Category of services– constant bit rate data (AAL1)

• for real time voice

• for real time video

– variable bit rate data (AAL2)– connection-oriented packet data (AAL3)– connectionless packet data (AAL4)– simple data (AAL5 or SEAL)

ATM LAN Network

Source : Figure 13.13 (Stallings, 5th edition)

ATM LAN Hub Configuration

Source : Figure 13.14 (Stallings, 5th Ed.)

ATM LAN Emulation Configuration

Source: Figure 14.13 (Stallings, 5th edition)