Telecommunication & Networking

Lesson 5

ISO Open System Interconnect (OSI)

Application

Presentation

Session

Transport

Network

Data Link

Physical

LLC

MAC

Routing

Move to TCP/IP and VPN

HTTP

Mainframe:IBM 370Minicomputer:VAX/VMS

ARPANET InternetOr other networks

Dumb Terminal

Dumb Terminal: No RAM, OS or CPU

File ServerFile Server

ARPANET Internet

Workstation: CPU, RAM, OS, Local Storage,… NIC

HDLC

• A bit-oriented data link protocol• Supports both half-duplex and full-duplex

communication over point-to-point and multipoint links

• ISO standard (1979)• Originated as IBM’s S(ynchronous)DLC in 1975• ITU-T has developed other (link access) protocols

(e.g., LAPB, PAPD, LAPM, etc.)

HDLC

Flag

Address

Control

Frame Check Sequence

Characteristics of HDLC

• Station Types– primary : complete control of the link; sends

commands to secondary. For example, a computer to a terminal

– secondary – combined : sends commands and responses

Primary Secondary Secondary

Configurations

• Unbalanced

• Symmetrical

• Balanced

Modes of Communication

• NRM - Normal Response Mode– standard primary-secondary relationship; a

secondary must have permission from the primary device before transmitting

• ARM - Asynchronous Response Mode– secondary may initiate transmissions without

permission from primary if the link is idle– still primary-secondary relationship

Modes of Communication

• ABM - Asynchronous Balanced Mode– all stations are equal (thus, combined

configuration)

Frames

• U-frames– unnumbered frames : reserved for management

of frames

• I-frames– information : data and control information

• S-frames– supervisory : data link layer flow and error

control information

Bit Stuffing - HDLC

• A flag field is used for synchronization

• 8-bit pattern : 01111110

• To prevent the same bit pattern to appear in the ‘data,’ a bit 0 is inserted after the fifth consecutive 1’s

Flag FlagAddress Control

IEEE 802 Standards• 802.1: High-Level Interface – addresses

matters relating to networking…

• 802.2: Logical Link Control

Data Link Layer

Physical Layer

Network LayerLogical Link Control (LLC)

Media Access Control (MAC)

• 802.3: CSMA/CD– 1Base5: 1-Mbps baseband medium with a max

segment length of 500 m– 10Base5– 10Base2: 185 m with cable of Thinnet or

Cheapernet– 10BaseT: with twisted-pair– 10Broad36: 10Mbps broadband medium, 3600-

m– 100Base-TX: 100 Mbps baseband, twisted-pair– 100Base-FX: 100, baseband, fiber optic– 1000Base-Sx: 1000, baseband, fiber optic

• 802.4: Token Bus• 802.5: Token Ring• 802.6: MANs – Distributed Queue Dual Bus

(DQDB); two one-directional buses• 802.7: Broadband Technical Advisory Group• 802.8: Fiber Optic Technical Advisory Group• 802.9: Integrated Data and Voice Networks –

setting standards to integrate with ISDN• 802.10: LAN Security• 802.11: Wireless LANS• 802.12: Demand Priority Access Method -

standards for 100 Mbps over twisted pair wires

DLC

Data Field

32-bit CRC

Length Field

Source Address

Destination Address

Start Frame Delimiter

Preamble: Synchronization

MAC• To gain access to a medium, it uses a

contention approach:– Each node ‘listens’ to the medium to see if it is

being used (a message is passing through) [Carrier Sensing]

– If no message, node begins transmission

• [Multiple Access] occurs when more than one node performs step 2 above: begin sending; thus creating a collision

MAC• CSMA/CD: Carrier Sense with Multiple

Access and Collision Detection – most commonly used MAC protocol in Ethernet LAN’s

• A fair protocol: each node has an equal access; random intervals between access if collision occurs

• CSMA/CA: Avoidance; time space out transmission for each node

Token Passing in MAC• One token goes around

• Each node waits for the token– No message to send, send the token to next

node– Message to send, send the message, wait for

acknowledgement, and then send the token to next node

• Bus versus Ring

Multiplexing

• When two communicating devices/computers do not completely utilize the full capacity of a data link

• FDM - frequency-division multiplexing

• TDM - time-division multiplexing

• statistical/asynchronous/intelligent TDM

Asynchronous TDM

1

2

3

4

5

MU

X 1 4 3 4 1

Synchronous TDM

1

2

3

4

5

MU

X 5 4 3 2 1

Digital Carrier Systems

• AT&T used synchronous TDM to structure a hierarchy of multiplex formats

• DS-1 (24 channels at 1.544 Mbps): provided by a T-1 carrier system

• DS-4 (4032 channels at 274.176 Mbps)• OC-1: 51.84 Mbps (Optical Carrier) – SONET

(synchornous optical network:ANSI) SDH (synchronous digital hierarchy: ITU-T)

Compression

• Lempel-Ziv (LZ) algorithm

• Used in ITU-T standard V.42bisV.42bis

• Also in ZIP

• Maintain a dictionary of character strings, starting from the alphabet

• Text-based compression

This course reviews data commcommunication issues businesses have to face with in this new economy.  Information is pushing through in unforgiving speed, internally in a corporation to facilitate effective and efficient commcommunications, and externally to consumers and business partners to fuel the frantic entry to B2C and B2B electronic commcommerce.   We will begin our investigation with transmission media that pave the 'road' for information to flow through in a complex networking environment with different levels of abstraction.  We will study some commcommon protocol standards, the wireless commcommunication trend and of course the networking technology behind the World Wide Web

Compression

• M-JPEG (Motion Joint Photographic Experts Group): compress by individual frame (intraframe)

• ITU-T H.261

• MPEG: interframe and intraframe

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