"Computer network" Chapter3- data link layer

Class A:

The data link layer: the possibility of error by the physical layer the physical connection is error-free data transformation logic link

Provide services to the network layer

Unacknowledged connectionless service: for real-time communication in the communication channel or the bit error rate. Such as Ethernet

Confirmation connectionless service: for error rate of a communication channel, such as a wireless communication

Confirmation of connection-oriented services: a communication suitable for high demanding applications

Link Management

Frame delimiting : determining the boundaries of the frame

Frame synchronization : receiving a frame in order to distinguish from the received binary bit stream start and stop

Transparent transmission : no matter what data should be able to pass transfer

flow control

Error Control

ARQ (Automatic Repeat Request) method: the data to be transmitted to the sender add a certain frame CRC error detection redundancy code transmitted together, the receiver detects the data frame error according to the error detection code, the error was found is discarded, the sender retransmission timeout transmit the data frame

SAP data link layer: the MAC address

MAC address length is 48 (6 bytes) , usually expressed as hexadecimal 12

Data link layer device:

Bridge:

A frame processing data objects

Media Access Control (MAC sublayer)

A sublayer of the data link layer

Objective: to make the communication channel between nodes without mutual interference

The three ways:

Channel is divided

Random access

polling

Channel is divided manner:

A: FDM plurality baseband signal is modulated to a different frequency carrier on the then overlapped to form a composite signal multiplexing technique

TDM: Time Division Multiplexing. Be provided to the whole transport channel information is time divided into time slices (referred to as time slot), and the time slots assigned to each signal source using

WDM: Wavelength Division Multiplexing . The two or more different wavelengths of the optical carrier signal (carrying various kinds of information) at the transmitting end by the confluence of a multiplexer (also known as a multiplexer, Multiplexer), and coupled to the same optical fiber line for transmission. Technology

CDM:

Examples: A understood as a loading station and B station mung beans means a vehicle, the article passes through the channel assembled together. Then separated again to the destination

CDMA Code Division Multiple Access: represented by the above ideas two vectors, while the requirement of mutually orthogonal vectors

Random Access: All users can send messages randomly according to their wishes

The core idea: Contention

Random access MAC protocol, also known as the contention-based protocol

The ALOHA : not detected directly transmit , not receive acknowledgment retransmission

Slot ALOHA: synchronous time divided several time slots, it can only be sent at the beginning slot

CSMA (carrier sense multiple access):

The core idea: listen, then send idle

classification:

1- insist CSMA: adhere to the listener, the channel is idle sent immediately

Non-stick CSMA: busy to give listeners, we will hear a clear channel to send immediately

p- adhere CSMA: if free, to transmit data a probability p, 1-p deferred to a time slot

CSMA / CD (carrier sense multiple access / collision detection) :

CSMA improvements for the bus network or half-duplex environment. Unlikely to be a full-duplex communication

Applied LAN wired connection

Listen Before hair, listened hair (different from the CSMA protocol), conflict suspended, random retransmission (binary backoff algorithm)

Binary exponential backoff algorithm to wait for some time

Contention period (collision window or collision window): * 2-way propagation delay

The minimum frame size: Ethernet provisions 64B, too short can not determine whether the conflict issue this time frame, frame pre-listening sent not come back

Binary backoff algorithm: Up to 16 times retransmission

CSMA / CA (carrier sense multiple access / collision avoidance): Broadcast tell the other nodes, so that other nodes do not transmit data at a certain time

Why should this agreement: wireless medium to achieve collision detection hardware cost is too large ; "hidden station"

It applies to wireless LANs

Binary backoff algorithm

Implementation:

The reservation channel : how fast I

ACK : I received a reply to

RTS / CTS frame : "hidden station"

Dial-up or leased line: PPP protocol to establish point to point connection

Serial communication line byte-oriented protocol

Three components:

Link Control Protocol LCP : create, configure, test, and manage data link

Network Control Protocol the NCP : establishing a network layer protocol and is configured logical connection

An IP datagrams encapsulated into serial link methods

Working principle: In order to establish a communication connection point on the link, the sender first sends PPP LCP frames to configure and test the data link .

After the LCP is set up a data link and coordinate the selected device, the transmitting side transmits PPP NCP frames to choose and configure one or more network layer protocols .

When the selected network layer protocol is configured, each data packet can be sent to the network layer protocol data link. After the link has been configured to maintain communications until LCP or NCP frame frame clearly suggest close links, or other external events.

PPP protocol Precautions:

It does not provide error correction , only to ensure error-free reception

Only support point to point

Only supports full duplex

Ends can run different network-layer protocols

Byte-oriented

The HDLC (High Level Data Link Control protocol) set for transmission of data between network nodes protocol

Bit oriented

Full duplex communication , a higher data link transmission efficiency

All frames use the CRC test , sequentially numbered, reliable transmission

Transmission control processing function separation, greater flexibility in

basic configuration:

Balanced configuration: a master station controlling the entire link

Unbalanced: two stations are complex station, initiates a data transfer equal

Station: master, slave, complex station

Data operation

Normal Response Mode: unbalanced. The master station to the slave data transmission

Asynchronous Balanced Mode: balance.

Asynchronous response mode: unbalanced. Not received from the master station to be allowed to transmit

HDLC and PPP comparison:

Bit oriented VS byte-oriented

PPP does not use the serial number and the acknowledgment mechanism only ensure error-free reception . HDLC uses numbers and acknowledgment mechanism to provide reliable transmission

Bridge : Ethernet connection operating at the link layer of the MAC sublayer

It is processed frame , working in the data link layer

You can isolate collision domains

Transparent Bridging: Spanning Tree algorithm generates is not the best route

Source routing bridge: choose the best route

LAN Switch : is essentially a multi-port bridge

Features: Each port directly connected with a single host, can simultaneously communicate with many ports

Two exchange mode:

Straight-through

Store-and-forward: inspection data is correct

Bridges and switches in common:

By MAC address forwarding

Isolate the conflict domain, can not isolate broadcast domains

*** STP Spanning Tree Protocol (Spanning Tree Protocol):

The main role is to prevent redundant links in the network bridge formed loop operation

The protocol can be used to establish a tree topology network, eliminating loops in the network. The basic principle of STP is passed through a special protocol packets between switches, bridge protocol data unit (Bridge Protocol Data Unit, referred to as the BPDU), to determine the network topology

Spanning Tree Protocol Spanning Tree Algorithm (STA) spanning tree algorithm is complex, but the process can be summarized into the following three steps:

(1) selecting the root bridge

(2) select a root port

(3) select the specified port : is assigned forward traffic for a network switch port

Inadequate: topology convergence is slow, can not provide load balancing functionality

Status of the port: blocking, learning, listening, forwarding prohibited

Class B:

A single frame of the sliding window and stop - wait protocol

Active window and a GBN multi-frame (reverse frame N) protocol

N represents an acknowledgment frame, 1 ~ n frames are received correctly

Receiving window 1

Transmitting window 1≤W≤2 ^ n-1

Active window and select the multi-frame retransmission protocols

NAK frame: sender is required to specify the retransmission frame

+ Receiving window send window ≤2 ^ n (generally transmit window = receive window = 2 ^ (n-1))

Media Access Control Polling: token-passing protocol

Mainly used in token ring LAN

User information can not be sent at random, but through a centralized monitoring station control. A cyclic manner poll each node, and then determines the allocation of channels

Token among the nodes in a fixed order exchange

The physical topology of the transmission medium need not be a ring, token on the transfer path between the logic device must be a ring

Very suitable for high load broadcast channel: the same time a great probability that the data transmission channel

Local area network (LAN)

Topology

Star

Annulus

Bus

Star and bus bound composite structure

Mainstream transmission media: twisted pair, copper cable, optical fiber

MAC method:

Bus CSMA / CD, token bus

Ring LAN: Token Ring

Topology enables:

Logical bus topology, the physical topology is a star

Token Ring: logical topology is a ring, the physical topology is a star

Only a LAN reference model data link layer and the physical layer

data link layer:

LLC sublayer logic control: the network layer to provide unacknowledged connectionless service, connectionless service confirmation, confirmation of the connection-oriented service

Medium Access MAC sublayer: framing, error control, transparent transmission